Category Archives: Security

curl and h2 on mac

$ curl ‐‐http2
curl: (1) Unsupported protocol

curl on mac

curcurl-symboll has been shipped by default on Mac OS X since many years – I actually couldn’t even manage to figure out exactly how many. It is built and bundled with the operating system by Apple itself and on Apple’s own terms and even though I’m the main curl developer I’ve never discussed this with them or even been asked or told about their plans. I’m not complaining, our license allows this and I’m nothing but happy with them shipping curl to millions of Mac users.

Leaving OpenSSL

osxOriginally, curl on Mac was built against OpenSSL for the TLS and SSL support, but over time our friends at Apple have switched more and more of their software over to use their own TLS and crypto library Secure Transport instead of OpenSSL. A while ago Apple started bundling curl built to use the native mac TLS library instead of OpenSSL.

As you may know, when you build curl you can select from eleven different TLS libraries and one of them of course is Secure Transport. Support for this TLS back-end in curl was written by curl hackers, but it apparently got to a quality level good enough for Apple to decide to build curl with this back-end and ship it like that.

The Secure Transport back-end is rather capable and generally doesn’t cause many reasons for concern. There’s however one notable little glitch that people keep asking me about…

curl doesn’t support HTTP/2 on mac!

There are two obvious reasons why not, and they are:

1. No ALPN with Secure Transport

Secure Transport doesn’t offer any public API to enable HTTP/2 with ALPN when speaking HTTPS. Sure, we know Apple supports HTTP/2 already in several other aspects in their ecosystem and we can check their open code so we know there’s support for HTTP/2 and ALPN. There’s just no official APIs for us to use to switch it on!

So, if you insist on building curl to use Secure Transport instead of one of the many alternatives that actually support ALPN just fine, then you can’t negotiate HTTP/2 over TLS!

2. No nghttp2 with Mac OS

Even without ALPN support, you could actually still negotiate HTTP/2 over plain text TCP connections if you have a server that supports it. But even then curl depends on the awesome nghttp2 library to provide the frame level protocol encoding/decoding and more. If Apple would decide to enable HTTP/2 support for curl on Mac OS, they need to build it against nghttp2. I really think they should.

Homebrew and friends to the rescue!

Correct. You can still install your own separate curl binary (and libcurl library) from other sources, like for example Homebrew or Macports and they do offer versions built against other TLS back-ends and nghttp2 and then of course HTTP/2 works just fine with curl on mac.

Did I file a bug with Apple?

No, but I know for certain that there has been a bug report filed by someone else. Unfortunately it isn’t public so I can’t link nor browse it.

The most popular curl download – by a malware

During October 2015 the curl web site sent out 1127 gigabytes of data. This was the first time we crossed the terabyte limit within a single month.

Looking at the stats a little closer, I noticed that in July 2015 a particular single package started to get very popular. The exact URL was

Curious. In October it alone was downloaded more than 300,000 times, accounting for over 70% of the site’s bandwidth. Why?

The downloads came from what appears to be different locations. They don’t use any HTTP referer headers and they used different User-agent headers. I couldn’t really see a search bot gone haywire or a malicious robot stuck in a crazy mode.

After I shared some of this data over in our IRC channel (#curl on freenode), Björn Stenberg stumbled over this AVG slide set, describing how a particular malware works when it infects a computer. Downloading that particular file is thus a step in its procedures to create a trojan that will run on the host system – see slide 11 for the curl details. The slide also mentions that an updated version of the malware comes bundled with the curl library already, which then I guess makes the hits we see on the curl site being done by the older versions still being run.

Of course, we can’t be completely sure this is the source for the increased download of this particular file but it seems highly likely.

I renamed the file just now to see what happens.

Evil use of good code

We can of course not prevent evil uses of our code. We provide source code and we even host some binaries of curl and libcurl and both good and bad actors are able to take advantage of our offers.

This rename won’t prevent a dedicated hacker, but hopefully it can prevent a few new victims from getting this malware running on their machines.

Update: the hacker news discussion about this post.

http2 explained in markdown

http2 explainedAfter twelve  releases and over 140,000 downloads of my explanatory document “http2 explained“, I eventually did the right thing and converted the entire book over to markdown syntax and put the book up on

Better output formats, now epub, MOBI, PDF and everything happens on every commit.

