Tag Archives: cURL and libcurl

cURL and libcurl

My BDFL guiding principles

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The thing about me being a BDFL for curl is that it has the D in there. I have the means and ability to push for or veto just about anything I like or don’t like in the project, should I decide to. In my public presentations about curl I emphasize that I truly try to be a benevolent dictator, but then I also presume quite a few dictators would say and believe so whether that is true or not to the outside world.

I think we can say with some certainty that dictatorships are not the ideal way of running a country, and it might also go for Open Source projects.

In curl we remain using this model because it works and changing it to something else is a large and complicated process which we have not wanted to get to because we have not had any strong reason. There is anecdotal evidence that this way of running the project works somewhat.

A significant difference between being a dictator for an Open Source project compared to a country is however the ease with which every citizen could just leave one day and start a new clone country, with all the same citizens and the same layout, just without the dictator. I’m easily replaceable and made into past tense if I would abuse my role.

So there is this inherent force to push me to do good for the project even if I am a “dictator”.

As a BDFL of curl…

This is what I think the curl project should focus on. What I want the curl project to be. These are the ten commandments I think should remain our guiding principles. I think this is what makes curl. My benevolent guidelines.

My ten guiding principles for curl

  1. Be open and friendly
  2. Ship rock-solid products
  3. Be a leader in Open Source
  4. Maintain a security first focus
  5. Provide top-notch documentation
  6. Remain independent
  7. Respond timely
  8. Keep up with the world
  9. Stay on the bleeding edge
  10. Respect feedback

This list of ten areas are perhaps things every open source projects want to focus and excel in, but I think that is irrelevant here. These are then ten key and core focus points for me when I work on curl. We should be best-in-class in each and every one of them.

Let me elaborate

1. Be open and friendly to all contributors, new and old

I think open source projects have a lot to gain by making efforts in being friendly and approachable. We were all newcomers into a project once. We gain more contributors, better, by remaining a friendly and open project.

This does not mean that we should tolerate abuse or trolling.

I try to lead this by example. I do not always succeed.

2. Ship rock-solid products for the universe to depend upon

Reviews, tests, analyzers, fuzzers, audits, bug bounty programs etc are means to make sure the code runs smoothly everywhere. Studying protocol specs and inter-operating with servers and other clients on the Internet ensure that the products we make work as expect by billions of end users. On this planet and beyond. If our products cannot carry the world on their shoulders, we fail.

Rock-solid also means we are reliable over time – we do not break users’ scripts and applications. We maintain ABI/API compatibility. The command line options the curl tool introduces are supported until the end of time.

3. Be a leader in Open Source, follow every best practice

As true believers in the powers of Open Source we lead by example. We are here to show that you not only can do all development and everything in the open and using open source practices, but that it it also makes the project thrive and deliver state of the art outcomes.

4. Always keep users secure, maintain a security first focus

Provide features and functionality with user and protocol security in focus. Address security concerns and reports without delays. Document every past mistake in thorough detail. Help users do secure and safe Internet transfers – by default.

5. Provide industry-leading quality documentation

A key to successful usage of our products, to give users the means and ability to use our project fully, we need to document how it works and how to use it. Everything needs to be documented with clarity and detail enough so that users understand and are empowered.

No comparable software project, open or proprietary, can compete with the quality and amount of documentation we provide.

6. Roam free, independent from all companies and organizations

curl shall forever remain independent. curl is not part of any umbrella organization, it is not owned or controlled by any company. It makes us entirely independent and free to do what we think is best for the community, for our users and for Internet transfers in general. Its license shall remain set and it ensures that curl remains free. Copyright holders are individuals, there is no assignment or licensing of copyrights involved.

7. Respond timely on issues and questions

We shall strive to respond to issues, reports and questions sent to the project within a reasonable time. To show that we care and to help users solve their problems. We want users to solve their problems sooner rather than later.

It does not mean that we always can fix the problems or give a good answer immediately. Sometimes we just have to say that we can’t fix it for now.

8. Remain the internet transfer choice, keep up with the world

In the curl project we should keep up with protocol development, updates and changes. The way we do Internet transfers changes over time and curl needs to keep up to remain relevant.

