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cURL and libcurl, Security

Increased CVE activity in curl?

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Recently I have received curious questions from users, customers and bystanders.

Can you explain the seemingly increased CVE activity in curl over like the last year or so?

(data and stats for this post comes mostly from the curl dashboard)

Pointless but related poll I ran on Twitter

Frequency

In 2022 we have already had 14 CVEs reported so far, and we will announce the 15th when we release curl 7.85.0 at the end of August. Going into September 2022, there have been a total of 18 reported CVEs in the last 12 months.

During the whole of 2021 we had 13 CVEs reported – and already that was a large amount and the most CVEs in a single year since 2016.

There has clearly been an increased CVE issue rate in curl as of late.

Finding and fixing problems is good

While every reported security problem stings my ego and makes my soul hurt since it was yet another mistake I feel I should have found or not made in the first place, the key take away is still that it is good that they are found and reported so that they can be fixed properly. Ideally, we also learn something from each such report and make it less likely that we ever introduce that (kind of) problem again.

That might be the absolutely hardest task around each CVE. To figure out what went wrong, detect a pattern and then lock it down. It’s almost amusing how all bugs look a like a one-off mistake with nothing to learn from…

Ironically, the only way we know people are looking really hard at curl security is when someone reports security problems. If there was no reports at all, can we really be sure that people are scrutinizing the code the way we want?

Who is counting?

Counting the amount of CVEs and giving that a meaning, or even comparing the number between projects, is futile and a bad idea. The number does not say much and comparing two projects this way is impossible and will not tell you anything. Every project is unique.

Just counting CVEs disregards the fact that they all have severity levels. Are they a dozen “severity low” or are they a handful of critical ones? Even if you take severity into account, they might have gotten entirely different severity for virtually the same error, or vice versa.

Further, some projects attract more scrutinize and investigation because they are considered more worthwhile targets. Or perhaps they just pay researchers more for their findings. Projects that don’t get the same amount of focus will naturally get fewer security problems reported for them, which does not necessarily mean that they have fewer problems.

Incentives

The curl bug-bounty really works as an incentive as we do reward security researchers a sizable amount of money for every confirmed security flaw they report. Recently, we have handed over 2,400 USD for each Medium-severity security problem.

In addition to that, finding and getting credited for finding a flaw in a widespread product such as curl is also seen as an extra “feather in the hat” for a lot of security-minded bug hunters.

Over the last year alone, we have paid about 30,000 USD in bug bounty rewards to security researchers, summing up a total of over 40,000 USD since the program started.

Accumulated bug-bounty rewards for curl CVEs over time

Who’s looking?

We have been fortunate to have received the attention of some very skilled, patient and knowledgeable individuals. To find security problems in modern curl, the best bug hunters both know the ins and outs of the curl project source code itself while at the same time they know the protocols curl speaks to a deep level. That’s how you can find mismatches that shouldn’t be there and that could lead to security problems.

The 15 reports we have received in 2022 so far (including the pending one) have been reported by just four individuals. Two of them did one each, the other two did 87% of the reporting. Harry Sintonen alone reported 60% of them.

Hardly any curl security problems are found with source code analyzers or even fuzzers these days. Those low hanging fruits have already been picked.

We care, we act

Our average response time for security reports sent to the curl project has been less than two hours during 2022, for the 56 reports received so far.

We give each report a thorough investigation and we spend a serious amount of time and effort to really make sure we understand all the angles of the claim, that it really is a security problem, that we produce the best possible fix for it and not the least: that we produce a mighty fine advisory for the issue that explains it to the world with detail and accuracy.

Less than 8% of the submissions we get are eventually confirmed actual security problems.

As a general rule all security problems we confirm, are fixed in the pending next release. The only acceptable exception would be if the report arrives just a day or two from the next release date.

We work hard to make curl more secure and to use more ways of writing secure code and tools to detect mistakes than ever, to minimize the risk for introducing security flaws.

Judge a project on how it acts

Since you cannot judge a project by the number of CVEs that come out of it, what you should instead pay more focus on when you assess the health of a software project is how it acts when security problems are reported.

Most problems are still very old

In the curl project we make a habit of tracing back and figuring out exactly in which release each and every security problem was once introduced. Often the exact commit. (Usually that commit was authored by me, but let’s not linger on that fact now.)

One fun thing this allows us to do, is to see how long time the offending code has been present in releases. The period during which all the eyeballs that presumably glanced over the code missed the fact that there was a security bug in there.

On average, curl security problems have been present an extended period of time before there are found and reported.

On average for all CVEs: 2,867 days

The average time bugs were present for CVEs reported during the last 12 months is a whopping 3,245 days. This is very close to nine years.

How many people read the code in those nine years?

The people who find security bugs do not know nor do they care about the age of a source code line when they dig up the problems. The fact that the bugs are usually old could be an indication that we introduced more security bugs in the past than we do now.

Finding vs introducing

Enough about finding issues for a moment. Let’s talk about introducing security problems. I already mentioned we track down exactly when security flaws were introduced. We know.

All CVEs in curl, green are found, red are introduced

With this, we can look at the trend and see if we are improving over time.

The project has existed for almost 25 years, which means that if we introduce problems spread out evenly over time, we would have added 4% of them every year. About 5 CVE problems are introduced per year on average. So, being above or below 5 introduced makes us above or below an average year.

Bugs are probably not introduced as a product of time, but more as a product of number of lines of code or perhaps as a ratio of the commits.

75% of the CVE errors were introduced before March 2014 – and yet the code base “only” had 102,000 lines of code at the end of that period. At that time, we had done 61% of the git commits (17684). One CVE per 189 commits.

15% of the security problems were introduced during the last five years and now we are at 148,000 lines of code. Finding needles in a growing haystack. With more code than ever before, we introduce bugs at a lower-than average rate. One CVE per 352 commits. This probably is also related to the ever-growing number of tests and CI jobs that help us detect more problems before we merge them.

Number of lines of code. Includes comments, excludes blank lines

The commit rate has remained between 1,100 and 1,700 commits per year since 2007 with ups and downs but no obvious growing or declining trend.