Better collaboration, github and regular pull requests work fine with text content instead of weird binary word processor file formats.

Easier for translators. With plain text commits to aid in tracking changes, and with the images in a separate directory etc writing and maintaining translated versions of the book should be less tedious.

I’m amazed and thrilled that we already have Chinese, Russian, French and Spanish translations and I hear news about additional languages in the pipe.

I haven’t yet decided how to do with “releases” now, as now we update everything on every push so the latest version is always available to read. Go to to find out the latest about the document and the most updated version of the document.

Thanks everyone who helps out. You’re the best!

The TLS trinity dance

In the curl project we currently support eleven different TLS libraries. That is 8 libraries and the OpenSSL “trinity” consisting of BoringSSL, libressl and of course OpenSSL itself.

You could easily be mislead into believing that supporting three libraries that all have a common base would be reallytrinity easy since they have the same API. But no, it isn’t. Sure, they have the same foundation and they all three have more in common that they differ but still, they all diverge in their own little ways and from my stand-point libressl seems to be the one that causes us the least friction going forward.

Let me also stress that I’m but a user of these projects, I don’t participate in their work and I don’t have any insights into their internal doings or greater goals.


Easy-peacy, very similar to OpenSSL. The biggest obstacle might be that the version numbering is different so an old program that might be adjusted to different OpenSSL features based on version numbers (like curl was) needs some adjusting. There’s a convenient LIBRESSL_VERSION_NUMBER define to detect libressl with.


I regularly build curl against OpenSSL from their git master to get an early head-start when they change things and break backwards compatibility. They’ve increased that behavior since Heartbleed and while I generally agree with their ambitions on making more structs opaque instead of exposing all internals, it also hurts us over and over again when they remove things we’ve been using for years. What’s “funny” is that in almost all cases, their response is “well use this way instead” and it has turned out that there’s an equally old API that is still there that we can use instead. It also tells something about their documentation situation when that is such a common pattern. It’s never been possible to grasp this from just reading docs.


BoringSSL has made great inroads in the market and is used on Android now and more. They don’t do releases(!) and have no version numbers so the only thing we can do is to build from git and there’s no install target in the makefile. There’s no docs for it, they remove APIs from OpenSSL (curl can’t support NTLM nor OCSP stapling when built with it), they’ve changed several data types in the API making it really hard to build curl without warnings. Funnily, they also introduced non-namespaced typedefs prefixed with X509_* that collide with other common headers.

How it can play out in real life

A while ago we noticed BoringSSL had removed the DES_set_odd_parity function which we use in curl. We changed the configure script to look for it and changed the code to survive without it. The lack of that function then also signaled that it wasn’t OpenSSL, it was BoringSSL

BoringSSL moved around things that caused our configure script to no longer detect it as “OpenSSL compliant” because CRYPTO_lock could no longer be found by configure. We changed it to instead search for HMAC_Init and we were fine again.

Time passed and BoringSSL brought back DES_set_odd_parity, so our configure script no longer saw it as BoringSSL (the Android fixed this problem in their git but never sent as the fix). We changed the configure script accordingly to properly use OPENSSL_IS_BORINGSSL instead to detect BoringSSL which was the correct thing anyway and now as a bonus it can thus detect and work with both new and old BoringSSL versions.

A short time after, I again try to build curl against the OpenSSL master branch only to realize they’ve deprecated HMAC_Init that we just recently switched to for detection (since the configure script needs to check for a particular named function within a library to really know that it has detected and can use said library). Sigh, we switched “detect function” again to HMAC_Update. Hopefully this exists in all three and will stick around for a while…

Right now I think we can detect and use all three. It is only a matter of time until one of them will ruin that and we will adapt again.

HTTPS and HTTP/2 plans for my sites

I produce a fair amount of open source code. I make that code available online. curl is probably the most popular package.

People ask me how they can trust that they are actually downloading what I put up there. People ask me when my source code can be retrieved over HTTPS. Signatures and hashes don’t add a lot against attacks when they all also are fetched over HTTP…


SSL padlockI really and truly want to offer HTTPS (only) for all my sites.  I and my friends run a whole busload of sites on the same physical machine and IP address (,,,,, and many more) so I would like a solution that works for all of them.