We also need to write our protocol implementations sensibly, knowing that we are being watched and our way of doing things are often copied, referred to and relied upon by other Internet clients.

This also implies that we are never done and that we can always improve. In every aspect of the project.

9. Offer bleeding edge protocol support to aid early adopters

When new protocols or ways to do protocols are introduced to the world, curl can play a great role in providing and offering early support of such protocols. This has through the years helped countless of other implementers or even protocol authors of these protocols and with this, we help improving the world around us. It also helps us get early feedback on our implementation and thus ship better code earlier.

10. Listen to and respect community feedback

I might be a dictator, but this dictatorship would not work if I and the rest of the curl maintainers did not listen to what the users and the greater curl community have to say. We need to stay agile and have a sense of what people want our products to do and to not do. Now and in the future.

An open source project can always get forked the second it makes a bad turn and somehow gives up on, sells out or betrays its users. Me being a dictator does not protect us from that. We need to stay responsive, listening and caring. We are here for our users.

Flag?

If curl was an evil empire, I figure we would sport this flag:

cURL and libcurl

Google Peer Bonus number five

It is not quite a gold medal, but it is now the fifth time I have the honor of receiving a Google Open Source Peer Bonus. I might soon start to think I have some fans over there.

There is a monetary component to this bonus. Last time it was to the amount of 500 USD. I have not seen the amount for this time as it has not been transferred to me yet. I trust it will buy me a few good beers anyway.

Update June 10: it was 500 USD this time as well.

cURL and libcurl

curl 8.8.0

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Numbers

the 257th release
8 changes
56 days (total: 9,560)
220 bug-fixes (total: 10,271)
348 commits (total: 32,280)
1 new public libcurl function (total: 94)
1 new curl_easy_setopt() option (total: 305)
1 new curl command line option (total: 259)
84 contributors, 41 new (total: 3,173)
49 authors, 20 new (total: 1,272)
0 security fixes (total: 155)

Download the new curl release from curl.se as always.

Release presentation

Security

It feels good to be able to say that this time around we do not have a single security vulnerability to announce and we in fact do not have any in the queue either.

Changes

Bugfixes

Some of the bugfixes from this cycle that might be worth noticing:

dist and build

  • reproducible tarballs. I will do a separate post with details later, but now it is easy for anyone who wants to, to generate an identical copy to verify what we ship.
  • docs/RELEASE-TOOLS.md into the tarball. This documents the tools and versions used to generate the files included in the tarball that are not present in git.
  • drop MSVC project files for recent versions. If you need to generate them for more recent versions, cmake can do it for you.
  • configure fix HAVE_IOCTLSOCKET_FIONBIO test for gcc 14. It runs more picky by default so it would always fail the check.
  • add -q as first option when invoking curl for tests. To reduce the risk of people having a ~/.curlrc file that ruins things.
  • fix make install with configure –disable-docs

tool

  • make –help adapt to the terminal width. Makes it easier on the eye when the terminal is wider.
  • limit rate unpause for -T . uploads. Avoids busy-looping
  • curl output warning for leading unicode quote character. Because it seems like a fairly common mistake when people copy and paste command lines from random sources
  • don’t truncate the etag save file by default. A regression less.

TLS

  • bearssl: use common code for cipher suite lookup
  • mbedtls: call mbedtls_ssl_setup() after RNG callback is set. Otherwise, more recent versions of mbedTLS will just return error.
  • mbedtls: support TLS 1.3. If you use a new enough version.
  • openssl: do not set SSL_MODE_RELEASE_BUFFERS. Uses slightly more memory, but uses fewer memory allocation calls.
  • wolfssl: plug memory leak in wolfssl_connect_step2()

bindings

  • openldap: create ldap URLs correctly for IPv6 addresses, doing LDAP with IPv6 numerical IP addresses in the URL just did not work previously.
  • quiche: expire all active transfers on connection close
  • quiche: trust its timeout handling