Number of commits per year

Harry Sintonen

Harry reported a large amount of the recent curl CVEs as mentioned above, and is credited for a total of 17 reported curl CVEs – no one else is even close to that track record. I figured it is apt to ask him about the curl situation of today, to make sure this is not just me hallucinating things up. What do you think is the reason for the increased number of CVEs in curl in 2022?

Harry replied:

  1. The news of good bounties being paid likely has been attracting more researchers to look at curl.
  2. I did put considerable effort in doing code reviews. I’m sure some other people put in a lot of effort too.
  3. When a certain before unseen type of vulnerability is found, it will attract people to look at the code as a whole for similar or surrounding issues. This quite often results in new, similar CVEs bundling up. This is kind of a clustering effect.

It’s worth mentioning, I think, that even though there has been more CVEs found recently, they have been found (well most of them at least) as part of the bug bounty program and get handled in a controlled manner. While it would be even better to have no vulnerabilities at all, finding and handling them in controlled manner is the 2nd best option.

This might be escaping a random observer who just looks at the recent amount of CVEs and goes “oh this is really bad – something must have gone wrong!”. Some of the issues are rather old and were only found now due to the increased attention.

Conclusion

We introduce CVE problems at a slower rate now than we did in the past even though we have gotten problems reported at a higher than usual frequency recently.

The way I see it, we are good. I suppose the future will tell if I am right.

cURL and libcurl, Network, Security

QUIC and HTTP/3 with wolfSSL

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Disclaimer: I work for wolfSSL but I don’t speak for wolfSSL. I state my own opinions and I try to be as honest and transparent as possible. As always.

QUIC API

Back in the summer of 2020 I blogged about QUIC support coming in wolfSSL. That work never actually took off, primarily I believe because the team kept busy with other projects and tasks that had more customer focus and interest and yeah, there was not really any noticeable customer demand for QUIC with wolfSSL.

Time passed.

On July 21 2022, Stefan Eissing submitted his work on introducing a QUIC API and after reviews and updates, it was merged into the wolfSSL master branch on August 9th.

The QUIC API is planned to appear “for real” in a coming wolfSSL release version. Until then, we can play with what is available in git.

Let me be clear here: the good people at wolfSSL has not decided to write a full QUIC implementation, because that would be insane when so many good alternatives are already being worked on. This is just a set of new functions to allow wolfSSL to be used as TLS component when a QUIC stack is created.

Having QUIC support in wolfSSL is just one (but important) step along the way as it makes it possible to use wolfSSL to build a QUIC implementation but there are some more steps needed to turn this baby into full HTTP/3.

ngtcp2

Luckily, ngtcp2 exists and it is an established QUIC implementation that was written to be TLS agnostic from the beginning. This “only” needs adaptions provided to make sure it can be built and used with wolSSL as the TLS provider.

Stefan brought wolfSSL support to ngtcp2 in this PR. Merged on August 13th.

nghttp3

nghttp3 is the HTTP/3 library that uses ngtcp2 for QUIC, so once ngtcp2 supports wolfSSL we can use nghttp3 to do HTTP/3.

curl

curl can (as one of the available options) get built to use nghttp3 for HTTP/3, and if we just make sure we use an underlying ngtcp2 built to use a wolfSSL version with QUIC support, we can now do proper curl HTTP/3 transfers powered by wolfSSL.

Stefan made it possible to build curl with the wolfSSL+ngtcp2 combo in this PR. Merged on August 15th.

Available HTTP/3 components

With this new ecosystem addition, the chart of HTTP/3 components for curl did not get any easier to parse!

If you start by selecting which HTTP/3 library (or maybe I should call it HTTP/3 vertical) to use when building, there are three available options to go with: quiche, msh3 or nghttp3. Depending on that choice, the QUIC library is given. quiche does QUIC as well, but the two other HTTP/3 libraries use dedicated QUIC libraries (msquic and ngtcp2 respectively).

Depending on which QUIC solution you use, there is a limited selection of TLS libraries to use. The image above shows TLS libraries that curl also supports for other protocols, meaning that if you pick one of those you can still use that curl build to for example do HTTPS for HTTP version 1 or 2.

TLS options

If you instead rather pick TLS library first, only quictls and BoringSSL are supported by all QUIC libraries (quictls is an OpenSSL fork with a BoringSSL-like QUIC API patched in). If you rather build curl to use Schannel (that’s the native Windows TLS API), GnuTLS or wolfSSL you have also indirectly chosen which QUIC and HTTP/3 libraries to use.

Picotls

ngtcp2 supports Picotls shown in orange in the image above because that is a TLS 1.3-only library that is not supported for other TLS operations within curl. If you build curl and opt to go with a ngtcp2 build using Picotls for QUIC, you would need to have use an second TLS library for other TLS-using protocols. This is possible, but is rarely what users prefer.

No OpenSSL option

It should probably be especially highlighted that the plain vanilla OpenSSL is not an available option. Primarily because they decided that the already created API was not good enough for them so they will instead work on implementing their own QUIC library to be released at some point in the future. That also implies that if we want to build curl to do HTTP/3 with OpenSSL in the future, we probably need to add support for a forth QUIC library – and someone would also have to write a HTTP/3 library to use OpenSSL for QUIC.

Why wolfSSL adding QUIC is good for HTTP/3

People in general want to build applications and infrastructure using released, official and supported libraries and the sad truth is that there is a clear shortage in such TLS libraries with QUIC support.

In your typical current Linux distribution, quictls and BoringSSL are usually not viable options. The first since it is an OpenSSL fork not many even ship as a package and the second because it is done by Google for Google and they don’t do releases and generally care little for outside-Google users.

For the situations where those two TLS options are out of the game, the image above shows you the grim reality: your HTTP/3 options are limited. On Windows you can go with msh3 since it can use Schannel there, but on non-Windows you can only use ngtcp2/nghttp3 and before this wolfSSL support the only TLS option was GnuTLS.

For many embedded solutions, or even FIPS requirements, wolfSSL is now the only viable option for doing HTTP/3 with curl.

cURL and libcurl, Network, Open Source

The dream of auto-detecting proxies

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curl, along with every other Internet tool with aspirations, supports proxies. A proxy is a (usually known) middle-man in a network operation; instead of going directly to the remote end server, the client goes via a proxy.

curl has supported proxies since the day it was born.