I can do this by buying certs, either a lot of individual ones or a few wildcard ones and then all servers would be covered. But the cost and the inconvenience of needing a lot of different things to make everything work has put me off. Especially since I’ve learned that there is a better solution in the works!

Let’s Encrypt will not only solve the problem for us from a cost perspective, but they also promise to solve some of the quirks on the technical side as well. They say they will ship certificates by September 2015 and that has made me wait for that option rather than rolling up my sleeves to solve the problem with my own sweat and money. Of course there’s a risk that they are delayed, but I’m not running against a hard deadline myself here.


Related, I’ve been much involved in the HTTP/2 development and I host my “http2 explained” document on my still non-HTTPS site. I get a lot of questions (and some mocking) about why my HTTP/2 documentation isn’t itself available over HTTP/2. I would really like to offer it over HTTP/2.

Since all the browsers only do HTTP/2 over HTTPS, a prerequisite here is that I get HTTPS up and running first. See above.

Once HTTPS is in place, I want to get HTTP/2 going as well. I still run good old Apache here so it might be done using mod_h2 or perhaps with a fronting nghttp2 proxy. We’ll see.


SSL padlockI’ve written the http2 explained document and I’ve done several talks about HTTP/2. I’ve gotten a lot of questions about TLS in association with HTTP/2 due to this, and I want to address some of them here.

TLS is not mandatory

In the HTTP/2 specification that has been approved and that is about to become an official RFC any day now, there is no language that mandates the use of TLS for securing the protocol. On the contrary, the spec clearly explains how to use it both in clear text (over plain TCP) as well as over TLS. TLS is not mandatory for HTTP/2.

TLS mandatory in effect

While the spec doesn’t force anyone to implement HTTP/2 over TLS but allows you to do it over clear text TCP, representatives from both the Firefox and the Chrome development teams have expressed their intents to only implement HTTP/2 over TLS. This means HTTPS:// URLs are the only ones that will enable HTTP/2 for these browsers. Internet Explorer people have expressed that they intend to also support the new protocol without TLS, but when they shipped their first test version as part of the Windows 10 tech preview, that browser also only supported HTTP/2 over TLS. As of this writing, there has been no browser released to the public that speaks clear text HTTP/2. Most existing servers only speak HTTP/2 over TLS.

The difference between what the spec allows and what browsers will provide is the key here, and browsers and all other user-agents are all allowed and expected to each select their own chosen path forward.

If you’re implementing and deploying a server for HTTP/2, you pretty much have to do it for HTTPS to get users. And your clear text implementation will not be as tested…

A valid remark would be that browsers are not the only HTTP/2 user-agents and there are several such non-browser implementations that implement the non-TLS version of the protocol, but I still believe that the browsers’ impact on this will be notable.

Stricter TLS

When opting to speak HTTP/2 over TLS, the spec mandates stricter TLS requirements than what most clients ever have enforced for normal HTTP 1.1 over TLS.

It says TLS 1.2 or later is a MUST. It forbids compression and renegotiation. It specifies fairly detailed “worst acceptable” key sizes and cipher suites. HTTP/2 will simply use safer TLS.

Another detail here is that HTTP/2 over TLS requires the use of ALPN which is a relatively new TLS extension, RFC 7301, which helps us negotiate the new HTTP version without losing valuable time or network packet round-trips.

TLS-only encourages more HTTPS

Since browsers only speak HTTP/2 over TLS (so far at least), sites that want HTTP/2 enabled must do it over HTTPS to get users. It provides a gentle pressure on sites to offer proper HTTPS. It pushes more people over to end-to-end TLS encrypted connections.

This (more HTTPS) is generally considered a good thing by me and us who are concerned about users and users’ right to privacy and right to avoid mass surveillance.

Why not mandatory TLS?

The fact that it didn’t get in the spec as mandatory was because quite simply there was never a consensus that it was a good idea for the protocol. A large enough part of the working group’s participants spoke up against the notion of mandatory TLS for HTTP/2. TLS was not mandatory before so the starting point was without mandatory TLS and we didn’t manage to get to another stand-point.

When I mention this in discussions with people the immediate follow-up question is…

No really, why not mandatory TLS?