libcurl

  • fix curl_global_cleanup crash in Windows. A regression coming from the introduction of the async name resolver function.
  • brotli and others, pass through 0-length writes
  • ignore duplicate chunked encoding. Apparently some sites do this and browsers let them so we need to let it slide…
  • CURLINFO_REQUEST_SIZE: fixed
  • ftp: add tracing support. Gives us better tooling to track down FTP problems.
  • http2: emit RST when client write fails. Previously it would just silently leave the stream there…
  • http: reject HTTP major version switch mid connection. This should of course never happen, but if it does, curl will error out correctly.
  • multi: introduce SETUP state for better timeouts. This adds a proper separation for when the existing transfer is retried or when the state machine is restarted because it make as a new transfer.
  • multi: timeout handles even without connection. They would previously often be exempted from checks and would linger for too long until stopped.
  • fix handling of paused upload on completed download
  • do not URL decode proxy credentials
  • allow setting port number zero. Remember this old post?
  • fix relative redirects to fragment-only
  • fix memory leak in websocket error path
cURL and libcurl, Security

curl, Tor, dot onion and SOCKS

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You can of course use curl to access hosts through Tor. (I know you know Tor so I am not going to explain it here.)

SOCKS

The typical way to access Tor is via a SOCKS5 proxy and curl has supported that since some time during 2002. Like this:

curl --socks5-hostname localhost:5432 https://example.com

or

curl --proxy socks5h://localhost:5432 https://example.com

or

export HTTPS_PROXY=socks5h://localhost:5432
curl https://example.com

Name resolving with SOCKS5

You know Tor, but do you know SOCKS5? It is an old and simple protocol for setting up a connection and when using it, the client can decide to either pass on the full hostname it wants to connect to, or it can pass on the exact IP address.

(SOCKS5 is by the way a minor improvement of the SOCKS4 protocol, which did not support IPv6.)

When you use curl, you decide if you want curl or the proxy to resolve the target hostname. If you connect to a site on the public Internet it might not even matter who is resolving it as either party would in theory get the same set of IP addresses.

The .onion TLD

There is a concept of “hidden” sites within the Tor network. They are not accessible on the public Internet. They have names in the .onion top-level domain. For example. the search engine DuckDuckGo is available at https://duckduckgogg42xjoc72x3sjasowoarfbgcmvfimaftt6twagswzczad.onion/.

.onion names are used to provide access to end to end encrypted, secure, anonymized services; that is, the identity and location of the server is obscured from the client. The location of the client is obscured from the server.

To access a .onion host, you must let Tor resolve it because a normal DNS server aware of the public Internet knows nothing about it.

This is why we recommend you ask the SOCKS5 proxy to resolve the hostname when accessing Tor with curl.

The proxy connection

The SOCKS5 protocol is clear text so you must make sure you do not access the proxy over a network as then it will leak the hostname to eavesdroppers. That is why you see the examples above use localhost for the proxy.

You can also step it up and connect to the SOCKS5 proxy over unix domain sockets with recent curl versions like this:

curl --proxy socks5h://localhost/run/tor/socks https://example.com

.onion leakage

Sites using the .onion TLD are not on the public Internet and it is pointless to ask your regular DNS server to resolve them. Even worse: if you in fact ask your normal resolver you practically advertise your intention of connection to a .onion site and you give the full name of that site to the outsider. A potentially significant privacy leak.

To combat the leakage problem, RFC 7686 The “.onion” Special-Use Domain Name was published in October 2015. With the involvement and consent from people involved in the Tor project.

It only took a few months after 7686 was published until there was an accurate issue filed against curl for leaking .onion names. Back then, in the spring of 2016, no one took upon themselves to fix this and it was instead simply added to the queue of known bugs.

This RFC details (among other things) how libraries should refuse to resolve .onion host names using the regular means in order to avoid the privacy leak.

After having stewed in the known bugs lists for almost five years, it was again picked up in 2023, a pull-request was authored, and when curl 8.1.0 shipped on May 17 2023 curl refused to resolve .onion hostnames.

Tor still works remember?

Since users are expected to connect using SOCKS5 and handing over the hostname to the proxy, the above mention refusal to resolve a .onion address did not break the normal Tor use cases with curl.