Which proxy?

Applications that do Internet transfers often would like to automatically be able to do their transfers even when users are trapped in an environment where they use a proxy. To figure out the proxy situation automatically.

Many proxy users have to use their proxy to do Internet transfers.

A library to detect which proxy?

The challenge to figure out the proxy situation is of course even bigger if your applications can run on multiple platforms. macOS, Windows and Linux have completely different ways of storing and accessing the necessary information.

libproxy

libproxy is a well-known library used for exactly this purpose. The first feature request to add support for this into libcurl that I can find, was filed in December 2007 (I also blogged about it) and it has been popping up occasionally over the years since then.

In August 2016, David Woodhouse submitted a patch for curl that implemented support for libproxy (the PR version is here). I was skeptical then primarily because of the lack of tests and docs in libproxy (and that the project seemed totally unresponsive to bug reports). Subsequently we did not merge that pull request.

Almost six years later, in June 2022, Jan Brummer revived David’s previous work and submitted a fresh pull request to add libproxy support in curl. Another try.

The proxy library dream is clearly still very much alive. There are also a fair amount of applications and systems today that are built to use libproxy to figure out the proxy and then tell curl about it.

What is unfortunately also still present, is the unsatisfying state of libproxy. It seems to have changed and improved somewhat since the last time I looked at it (6 years ago), but there several warning signs remaining that make me hesitate.

This is not a dependency I want to encourage curl users to lean and depend upon.

I greatly appreciate the idea of a libproxy but I do not like the (state of the) implementation.

My responsibility

Or rather, the responsibility I think we have as curl maintainers. We ship a product that is used and depended upon by an almost unfathomable amount of users, tools, products and devices.

Our job is to help guide our users so that the entire product, including third party dependencies, become an as safe and secure solution as possible. I am not saying that we can take full responsibility for the security of code outside of our own domain, but I think we should recognize that what we condone and recommend will be used. People read our support for library X as some level of approval.

We should only add support for third party libraries that meets a certain quality threshold. This threshold or bar maybe is (unfortunately) not written down anywhere but is still mostly a soft “gut feeling” based on human reviews of the situation.

What to do

I think the sensible thing to do first, before trying to get curl to use libproxy, is to make sure that there is a library for proxies that we can and want to lean on. That library can be the libproxy of today but it could also be something else.

Some areas in need of attention in libproxy that I recently also highlighted in the curl PR, that would take it closer to meeting the requirements we can ask of a dependency:

  1. Improve the project with docs. We cannot safely rely on a library and its APIs if we don’t know exactly what to expect.
  2. There needs to be at least basic tests that verify its functionality. We cannot fix and improve the library safely if we cannot check that it still works and behaves as expected. Tests is the only reliable way to make sure of this.
  3. Add CI jobs that build the project and run those tests. To help the project better verify that things don’t break along the way.
  4. Consider improving the API. To be fair, it is extremely simple now, but it’s also so simple that that it becomes ineffective and quirky in some use cases.
  5. Allow for external URL parser and URL retrieving etc to avoid blocking, double-parsing and to reuse caches properly. It would be ridiculous for curl to use a library that has its own separate (semi-broken) HTTP transfer and its own (synchronous) name resolving process.

A solid proxy library?

The current libproxy is not even a lot of code, it could perhaps make more sense to just plainly write a new library based on how you would want a library like this to work – and use all the knowledge, magic and experience from libproxy to get the technical parts done correctly. A libproxy-next-generation.

But

This would require that someone really wanted to see this development take place and happen. It would take someone to take lead in the project and push for changes (like perhaps the 5 bullets I listed above). The best would be if someone who would like to use this kind of setup could sponsor a developer half/full time for a while to get a good head start on this.

Based on history, seeing we have had this known use case and this library around for well over a decade and this is the best we have accomplished so far, I am not optimistic that we can turn this ship around. Until then, we simply cannot allow curl to use this dependency.

cURL and libcurl, Network, Security

IPFS and their gateways

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The InterPlanetary File System (IPFS) is according to the Wikipedia description: “a protocol, hypermedia and file sharing peer-to-peer network for storing and sharing data in a distributed file system.”. It works a little like bittorrent and you typically access content on it using a very long hash in an ipfs:// URL. Like this:

ipfs://bafybeigagd5nmnn2iys2f3doro7ydrevyr2mzarwidgadawmamiteydbzi

HTTP Gateways

I guess partly because IPFS is a rather new protocol not widely supported by many clients yet, people came up with the concept of IPFS “gateways”. (Others might have called it a proxy, because that is what it really is.)

This gateway is an HTTP server that runs on a machine that also knows how to speak and access IPFS. By sending the right HTTP request to the gateway, that includes the IPFS hash you are interested in, the gateway can respond with the contents from the IPFS hash.

This way, you just need an ordinary HTTP client to access IPFS. I think it is pretty clever. But as always, the devil is in the details.

Rewrite ipfs:// into https://

The quest for IPFS aficionados have seemingly become to add support for IPFS using this gateway approach to multiple widely used applications that know how to speak HTTP(S). Just rewrite the URL internally from IPFS to HTTPS.

ffmpeg got “native” IPFS support this way, and there is ongoing work to implement the same kind of URL rewrite for curl. (into curl, not libcurl)

So far so good I guess.

The URL rewrite

The IPFS “URL rewrite” is done in such so that the example IPFS URL is converted into “https://$gateway/$hash”. The transfer is done by the client as if it was a plain old HTTPS transfer.

What if the gateway is not local?

This approach has its biggest benefit of course when you can actually use a remote IPFS gateway. I presume most random ordinary users who want to access IPFS does not actually want to download, install and run an IPFS gateway on their machine to use this new power. They might very well appreciate the idea and convenience of accessing a remote IPFS gateway.

Remote IPFS gateways illustration

After all, the gateways are using HTTPS so at least the transfers are secure, right?

Leading people behind IPFS are even running public IPFS gateways for anyone to use. Both dweb.link and ipfs.io have been mentioned and suggested for default use. Possibly they are the same physical host as they appear to have the same IP addresses (owned by Protocol Labs, one of the big proponents behind IPFS).