The motivations why anyone would be against TLS for HTTP/2 are plentiful. Let me address the ones I hear most commonly, in an order that I think shows the importance of the arguments from those who argued them.

1. A desire to inspect HTTP traffic

looking-glassThere is a claimed “need” to inspect or intercept HTTP traffic for various reasons. Prisons, schools, anti-virus, IPR-protection, local law requirements, whatever are mentioned. The absolute requirement to cache things in a proxy is also often bundled with this, saying that you can never build a decent network on an airplane or with a satellite link etc without caching that has to be done with intercepts.

Of course, MITMing proxies that terminate SSL traffic are not even rare these days and HTTP/2 can’t do much about limiting the use of such mechanisms.

2. Think of the little ones

small-big-dogSmall devices cannot handle the extra TLS burden“. Either because of the extra CPU load that comes with TLS or because of the cert management in a billion printers/fridges/routers etc. Certificates also expire regularly and need to be updated in the field.

Of course there will be a least acceptable system performance required to do TLS decently and there will always be systems that fall below that threshold.

3. Certificates are too expensive

The price of certificates for servers are historically often brought up as an argument against TLS even it isn’t really HTTP/2 related and I don’t think it was ever an argument that was particularly strong against TLS within HTTP/2. Several CAs now offer zero-cost or very close to zero-cost certificates these days and with the upcoming efforts like, chances are it’ll become even better in the not so distant future.

pile-of-moneyRecently someone even claimed that HTTPS limits the freedom of users since you need to give personal information away (he said) in order to get a certificate for your server. This was not a price he was willing to pay apparently. This is however simply not true for the simplest kinds of certificates. For Domain Validated (DV) certificates you usually only have to prove that you “control” the domain in question in some way. Usually by being able to receive email to a specific receiver within the domain.

4. The CA system is broken

TLS of today requires a PKI system where there are trusted certificate authorities that sign certificates and this leads to a situation where all modern browsers trust several hundred CAs to do this right. I don’t think a lot of people are happy with this and believe this is the ultimate security solution. There’s a portion of the Internet that advocates for DANE (DNSSEC) to address parts of the problem, while others work on gradual band-aids like Certificate Transparency and OCSP stapling to make it suck less.

please trust me

My personal belief is that rejecting TLS on the grounds that it isn’t good enough or not perfect is a weak argument. TLS and HTTPS are the best way we currently have to secure web sites. I wouldn’t mind seeing it improved in all sorts of ways but I don’t believe running protocols clear text until we have designed and deployed the next generation secure protocol is a good idea – and I think it will take a long time (if ever) until we see a TLS replacement.

Who were against mandatory TLS?

Yeah, lots of people ask me this, but I will refrain from naming specific people or companies here since I have no plans on getting into debates with them about details and subtleties in the way I portrait their arguments. You can find them yourself if you just want to and you can most certainly make educated guesses without even doing so.

What about opportunistic security?

A text about TLS in HTTP/2 can’t be complete without mentioning this part. A lot of work in the IETF these days are going on around introducing and making sure opportunistic security is used for protocols. It was also included in the HTTP/2 draft for a while but was moved out from the core spec in the name of simplification and because it could be done anyway without being part of the spec. Also, far from everyone believes opportunistic security is a good idea. The opponents tend to say that it will hinder the adoption of “real” HTTPS for sites. I don’t believe that, but I respect that opinion because it is a guess as to how users will act just as well as my guess is they won’t act like that!

Opportunistic security for HTTP is now being pursued outside of the HTTP/2 spec and allows clients to upgrade plain TCP connections to instead do “unauthenticated TLS” connections. And yes, it should always be emphasized: with opportunistic security, there should never be a “padlock” symbol or anything that would suggest that the connection is “secure”.

Firefox supports opportunistic security for HTTP and it will be enabled by default from Firefox 37.


Пост доступен на сайте Восстановление: TLS в HTTP/2. (Russian)

TLS in HTTP/2 (Kazakh)

Bug finding is slow in spite of many eyeballs

“given enough eyeballs, all bugs are shallow”

The saying (also known as Linus’ law) doesn’t say that the bugs are found fast and neither does it say who finds them. My version of the law would be much more cynical, something like: “eventually, bugs are found“, emphasizing the ‘eventually’ part.