Turns out there are other common ways to do it.

A few days before the 8.1.0 release shipped a discussion thread was created: I want to resolve onion addresses.

Every change breaks someone’s workflow

XKCD 1172 – we hear you

Transparent proxies

Turns out there is a group of people who runs transparent proxies who automatically “catches” all local traffic and redirects it over Tor. They have a local DNS server who can resolve .onion host names and they intercept outgoing traffic to instead tunnel it through Tor.

With this setup now curl no longer works because it will not send .onion addresses to the local resolver because RFC 7686 tells us we should not,

curl of course does not know when it runs in a presumed safe and deliberate transparent proxy network or when it does not. When a leak is not a leak or when it actually is a leak.

torsocks

A separate way to access tor is to use the torsocks tool. Torsocks allows you to use most applications in a safe way with Tor. It ensures that DNS requests are handled safely and explicitly rejects any traffic other than TCP from the application you’re using.

You run it like 

torsocks curl https://example.com

Because of curl’s new .onion filtering, the above command line works fine for “normal” hostnames but no longer for .onion hostnames.

Arguably, this is less of a problem because when you use curl you typically don’t need to use torsocks since curl has full SOCKS support natively.

Option to disable the filter?

In the heated discussion thread we are told repeatedly how silly we are who block .onion name resolves – exactly in the way the RFC says, the RFC that had the backing and support from the Tor project itself. There are repeated cries for us to add ways to disable the filter.

I am of course sympathetic with the users whose use cases now broke.

A few different ways to address this have been proposed, but the problem is difficult: how would curl or a user know that it is fine to leak a name or not? Adding a command line option to say it is okay to leak would just mean that some scripts would use that option and users would run it in the wrong conditions and your evil malicious neighbors who “help out” will just add that option when they convince their victims to run an innocent looking curl command line.

The fact that several of the louder voices show abusive tendencies in the discussion of course makes these waters even more challenging to maneuver.

Future

I do not yet know how or where this lands. The filter has now been in effect in curl for a year. Nothing is forever, we keep improving. We listen to feedback and we are of course eager to make sure curl remains and awesome tool and library also for content over Tor.

This discussion is also held within the more proper realms of the tor project itself.

Credits

Image by Couleur from Pixabay

cURL and libcurl

curl user survey 2024

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Welcome to the 11th annual curl user survey. This is a once a year poll that we ask as many curl and libcurl users as possible to respond to.

>> Take the survey <<

This is in many ways the only real way we get to know what curl users think about all sorts of curl matters. Our website does not log, it has no adds, it uses no cookies and it does no tracking. We do not count downloads, we do not know which man pages are read the most. We mostly ship our code into the void without knowing a whole about what people do with and think about it.

Asking our users directly is in effect our only and best way to get proper answers. So we do this every year, and we ask a lot of questions in the same fashion as last year so that we can better detect trends and changes in the community.

Your help is not only appreciated, it is crucial. Tell us your honest opinion. And if you have friends you know use curl or libcurl, please ask them to submit a set of answers as well. You help us greatly by donating several minutes of your busy life.

>> Take the survey <<

The survey will be up during 14 days from May 14th until the end of May 28th 2024. It would be awesome to try to beat the last year’s submission numbers when 606 persons responded.

See also: the curl user survey 2023 analysis, which details the results and thoughts after last year’s edition.

cURL and libcurl

I survived curl up 2024

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On Friday May 3, 2024 I had several of my curl friends over for dinner in my house. An unusually warm and sunny spring day with a temperature reaching twenty degrees centigrade.

The curl up 2024 weekend started excellently and the following morning we all squeezed ourselves into a conference room in downtown Stockholm. I had rented a room in a hotel in the city center for two days.

curl up is never a big meeting/conference but we have in the past sometimes been around twenty-five attendees. This year’s amount of fifteen was the smallest so far, but in this small set of people we have a set of long-term well-known curl contributors. It is not a big list of attendees that creates a good curl up.

Swag

We started by making sure every attendee got their needs of curl t-shirts, curl mugs, curl stickers and curl coasters satisfied. The t-shirts of the year are “forest green” with the curl logo in white on the front and the curl symbol slightly larger on the back.