The IPFS project also provides a list of public gateways.

No gateway set, use a default!

In an attempt to make this even easier for users, ffmpeg is made to use a built-in default gateway if none is set by the user.

I would expect very few users to actually have an IPFS gateway set. And even fewer to actually specify a local one.

So, when users want to watch a video using ffmpeg and an ipfs:// URL what happens?

The gateway sees it all

The remote gateway, that is administered by someone, somewhere, gets to see the full incoming request, and all traffic (video, whatever) that is sent back to the client (ffmpeg in this example) goes via the gateway. Sure, the client accesses the gateway via HTTPS so nobody can tamper with the traffic along the way, but the gateway is in full control and can inspect and tamper with the data as much as it likes. And there is no way for the client to know or detect if it is happening.

I am not involved in ffmpeg, nor do I have any insights into how the discussions evolved and were done around this subject, but for curl I have put my foot down and said that we must not blindly use and trust any default remote gateway like this. I believe it is our responsibility to not lure users into this traffic-monitoring setup.

Make sure you trust the gateway you use.

The curl way

curl will probably output an error message if there was no gateway set when an ipfs:// URL is used, and inform the user that it needs a gateway set to function and probably also show a URL for a page that contains more information about what this means and how to specify a gateway.

Bouncing gateways!

During the work of making the IPFS support for curl (which I review and comment on, not written or authored myself) I have also learned that some of the IPFS gateways even do regular HTTP 30x redirects to bounce over the client to another gateway.

Meaning: not only do you use and rely a total rando’s gateway on the Internet for your traffic. That gateway might even, on its own discretion, redirect you over to another host. Possibly run somewhere else, monitored by a separate team.

I have insisted, in the PR for ipfs support to curl, that the IPFS URL handling code should not automatically follow such gateway redirects, as I believe that adds even more risk to the user so if a user wants to allow this operation, it should be opt-in.

Imagine rising use of this

I would imagine that the ones hosting a popular default gateway either becomes overloaded and slow, or they need to scale up. If they scale up, they risk to leak traffic even wider. Scaling up also makes the operation more expensive, leading to incentives to make money somehow to finance it. Will the cookie car in the form of a massive data trove perhaps then be used/sold?

Users of these gateways get no promises, no rights, no contracts.

Other IPFS privacy concerns

Brave, the browser, has actual native IPFS support (not using any gateway) and they have an informative page called How does IPFS Impact my Privacy?

Final verdict

I am not dissing the idea nor implementation of IPFS itself. I think using HTTP gateways to access IPFS is a good idea in general as it makes the network more accessible. It is just a fragile solution that easily misleads users to do things they maybe shouldn’t. So maybe a little too accessible?

Credits

Image by Hans Schwarzkopf from Pixaba

Follow-up

I poked the ffmpeg project over Twitter, there was immediate reaction and there is already a proposed patch for ffmpeg that removes the use of a default IPFS gateway. “it’s a security risk”

cURL and libcurl, Network, Security

.netrc pains

The .netrc file is used to hold user names and passwords for specific host names and allows tools to login to those systems automatically without having to prompt the user for the credentials while avoiding having to use them in command lines. The .netrc file is typically set without group or world read permissions (0600) to reduce the risk of leaking those secrets.

History

Allegedly, the .netrc file format was invented and first used for Berknet in 1978 and it has been used continuously since by various tools and libraries. Incidentally, this was the same year Intel introduced the 8086 and DNS didn’t exist yet.

.netrc has been supported by curl (since the summer of 1998), wget, fetchmail, and a busload of other tools and networking libraries for decades. In many cases it is the only cross-tool way to provide credentials to remote systems.

The .netrc file use is perhaps most widely known from the “standard” ftp command line client. I remember learning to use this file when I wanted to do automatic transfers without any user interaction using the ftp command line tool on unix systems in the early 1990s.

Example

A .netrc file where we tell the tool to use the user name daniel and password 123456 for the host user.example.com is as simple as this:

machine user.example.com
login daniel
password 123456

Those different instructions can also be written on the same single line, they don’t need to be separated by newlines like above.

Specification

There is no and has never been any standard or specification for the file format. If you google .netrc now, the best you get is a few different takes on man pages describing the format in a high level. In general this covers our needs and for most simple use cases this is good enough, but as always the devil is in the details.

The lack of detailed descriptions on how long lines or fields to accept, how to handle special character or white space for example have left the implementers of the different code basis to decide by themselves how to handle those things.

The horse left the barn

Since numerous different implementations have been done and have been running in systems for several decades already, it might be too late to do a spec now.

This is also why you will find man pages out there with conflicting information about the support for space in passwords for example. Some of them explicitly say that the file format does not support space in passwords.

Passwords

Most fields in the .netrc work fine even when not supporting special characters or white space, but in this age we have hopefully learned that we need long and complicated passwords and thus having “special characters” in there is now probably more common than back in the 1970s.

Writing a .netrc file with for example a double-quote or a white space in the password unfortunately breaks tools and is not portable.

I have found at least three different ways existing tools do the parsing, and they are all incompatible with each other.

curl parser (before 7.84.0)

curl did not support spaces in passwords, period. The parser split all fields at the following space or newline and accepted whatever is in between. curl thus supported any characters you want, except space and newlines . It also did not “unquote” anything so if you wanted to provide a password like ""llo (with two leading double-quotes), you would use those five bytes verbatim in the file.

wget parser

This parser allows a space in the password if you provide it quoted within double-quotes and use a backslash in front of the space. To specify the same ""llo password mentioned above, you would have to write it as "\"\"llo".

fetchmail parser

Also supports spaces in passwords. Here the double-quote is a quote character itself so in order to provide a verbatim double-quote, it needs to be doubled. To specify the same ""llo password mentioned above, you would have to write it as """"llo – that is with four double-quotes.

What is the best way?

Changing any of these parsers in an effort to unify risk breaking existing use cases and scripts out in the wild with outraged users as a result. But a change could also generate a few happy users too who then could better share the same .netrc file between tools.

In my personal view, the wget parser approach seems to be the most user friendly one that works perhaps most closely to what I as a user would expect. So that’s how I went ahead and made curl work.