(Jim Zemlin apparently said the other day that it can work the Linus way, if we just fund the eyeballs to watch. I don’t think that’s the way the saying originally intended.)

Because in reality, many many bugs are never really found by all those given “eyeballs” in the first place. They are found when someone trips over a problem and is annoyed enough to go searching for the culprit, the reason for the malfunction. Even if the code is open and has been around for years it doesn’t necessarily mean that any of all the people who casually read the code or single-stepped over it will actually ever discover the flaws in the logic. The last few years several world-shaking bugs turned out to have existed for decades until discovered. In code that had been read by lots of people – over and over.

So sure, in the end the bugs were found and fixed. I would argue though that it wasn’t because the projects or problems were given enough eyeballs. Some of those problems were found in extremely popular and widely used projects. They were found because eventually someone accidentally ran into a problem and started digging for the reason.

Time until discovery in the curl project

I decided to see how it looks in the curl project. A project near and dear to me. To take it up a notch, we’ll look only at security flaws. Not only because they are the probably most important bugs we’ve had but also because those are the ones we have the most carefully noted meta-data for. Like when they were reported, when they were introduced and when they were fixed.

We have no less than 30 logged vulnerabilities for curl and libcurl so far through-out our history, spread out over the past 16 years. I’ve spent some time going through them to see if there’s a pattern or something that sticks out that we should put some extra attention to in order to improve our processes and code. While doing this I gathered some random info about what we’ve found so far.

On average, each security problem had been present in the code for 2100 days when fixed – that’s more than five and a half years. On average! That means they survived about 30 releases each. If bugs truly are shallow, it is still certainly not a fast processes.

Perhaps you think these 30 bugs are really tricky, deeply hidden and complicated logic monsters that would explain the time they took to get found? Nope, I would say that every single one of them are pretty obvious once you spot them and none of them take a very long time for a reviewer to understand.

Vulnerability ages

This first graph (click it for the large version) shows the period each problem remained in the code for the 30 different problems, in number of days. The leftmost bar is the most recent flaw and the bar on the right the oldest vulnerability. The red line shows the trend and the green is the average.

The trend is clearly that the bugs are around longer before they are found, but since the project is also growing older all the time it sort of comes naturally and isn’t necessarily a sign of us getting worse at finding them. The average age of flaws is aging slower than the project itself.

Reports per year

How have the reports been distributed over the years? We have a  fairly linear increase in number of lines of code but yet the reports were submitted like this (now it goes from oldest to the left and most recent on the right – click for the large version):


Compare that to this chart below over lines of code added in the project (chart from openhub and shows blanks in green, comments in grey and code in blue, click it for the large version):

curl source code growth

We received twice as many security reports in 2014 as in 2013 and we got half of all our reports during the last two years. Clearly we have gotten more eyes on the code or perhaps users pay more attention to problems or are generally more likely to see the security angle of problems? It is hard to say but clearly the frequency of security reports has increased a lot lately. (Note that I here count the report year, not the year we announced the particular problems, as they sometimes were done on the following year if the report happened late in the year.)

On average, we publish information about a found flaw 19 days after it was reported to us. We seem to have became slightly worse at this over time, the last two years the average has been 25 days.

Did people find the problems by reading code?

In general, no. Sure people read code but the typical pattern seems to be that people run into some sort of problem first, then dive in to investigate the root of it and then eventually they spot or learn about the security problem.

(This conclusion is based on my understanding from how people have reported the problems, I have not explicitly asked them about these details.)

Common patterns among the problems?

I went over the bugs and marked them with a bunch of descriptive keywords for each flaw, and then I wrote up a script to see how the frequent the keywords are used. This turned out to describe the flaws more than how they ended up in the code. Out of the 30 flaws, the 10 most used keywords ended up like this, showing number of flaws and the keyword:

8 cert-check
8 buffer-overflow

6 info-leak
3 URL-parsing
3 openssl
3 http-headers
3 cookie

I don’t think it is surprising that TLS, HTTP or certificate checking are common areas of security problems. TLS and certs are complicated, HTTP is huge and not easy to get right. curl is mostly C so buffer overflows is a mistake that sneaks in, and I don’t think 27% of the problems tells us that this is a problem we need to handle better. Also, only 2 of the last 15 flaws (13%) were buffer overflows.