I have spare t-shirts that I intend to distribute to people I meet over the coming year. Before you ask: no, there is no way to buy these.

Recordings

I had tested my external microphone setup at home but it just refused to work when at the venue. We struggled for a while until we had to surrender and fall back to using the built-in microphone in the webcam that we used for recording the video. This is why the sound is low in all recordings we did. A little disappointing. Sorry for this.

I live-streamed the entire event over twitch. We had in total over 460 unique viewers over the days and at times at least we had over 30 concurrent sustained viewers. This made us at least sometimes have twice the size audience online as in the room. In spite of the sound issue.

I also noticed that my trusty old laptop was maybe a little weak for this purpose as it struggled to stream and save the recordings at high frame rates.

I brought: two laptops, webcam on stand, mike, mike-stand, power for laptops, cable kit, repair kit, 12 curl mugs, eight packs with different curl stickers, carton coasters, pcb coasters, t-shirts, name tags + pens, two UCB-C to HDMI adapters.

Day one

The state of curl 2024

Where are we, what did we do last year or so? Who did the work? How often? How much?

Evolutions

Apparently this is not a real word, but Stefan Eissing pushes for language development in this presentation where he talks about changes and improvements he worked on in curl over the last few years.

Fuzzing curl

James Fuller talks about his work on generating “fun” curl command lines in order to find those that might not be handled correctly.

Implementing parallel testing

Dan Fandrich talks about the journey from serial to fully parallel tests in curl.

curl containers

James is back and talks about where the curl containers are right now.

Security

I talk about the security situation in curl as of right now and the last year.

End of day 1

We topped off this packed day with a twenty minute walk through a sunny Stockholm down to the water where we could sit outside and have a few drinks before we moved over to the restaurant where we ended the evening with a joint dinner. A great first day!

Day two

The nice weather was gone. The temperature dropped ten degrees and the rain poured down most of this day.

HTTP/1/2/3 Performance

Stefan Eissing warms up the day. About his work on HTTP refactors and related performance improvements.

trurl

This is the newcomer in the curl family and I talked a little about what it is and why it exists.

Apple Specialties

Christian has improved curl on Apple devices, which he talks about.

rust in curl

You can build curl to use third party components written in rust. This is where we are now and what might happen next. Or not.

Test clutch

Dan talks about his work on improving curl tests and their reliability.

Future

We don’t know much about the future but there are some plans and there are at least some ideas…

End of curl up 2024

The rest of day two was mostly spent hanging out and talking about life, the universe and various things curl. People started leaving and by five o’clock we shut the door for the last time this time around. We had survived curl up 2024.

After all these talks, discussions, dinners, beers, coffees, challenging questions, brainstorms over 48 hours, I was exhausted and drained of energy. Apart from the recording problem, I think almost everything else in the event organization went as smoothly as we could have wished for. The venue, the food, the coffee etc worked perfectly for us.

Planning ahead for 2025

We will most certainly run another curl up event in 2025 in roughly the same frame of the year as we did now. The idea is then to visit another capital city in Europe. Stay tuned for coming announcements of date and location for that.

cURL and libcurl

Six billion docker pulls

1 Comment

We provide an official curl container.

Why would you use curl in a container? We actually don’t ask, we just provide the image, but I can think of a few reasons…

  • it is an easy way to use a modern curl version in a system that otherwise ships an ancient version. So many people are stuck on legacy distros with ancient curl versions.
  • it is an easy way to make use of a consistent fixed version from many places independently of what particular curl versions those systems otherwise can offer
  • CI jobs
  • other elaborate explanations

Six billion as of now

The official curl docker repository now (as of 06:43 UTC April 24, 2024) reports that the curl container has been pulled more than six billion times. Currently, people seem to be pulling the curl image from docker.com at a rate of 2-3 million pulls per day (about 25 per second).

It shall be noted that a pull does not necessary imply a download. The pull is a a check and the client may already have the latest version downloaded. It is therefore not equal to six billion downloads.