What to do

Users will of course be stuck with ancient versions for a long time and this incompatibility situation will remain for the foreseeable future. I can think of a few work-arounds users can do to cope:

  • Avoid space, tabs, newline and various quotes in passwords
  • Use separate .netrc files for separate tools
  • Provide passwords using other means than .netrc – with curl you can for example explore using –config instead

Future curl supports quoting

We are changing the curl parser somewhat in the name of compatibility with other tools (read wget) and curl will allow quoted strings in the way wget does it, starting in curl 7.84.0. While this change risks breaking a few command lines out there (for users who have leading double-quotes in their existing passwords), I think the change is worth doing in the name of compatibility and the new ability to use spaces in passwords.

A little polish after twenty-four years of not supporting spaces in user names or passwords.

Hopefully this will not hurt too many users.

Credits

Image by Anja-#pray for ukraine# #helping hands# stop the war from Pixabay

cURL and libcurl, Network, Web

now on HTTP/3

1 Comment

The first mention of QUIC on this blog was back when I posted about the HTTP workshop of July 2015. Today, this blog is readable over the protocol QUIC subsequently would turn into. (Strictly speaking, it turned into QUIC + HTTP/3 but let’s not be too literal now.)

The other day Fastly announced that all their customers now can enable HTTP/3, and since this blog and the curl site are graciously running on the Fastly network I went ahead and enabled the protocol.

Within minutes and with almost no mistakes, I could load content over HTTP/3 using curl or browsers. Wooosh.

The name HTTP/3 wasn’t adopted until late 2018, and the RFC has still not been published yet. Some of the specifications for QUIC have however.

curling curl with h3

Haxx, Network, Web

Considered “18+”

12 Comments

Vodafone UK has taken it on themselves to make the world better by marking this website (daniel.haxx.se) “adult content”. I suppose in order to protect the children.

It was first reported to me on May 2, with this screenshot from a Vodafone customer:

And later followed up with some more details from another user in this screenshot

Customers can opt out of this “protection” and then apparently Vodafone will no longer block my site.

How

I was graciously given more logs (my copy) showing DNS resolves and curl command line invokes.

It shows that this filter is for this specific host name only, not for the entire haxx.se domain.

It also shows that the DNS resolves are unaffected as they returned the expected Fastly IP addresses just fine. I suspect they have equipment that inspects outgoing traffic that catches this TLS connection based on the SNI field.

As the log shows, they then make their server do a TLS handshake in which they respond with a certificate that has daniel.haxx.se in the CN field.

The curl verbose output shows this:

* SSL connection using TLSv1.2 / ECDHE-ECDSA-CHACHA20-POLY1305
* ALPN, server did not agree to a protocol
* Server certificate:
*  subject: CN=daniel.haxx.se
*  start date: Dec 16 13:07:49 2016 GMT
*  expire date: Dec 16 13:07:49 2026 GMT
*  issuer: C=ES; ST=Madrid; L=Madrid; O=Allot; OU=Allot; CN=allot.com/emailAddress=info@allot.com
*  SSL certificate verify result: self signed certificate in certificate chain (19), continuing anyway.
> HEAD / HTTP/1.1
> Host: daniel.haxx.se
> User-Agent: curl/7.79.1
> Accept: */*
>

The allot.com clue is the technology they use for this filtering. To quote their website, you can “protect citizens” with it.

I am not unique, clearly this has also hit other website owners. I have no idea if there is any way to appeal against this classification or something, but if you are a Vodafone UK customer, I would be happy if you did and maybe linked me to a public issue about it.

Update

I was pointed to the page where you can request to unblock specific sites so I have done that now (at 12:00 May 2).

Update on May 3

My unblock request for daniel.haxx.se is apparently “on hold” according to the web site.

I got an email from an anonymous (self-proclaimed) insider who says he works at Allot, the company doing this filtering for Vodafone. In this email, he says

Most likely, Vodafone is using their parental control a threat protection module which works based on a DNS resolving.

and then

After the business logic decides to block the website, it tells the DNS server to reply with a custom IP to a server that always shows a block page, because how HTTPS works, there is no way to trick it, either with Self-signed certificate, or using a signed certificate for a different domain, hence the warning.

What is weird here is that this explanation does not quite match what I have seen the logs provided to me. They showed this filtering clearly not being DNS based – since the DNS resolves got the exact same IP address a non-filtered resolver does.

Someone on Vodafone UK could of course easily test this by simply using a different DNS server, like 1.1.1.1 or 8.8.8.8.

Discussed on hacker news.

cURL and libcurl, Security

This busy-loop is not a security issue

One of the toughest jobs I have, is to assess if a reported security problem is indeed an actual security vulnerability or “just” a bug. Let me take you through a recent case to give you an insight…

Some background

curl is 24 years old and so far in our history we have registered 111 security vulnerabilities in curl. I’ve sided with the “security vulnerability” side in reported issues 111 times. I’ve taken the opposite stance many more times.

Over the last two years, we have received 129 reports about suspected security problems and less than 15% of them (17) were eventually deemed actual security vulnerabilities. In the other 112 cases, we ended up concluding that the report was not pointing out a curl security problem. In many of those 112 cases, it was far from easy to end up with that decision and in several instances the reporter disagreed with us. (But sure, in the majority of the cases we could fairly quickly conclude that the reports were completely bonkers.)

The reporter’s view

Many times, the reporter that reports a security bug over on Hackerone has spent a significant amount of time and effort to find it, research it, reproduce it and report it. The reporter thinks it is a security problem and there’s a promised not totally insignificant monetary reward for such problems. Not to mention that a found and reported vulnerability in curl might count as something of a feat and a “feather in the hat” for a security researcher. The reporter has an investment in this work and a strong desire to have their reported issue classified as a security vulnerability.

The project’s view

If the reported problem is a security problem then we must consider it as that and immediately work on fixing the issue to reduce the risk of users getting hurt, and to inform all users about the risk and ask them to upgrade or otherwise mitigate and take precautions against the risks.

Most reported security issues are not immediately obvious. At least not in my eyes. I usually need to object, discuss, question and massage the data for a while in order to land on how we should best view the issue. I’m a skeptic by nature and I need to be convinced before I accept it.