The discussion following this blog post is on hacker news.

curl 7.40.0: unix domain sockets and smb

curl and libcurl curl dot-to-dot7.40.0 was just released this morning. There’s a closer look at some of the perhaps more noteworthy changes. As usual, you can find the entire changelog on the curl web site.

HTTP over unix domain sockets

So just before the feature window closed for the pending 7.40.0 release of curl, Peter Wu’s patch series was merged that brings the ability to curl and libcurl to do HTTP over unix domain sockets. This is a feature that’s been mentioned many times through the history of curl but never previously truly implemented. Peter also very nicely adjusted the test server and made two test cases that verify the functionality.

To use this with the curl command line, you specify the socket path to the new –unix-domain option and assuming your local HTTP server listens on that socket, you’ll get the response back just as with an ordinary TCP connection.

Doing the operation from libcurl means using the new CURLOPT_UNIX_SOCKET_PATH option.

This feature is actually not limited to HTTP, you can do all the TCP-based protocols except FTP over the unix domain socket, but it is to my knowledge only HTTP that is regularly used this way. The reason FTP isn’t supported is of course its use of two connections which would be even weirder to do like this.


SMB is also known as CIFS and is an old network protocol from the Microsoft world access files. curl and libcurl now support this protocol with SMB:// URLs thanks to work by Bill Nagel and Steve Holme.

Security Advisories

Last year we had a large amount of security advisories published (eight to be precise), and this year we start out with two fresh ones already on the 8th day… The ones this time were of course discovered and researched already last year.

CVE-2014-8151 is a way we accidentally allowed an application to bypass the TLS server certificate check if a TLS Session-ID was already cached for a non-checked session – when using the Mac OS SecureTransport SSL backend.

CVE-2014-8150 is a URL request injection. When letting curl or libcurl speak over a HTTP proxy, it would copy the URL verbatim into the HTTP request going to the proxy, which means that if you craft the URL and insert CRLFs (carriage returns and linefeed characters) you can insert your own second request or even custom headers into the request that goes to the proxy.

You may enjoy taking a look at the curl vulnerabilities table.

Bugs bugs bugs

The release notes mention no less than 120 specific bug fixes, which in comparison to other releases is more than average.


Can curl avoid to be in a future funnily named exploit that shakes the world?

During this year we’ve seen heartbleed and shellshock strike (and a  few more big flaws that I’ll skip for now). Two really eye opening recent vulnerabilities in projects with many similarities:

  1. Popular corner stones of open source stacks and internet servers
  2. Mostly run and maintained by volunteers
  3. Mature projects that have been around since “forever”
  4. Projects believed to be fairly stable and relatively trustworthy by now
  5. A myriad of features, switches and code that build on many platforms, with some parts of code only running on a rare few
  6. Written in C in a portable style

Does it sound like the curl project to you too? It does to me. Sure, this description also matches a slew of other projects but I lead the curl development so let me stay here and focus on this project.

cURLAre we in jeopardy? I honestly don’t know, but I want to explain what we do in our project in order to minimize the risk and maximize our ability to find problems on our own before they become serious attack vectors somewhere!

previous flaws

There’s no secret that we have let security problems slip through at times. We’re right now working toward our 143rd release during our around 16 years of life-time. We have found and announced 28 security problems over the years. Looking at these found problems, it is clear that very few security problems are discovered quickly after introduction. Most of them linger around for several years until found and fixed. So, realistically speaking based on history: there are security bugs still in the code, and they have probably been present for a while already.

code reviews and code standards

We try to review all patches from people without push rights in the project. It would probably be a good idea to review all patches before they go in for real, but that just wouldn’t work with the (lack of) man power we have in the project while we at the same time want to develop curl, move it forward and introduce new things and features.

We maintain code standards and formatting to keep code easy to understand and follow. We keep individual commits smallish for easier review now or in the future.

test cases

As simple as it is, we test that the basic stuff works. We don’t and can’t test everything but having test cases for most things give us the confidence to change code when we see problems as we then remain fairly sure things keep working the same way as long as the test go through. In projects with much less test coverage, you become much more conservative with what you dare to change and that also makes you more vulnerable.