We started offering docker images to the world with curl 7.65.3, July 19 2019. Six billion pulls in 1832 days makes an average of 38 pulls/second through all this time. Less than five years.

How do I know the pull counter reached six billion? I asked their API:

curl https://hub.docker.com/v2/repositories/curlimages/ -s | jq .results[0].pull_count

Sponsored OSS

We do not pay Docker anything for this service of theirs. They also do not pay anything to us for our service. The Docker Sponsored OSS program lists conditions that might make us disqualified for being part of it, but as long as you don’t tell them I won’t. And hey, at least the first six billion pulls have been served.

Other repositories

You can also opt to pull the container from other repositories like quay and GitHub. I have not included their pull counters in this.

cURL and libcurl

curl is just the hobby

11 Comments

Jan Gampe took things to the next level by actually making this cross-stitch out of the pattern I previously posted online. The flowers really gave it an extra level of charm I think.

curl is just the hobby of some guy that has no business providing a service to a billion people
As a cross-stitch
As a pillow

This quote is from a comment by an upset user on my blog, replying to one of my previous articles about curl.

Fact check: while curl is my hobby, I also work on curl as a full-time job. It is a business and I serve and communicate with many customers on a daily basis. curl provides service to way more than a billion people. I claim that every human being on the planet that is Internet-connected uses devices or services every day that run curl.

The pattern

curl is just the hobby of some guy that has no business providing a service to a billion people

curl in San Francisco

Meanwhile, another “curl craft” seen in the wild recently is this ad in San Francisco (photo by diego).

The full command line looks like:

curl --request PUT \
  --url https://api.stytch.com/v1/b2b/organizations/{ID} \
  -d '{
        "mfa_policy": "REQUIRED_FOR_ALL",
        "mfa_methods": "RESTRICTED",
        "allowed_mfa_methods": ["totp", "sms_otp"]
       }'

I would personally perhaps protest against the use of PUT for POSTing JSON, but nobody asked me.

Update

Quote from the hacker news thread about the ad shown in the photo from San Francisco:

hi, Stytch team member here who worked on the PUT request ad from Daniel’s post — feel free to AMA

TL;DR on ‘why pay to put a curl request on an ad’ is what you all have already said — (1) unique concentration of tech in SF; and (2) to specifically engage software engineers. More on each…

(1) We wouldn’t have run this ad in Sydney, or New York, or LA. SF definitely has an uniquely tech-oriented culture, and in particular has lots of startups in our ideal customer persona (ICP) at Stytch – in this case, engineers building B2B SaaS apps.

But in addition to the people who live in SF, even more software engineers visit the city for conferences, or events, or to fundraise. For example, while our ads are up over the next month, SF will host Stripe Sessions and POSTCon (two entire conferences focused around APIs), plus RSA (security focused).

(2) And although even with that, only a small segment of the pop will understand the ads, those people might be intrigued enough to actually look at them – and our ultimate goal is to get more engineers to look at our code & our docs. Another perk is that engineers can’t use ad blockers if the ad is on a bus shelter 🙂

So that was a bit of the thinking for us – on why SF & why a PUT request on a billboard. We’re also making the physical ads into an anchor for a ‘marketing moment’ for Stytch — so pair offline ads with digital marketing, as well. So if we’re successful, maybe you’ll see more on that, soon.

cURL and libcurl

Verified curl

4 Comments

Don’t trust. Verify.

I could not resist making a fake book cover

Here follows a brief description on how you can detect if the curl package would ever make an xz.

xz (and its library liblzma) was presumably selected as a target because it is an often used component and by extension via systemd it often used by openssh in several Linux distros. libcurl is probably an even more widely used software component and if infected, could potentially serve as an effective vessel to distribute evil into the world.

Conceivably, the xz attackers have infiltrated more than one other Open Source project to cover their bases. Which ones?

No inexplicable binary blobs

First, you can verify that there are no binary blobs stored in git that could host an encrypted attack payload, planted there for the future.

Every file in the curl git repository has a benign meaning and purpose. As part of the products, the documentation, tooling or the test suites etc.