Labeling something a “security vulnerability” if it indeed is not, is rather hurting users and the entire community rather than helping it. We must not cry wolf for a problem that cannot hurt users or that in practical terms is impossible to occur. Or maybe it is a problem that users are already expected to deal with. Or a result of an explicit or implicit application choice rather than a mistake done by us.

But we must not ignore actual security problems!

This latest MQTT problem

On March 24, 2022 we got a new report filed over on hackerone with the title Denial of Service vulnerability in curl when parsing MQTT server response.

Here’s (roughly) what the issue is about:

  1. A bug in current libcurl makes it misbehave under certain conditions. When the MQTT connection gets closed mid message, libcurl refuses to acknowledge that and thinks the connection is still alive. Easily triggered by a malicious server.
  2. libcurl considers the connection readable non-stop
  3. Reading from the connection brings no more data
  4. Busy-looping in the event-loop. Goto 2

The loop stops only once it reaches the set timeout, the progress callback can stop it and the speed-limit options will stop it if the right conditions are met.

By default, none of those options are set for a transfer and therefore, by default this makes an endless busy-loop.

At the same time…

A transfer can always stall and take a very long time to complete. A server can basically always just stop delivering more data, making the transfer take an infinite amount of time to complete. Applications that have not set any options to stop such a transfer risk doing a transfer that never ends. An endless transfer.

Also: if libcurl makes a transfer over a really fast network, such as localhost or using a super fast local network, then it might also reach the same level of busy-loop due to never having to wait for data. Albeit for a limited amount of time – until the transfer is complete. This busy-loop is highly unlikely to actually starve out any important threads in a system.

Yes, a closed connection is a much “cheaper” attack from server’s point of view than maintaining a long-living connection, but the cost of the attack is not a factor here.

Where in this grey area do we land?

This is difficult one.

I can see the point of the reporter, but I can also see how this flaw will basically not hurt any existing curl user. Where is our responsibility here?

I ended up concluding that this issue not a security vulnerability. The reporter disagreed.

It is a terribly annoying bug for sure. But the only applications that are seriously affected by it, are the ones that already allow an endless transfer.

The bug-fix was instead submitted as a normal pull-request: PR 8644, targeted to be fixed and included in the pending curl 7.83.0 release.

We publicize the reports after the fact

We make all (non-rubbish) previously reported hackerone issues public, whether they ended up being a vulnerability or not. To give everyone involved time to object or redact sensitive details, the publication date is usually within a month after the issue was closed.

By making the reports public, we allow everyone interested enough the ability and chance to check out and follow past discussions and deliberations for going the directions we did. The idea is primarily to be completely open about the reported issues and how we classify them, to show that we are not hiding anything and it also provides a chance for us to get more feedback from the surrounding and from security people who might disagree with previous analyses.

Security is hard.

cURL and libcurl, Security

Anatomy of a ghost CVE

1 Comment

“The Lord giveth and the Lord taketh away.”

Job 1:21

On March 16 2022, the curl security team received an email in which the reporter highlighted an Apple web page. What can you tell us about this?

I hadn’t seen it before. On this page with the title “About the security content of macOS Monterey 12.3”, said to have been published just two days prior, Apple mentions recent package upgrades and the page lists a bunch of products and what security fixes that were done for them in this update. Among the many products listed, curl is mentioned.

This is what the curl section of the page looked like:

Screenshot from March 17, 2022

In the curl project we always make all CVEs public with as much detail as we can possibly extract and provide about them. We take great pride in being the best in class in security flaw information and transparency.

Apple listed four CVE fixed. The three first IDs we immediately recognized from the curl security page. The last one however, was a surprise. What was that?

CVE-2022-22623

This is not a CVE published by the curl project. The curl project has in fact not shipped any CVE at all in 2022 (yet) so that’s easy to spot. When we looked at the MITRE registration for the ID, it also didn’t disclose any clues really. Not that it was expected to. It did show it was created on January 5 though, so it wasn’t completely new.

Was it a typo?

I compared this number to other recent CVE numbers announced from curl and I laid eyes on CVE-2021-22923 which had just two digits changed. Did they perhaps mean that CVE?

The only “problem” with that CVE is that it was in regards to Metalink and I don’t think Apple ever shipped their curl package with metalink support so therefore they wouldn’t have fixed a Metalink problem. So probably not a typo for that number at least!

I reached out to a friend at Apple as well with an email to Apple Product Security.

Security is our number one priority

In the curl project, we take security seriously. The news that there might be a security problem in curl that we haven’t been told about and that looks like it was about to get public sooner or later was of course somewhat alarming and something we just needed to get to the bottom of. It was also slightly disappointing that a large vendor and packager of curl since over 20 years would go about it this way and jab this into our back.

No source code

Apple has not made the source code for their macOS 12.3 version and the packages they use in there public, so there was no way for us to run diffs or anything to check for the exact modifications that this claimed fix would’ve resulted in.

Apple said so

Several “security websites” (the quotes are there to indicate that clearly these sites are more security in the name than in reality) immediately posted details about this “vulnerability”. Some of them with CVSS scores and CWE numbers , explaining how this problem can hurt users. Obviously completely made up since none of that info was made available by any first party sources anywhere. Not from Apple and not from the curl project. If you now did a web search on that CVE number, several of the top search results linked to such sites providing details – obviously made up from thin air.

As I think these sites don’t add much value to humanity, I won’t link to them here but instead I will show you a screenshot from such an article to show you what a made up CVE number posted by Apple can make people claim:

Screenshot from exploitone.com

At 23:28 (my time zone) on the 17th, my Apple friend responded saying they had forwarded the issue to “the right team”.

The Apple Product Security team I also emailed about this issue, answered at 00:23 (still my time) on the 18th saying “we are looking into this and will provide an update soon when we have more information.”

The MITRE page got more details

The MITRE CVE page from March 21st

After the weekend passed with no response, I looked back again on the MITRE page for the CVE in question and it had then gotten populated with additional curl details; mentioning Apple as CNA and now featuring links back to the Apple page! Now it really started to look like the CVE was something real that Apple (or someone) had registered but not told us about. It included real curl related snippets like this:

Multiple issues were addressed by updating to curl version 7.79.1. This issue is fixed in macOS Monterey 12.3. Multiple issues in curl.