We always want more test cases and we want to improve on how we always add test cases when we add new features and ideally we should also add new test cases when we fix bugs so that we know that we don’t introduce any such bug again in the future.

static code analyzes

We regularly scan our code base using static code analyzers. Both clang-analyzer and coverity are good tools, and they help us by pointing out code that look wrong or suspicious. By making sure we have very few or no such flaws left in the code, we minimize the risk. A static code analyzer is better than run-time tools for cases where they can check code flows that are hard to repeat in my local environment.


bike helmetValgrind is an awesome tool to detect memory problems in run-time. Leaks or just stupid uses of memory or related functions. We have our test suite automatically use valgrind when it runs tests in case it is present and it helps us make sure that all situations we test for are also error-free from valgrind’s point of view.


Building and testing curl on a plethora of platforms non-stop is also useful to make sure we don’t depend on behaviors of particular library implementations or non-standard features and more. Testing it all is basically the only way to make sure everything keeps working over the years while we continue to develop and fix bugs. We would course be even better off with more platforms that would test automatically and with more developers keeping an eye on problems that show up there…

code complexity

Arguably, one of the best ways to avoid security flaws and bugs in general, is to keep the source code as simple as possible. Complex functions need to be broken down into smaller functions that are possible to read and understand. A good way to identify functions suitable for fixing is pmccabe,

essential third parties

curl and libcurl are usually built to use a whole bunch of third party libraries in order to perform all the functionality. In order to not have any of those uses turn into a source for trouble we must of course also participate in those projects and help them stay strong and make sure that we use them the proper way that doesn’t lead to any bad side-effects.

You can help!

All this takes time, energy and system resources. Your contributions and help will be appreciated where ever among these tasks that you can insert any. We could do more of all this, more often and more thorough if we only were more people involved!

curl is no POODLE

Once again the internet flooded over with reports and alerts about a vulnerability using a funny name: POODLE. If you have even the slightest interest in this sort of stuff you’ve already grown tired and bored about everything that’s been written about this so why on earth do I have to pile on and add to the pain?

This is my way of explaining how POODLE affects or doesn’t affect curl, libcurl and the huge amount of existing applications using libcurl.

Is my application using HTTPS with libcurl or curl vulnerable to POODLE?

No. POODLE really is a browser-attack.


The POODLE attack is a combination of several separate pieces that when combined allow attackers to exploit it. The individual pieces are not enough stand-alone.

SSLv3 is getting a lot of heat now since POODLE must be able to downgrade a connection to SSLv3 from TLS to work. Downgrade in a fairly crude way – in libcurl, only libcurl built to use NSS as its TLS backend supports this way of downgrading the protocol level.

Then, if an attacker manages to downgrade to SSLv3 (both the client and server must thus allow this) and get to use the sensitive block cipher of that protocol, it must maintain a connection to the server and then retry many similar requests to the server in order to try to work out details of the request – to figure out secrets it shouldn’t be able to. This would typically be made using javascript in a browser and really only HTTPS allows this so no other SSL-using protocol can be exploited like this.

For the typical curl user or a libcurl user, there’s A) no javascript and B) the application already knows the request it is doing and normally doesn’t inject random stuff from 3rd party sources that could be allowed to steal secrets. There’s really no room for any outsider here to steal secrets or cookies or whatever.

How will curl change

There’s no immediate need to do anything as curl and libcurl are not vulnerable to POODLE.

Still, SSLv3 is long overdue and is not really a modern protocol (TLS 1.0, the successor, had its RFC published 1999) so in order to really avoid the risk that it will be possible exploit this protocol one way or another now or later using curl/libcurl, we will disable SSLv3 by default in the next curl release. For all TLS backends.

Why? Just to be extra super cautious and because this attack helped us remember that SSLv3 is old and should be let down to die.

If possible, explicitly requesting SSLv3 should still be possible so that users can still work with their legacy systems in dire need of upgrade but placed in corners of the world that every sensible human has since long forgotten or just ignored.

In-depth explanations of POODLE

I especially like the ones provided by PolarSSL and GnuTLS, possibly due to their clear “distance” from browsers.