Please tell us more details

On Monday the 21st, I continued to get questions about this CVE. Among others, from a member of a major European ISP’s CERT team curious about this CVE as they couldn’t find any specific information about this issue either and they were concerned they might have this vulnerability in the curl versions they run. They of course (rightfully) assumed that I would know about curl CVEs.

It turns out that when a major company randomly mentions a new CVE, it actually has an impact on the world!

Gone!

At around 20:30 on March 21st, someone on Twitter spotted that the ghost CVE had been removed from Apple’s web page and it only listed three issues (and a mention that the section had been updated). At 21:39 I get an email response from Apple Product Security:

Thank you for reaching out to us about the error with this CVE on our security advisory. We’ve updated our site and requested that MITRE reject CVE-2022-22623 on their end.

Please let us know if you have any questions.

Screenshot from March 21, 2022

The reject request to MITRE is expected to be slow so that page will remains showing the outdated data for a while longer.

Exploit one

When Apple had retracted the wrong CVE, I figured I should maybe try to get exploitone.com to remove their “article” to maybe at least stop one avenue of further misinformation about this curl “issue”. I tweeted (in perhaps a tad bit inflammatory manner):

I get the feeling they didn’t quite understand my point. They replied:

What happened?

As I had questions about Apple’s mishap, I replied (sent off 22:28 on the 21st, still only early afternoon on the US west coast), asking for details on what exactly had happened here. If it was a typo, then how come it got registered with MITRE? It’s just so puzzling and mysterious!

When I’m posting this article on my blog (36 hours after I sent the question), I still haven’t gotten any response or explanation. I don’t expect to get any either, but if I do, I will update this post accordingly.

Update March 26

exploitone.com updated their page at some point after my tweet to remove the mention of the imaginary CVE, but the wording remains very odd:

cURL and libcurl, Security

Keeping curl safe

2 Comments

I’ve talked on this topic before but I realized I never did a proper blog post on the topic. So here it is: how we develop curl to keep it safe. The topic of supply chain security is one that is discussed frequently these days and every so often there’s a very well used (open source) component that gets a terrible weakness revealed.

Don’t get me wrong. Proprietary packages have their share of issues as well, and probably even more so, but for obvious reasons we never get the same transparency, details and insight into those problems and solutions.

curl

curl, in the shape of libcurl primarily, is one of the world’s most commonly used software components. It is installed in somewhere around ten billion installations world wide. It might even be forty billion. Nobody knows.

If we would find a critical vulnerability in curl, it could potentially exist in every internet-connected device on the globe. We don’t want that.

A critical security flaw in our products would be bad, but we also similarly need to make sure that we provide APIs and help users of our products to be safe and to use curl safely. To make sure users of libcurl don’t accidentally end up getting security problems, to the best of our ability.

In the curl project, we work hard to never have our own version of a “heartbleed moment“. How do we do this?

Always improving

Our method is not strange, weird or innovative. We simply apply all best practices, tools and methods that are available to us. In all areas. As we go along, we tighten the screws and improve our procedures, learning from past mistakes.

There are no short cuts or silver bullets. Just hard work and running tools.

Not a coincidence

Getting safe and secure code into your product is not something that happens by chance. We need to work on it and we need to make a concerned effort. We must care about it.

We all know this and we all know how to do it, we just need to make sure that we also actually do it.

The steps

  1. Write code following the rules
  2. Review written code and make sure it is clear and easy to read.
  3. Test the code. Before and after merge
  4. Verify the products and APIs to find cracks
  5. Bug-bounty to reward outside helpers
  6. Act on mistakes – because they will happen

Writing

For users of libcurl we provide an API with safe and secure defaults as we understand the power of the default. We also document everything with details and take great pride in having world-class documentation. To reduce the risk of applications becoming unsafe just because our API was unclear.

We also document internal APIs and functions to help contributors write better code when improving and changing curl.

We don’t allow compiler warnings to remain – on any platform. This is sometimes quite onerous since we build on such a ridiculous amount of systems.

We encourage use of source code comments and assert()s to make assumptions obvious. (curl is primarily written in C.)

Number of lines of (product) code in the curl project over time.

Review

All code should be reviewed. Maintainers are however allowed to review and merge their own pull-requests for practical reasons.

Code should be easy to read and understand. Our code style must be followed and encourages that: for example, no assignments in conditions, one statement per line, no lines longer than 80 columns and more.

Strict compliance with the code style also means that the code gets a flow and a consistent look, which makes it easier to read and manage. We have a tool that verifies most aspects of the code style, which takes away most of that duty away from humans. I find that PR authors generally take code style remarks better when pointed out by a tool than when humans do it.

A source code change is accompanied with a git commit message that need to follow the template. A consistent commit message style makes it easier to later come back and understand it proper when viewing source code history.

Test

We want everything tested.

  • Unit tests. We strive at writing more and more unit tests of internal functions to make sure they truly do what expected.
  • System tests. Do actual network transfers against test servers, and make sure different situations are handled.
  • Integration tests. Test libcurl and its APIs and verify that they handle what they are expected to.
  • Documentation tests. Check formats, check references and cross-reference with source code, check lists that they include all items, verify that all man pages have all sections, in the same order and that they all have examples.
  • “Fix a bug? Add a test!” is a mantra that we don’t always live up to, but we try.

curl runs on 80+ operating systems and 20+ CPU architectures, but we only run tests on a few platforms. This usually works out fine because most of the code is written to run on multiple platforms so if tested on one, it will also run fine on all the other.

curl has a flexible build system that offers many million different build combinations with over 30 different possible third-party libraries in countless version combinations. We cannot test all build combos, but we try to test all the popular ones and at least one for each config option enabled and disabled.

We have many tests, but there are unfortunately still gaps and details not tested by the test suite. For those things we simply have to rely on the code review and then that users report problems in the shipped products.

Number of test cases, test files really, over time.

Verify

We run all the tests using valgrind to make sure nothing leaks memory or do bad memory accesses.

We build and run with address, undefined behavior and integer overflow sanitizers.

We are part of the OSS-Fuzz project which fuzzes curl code non-stop, and we run CIFuzz in CI builds, which runs “a little” fuzzing on the curl code in the normal pull-request process.

We do “torture testing“: run a test case once and count the number of “fallible” function calls it makes. Those are calls to memory allocation, file operations, socket read/write etc. Then re-run the test that many times, and for each new iteration we make another one of the fallible functions fail and return error. Verify that no memory leaks or crashes occur. Do this on all tests.

We use several different static code analyzers to scan the code checking for flaws and we always fix or otherwise handle every reported defect. Many of them for each pull-request and commit, some are run regularly outside of that process:

  • scan-build
  • clang tidy
  • lgtm
  • CodeQL
  • Lift
  • Coverity

The exact set has varied and will continue to vary over time as services come and go.

Bug-bounty

No matter how hard we try, we still ship bugs and mistakes. Most of them of course benign and harmless but some are not. We run a bug-bounty program to reward security searchers real money for reported security vulnerabilities found in curl. Until today, we have paid almost 17,000 USD in total and we keep upping the amounts for new findings.

Accumulated bug-bounty payouts over time

When we report security problems, we produce detailed and elaborate advisories to help users understand every subtle detail about the problem and we provide overview information that shows exactly what versions are vulnerable to which problems. The curl project aims to also be a world-leader in security advisories and related info.

Act on mistakes

We are not immune, no matter how hard we try. Bad things will happen. When they do, we:

  • Act immediately.
  • Own the problem, responsibly
  • Fix it and announce it – as soon as possible
  • Learn from it
  • Make it harder to do the same or similar mistakes again

Does it work? Do we actually learn from our history of mistakes? Maybe. Having our product in ten billion installations is not a proof of this. There are some signs that might show we are doing things right:

  • We were reporting fewer CVEs/year the last few years but in 2021 we went back up. It could also be the result of more people looking, thanks to the higher monetary rewards offered. At the same time the number of lines of code have kept growing at a rate of around 6,000 lines per year.
  • We get almost no issues reported by OSS-Fuzz anymore. The first few years it ran it found many problems.
  • We are able to increase our bug-bounty payouts significantly and now pay more than one thousand USD almost every time. We know people are looking hard for security bugs.
Security vulnerabilities. Fixed vs Introduced over the years.

Continuous Integration

For every pull-request and commit done in the project, we run about 100 different builds + test rounds.

Total number of CI builds per pull-request and commit, over time
  • Test code style
  • Run thousands of tests per build
  • Build and test on tens of platforms
  • Over twenty hours of CPU time per commit
  • Done using several different CI services for maximum performance, widest possible coverage and shortest time to completion.

We currently use the following CI services: Cirrus CI, AppVeyor, Azure Pipelines, GitHub Actions, Circle CI and Zuul CI.

We also have a separate autobuild system with systems run by volunteers that checkout the latest code, build, run all the tests and report back in a continuous manner a few times or maybe once per day.

New habits past mistakes have taught us

We have done several changes to curl internals as direct reactions to past security vulnerabilities and their root causes. Lessons learned.

Unified dynamic buffer functions

These days we have a family of functions for working with dynamically sized buffers. Be using the same set for this functionality we have it well tested and we reduce the risk that new code messes up. Again, nothing revolutionary or strange, but as curl had grown organically over the decades, we found ourselves in need of cleaning this up one day. So we did.

Maximum string sizes

Several past mistakes came from possible integer overflows due to libcurl accepting input string sizes of unrestricted lengths and after doing operations on such string sizes, they would sometimes lead to overflows.

Since a few years back now, no string passed to curl is allowed to be larger than eight megabytes. This limit is somewhat arbitrarily set but is meant to be way larger than the largest user names and passwords ever used etc. We could also update the limit in a future, should we want. It’s not a limit that is exposed in the API or even mentioned. It is there to trap mistakes and malicious use.

Avoid reallocs

Thanks to the previous points we now avoid realloc as far as possible outside of those functions. History shows that realloc in combination with integer overflows have been troublesome for us. Now, both reallocs and integer overflows should be much harder to mess up.

Code coverage

A few years ago we ran code coverage reports for one build combo on one platform. This generated a number that really didn’t mean a lot to anyone but instead rather mislead users to drawing funny conclusions based on the report. We stopped that. Getting a “complete” and representative number for code coverage for curl is difficult and nobody has yet gone back to attempt this.

The impact of security problems

Every once in a while someone discovers a security problem in curl. To date, those security vulnerabilities have been limited to certain protocols and features that are not used by everyone and in many cases even disabled at build-time among many users. The issues also often rely on either a malicious user to be involved, either locally or remotely and for a lot of curl users, the environments it runs in limit that risk.

To date, I’m not aware of any curl user, ever, having been seriously impacted by a curl security problem.

This is not a guarantee that it will not ever happen. I’m only stating facts about the history so far. Security is super hard and I can only promise that we will keep working hard on shipping secure products.

Is it scary?

Changes done to curl code today will end up in billions of devices within a few years. That’s an intimidating fact that could truly make you paralyzed by fear of the risk that the world will “burn” due to a mistake of mine.

Rather than instilling fear by this outlook, I think the proper way to think it about it, is respecting the challenge and “shouldering the responsibility”. Make the changes we deem necessary, but make them according to the guidelines, follow the rules and trust that the system we have setup is likely to detect almost every imaginable mistake before it ever reaches a release tarball. Of course we plug holes in the test suite that we spot or suspect along the way.

The back-door threat

I blogged about that recently. I think a mistake is much more likely to slip-in and get shipped to the world than a deliberate back-door is.

Memory safe components might help

By rewriting parts of curl to use memory safe components, such as hyper for HTTP, we might be able to further reduce the risk of future vulnerabilities. That’s a long game to make reality. It will also be hard in the future to actually measure and tell for sure if it truly made an impact.

How can you help out?

  1. Pay for a curl support contract. This is what enables me to work full time on curl.
  2. Help out with reviews and adding new tests to curl
  3. Help out with fixing issues and improving the code
  4. Sponsor curl
  5. Report all bugs you find
  6. Upgrade your systems to run modern curl versions

Credits

Image by Dorian Krauss from Pixabay