Without any secret “hide-out” in the git repository, you know that any backdoor needs to be provided either in plain code or using some crazy weird steganography. Or get inserted into the tarballs with content not present in git – read on for how to verify that this is not happening.

No disabled fuzzers

The xz attack could have been detected by proper fuzzing (and valgrind use) which is why the attacker made sure to sneakily disable such automated checks of code.

While somewhat hard to verify, you can make sure that no such activities have been done in curl’s fuzzing or curl’s automated and CI testing,

No hidden payloads in tarballs

In the curl project we generate several files as part of the release process and those files end up in the release tarball. This means that not all files in the tarball are found in the git repository. (Because we don’t commit generated files.)

The generated files are produced with a small set of tools, and these tools use the source code available in git at the release tag. Anyone can check out the same code from that same release tag, make sure to have the corresponding tools (and versions) installed and then generate a version of the tarball themselves – to verify that this tarball indeed becomes an identical copy of the public release.

That process verifies that the tarballs we ship are generated only with legitimate tools and that the release contents originate only from the files present in git. No secret sauce added in the process. No malicious code can get inserted.

Reproducible tarballs

We have recently improved reproducibility as a direct result of the post xz-attack debate. To make sure that a repeated tarball creation actually produces the exact same results, but also to make it easier for others to verify our release tarballs. With more documentation (releases now contain documentation of exactly which tools and versions that generated the tarball) and by making it easier to run the exact same virtual machine and tool setup like the one that created the release. We aim to soon provide a Dockerfile to make this process even smoother.

We also verify tarball reproducibility in a CI job: generating a release tarball with a given timestamp produces the identical binary output when done again for the same timestamp.

Signed tarballs

As an extra detail, everyone can also verify that the released tarballs are in fact shipped by me Daniel personally, as they are always signed with my GPG key as part of the release process. This should at least prove that new releases are made with the same keys as previous ones were, which should with a reasonable probability be me.

The signatures also help verify that the tarballs have not been tampered with in transition, from the point I generated them to the moment they land in your download directory. curl downloads are normally distributed via a third-party CDN which we normally trust of course, but if it would ever be breached or similar, a modified tarball would be detected when the digital signature is verified.

We do not provide checksums for the tarballs simply because providing checksums next to the downloads adds almost no extra verification. If someone can tamper with the tarballs, they can probably update the webpage with a fake checksum as well.

Signed commits

All commits done to curl done by me are signed, You can verify that I did them. Not all committers in the project do them signed, unfortunately. We hope to increase the share going forward. Over the last 365 days, 73% of the curl commits were signed.

These signatures only verify that the commits where done by a maintainer of the curl project (or someone who controls that account). A maintainer you may not trust and who might not be known under their real name and you do not even know in which country they live. And of course, even a trusted maintainer can suddenly go rogue.

Is the content in git benign?

The process above only verifies that tarballs are indeed generated (only) from contents present in git and that they are unaltered from the moment I made them.

How do you know that the contents in git does not contain any backdoors or other vulnerabilities?

Without trusting anyone else’s opinions and without just relying on the fact that you can run the test suite, fuzzers and static code analyzers without finding anything, you can review it. Or pay someone else to review it.

We have had curl audited several times by external organizations, but can you trust claimed random audits?

Anonymous contributors

We regularly accept contributions from anonymous and pseudonymous contributors in curl – and we always have. Our policy says that if a contribution is good: if it passes review and all tests run green, we have no reason to deny it – in the name of progress and improvement. That is also why we accept even single-letter typo fixes: even a very small fix is a step in the right direction.

A (to me) surprisingly large amount of contributions are done by people who do not state a full real name. They may chose to be anonymous for various reasons – we do not ask. Maybe they fear retaliation if they would propose something that ends up buggy? Sometimes people want to hide their affiliation/origin so that their contribution is not associated with the organization they work at. Another reason sometimes mentioned is that women do it to avoid revealing themselves as female. etc. As I said: we do not ask so I cannot tell for sure.

Anonymous maintainers

We do not have anonymous maintainers, but we don’t actually have rules against it.

Right now, we have 18 members in the GitHub curl organization with the rights to push commits. I have not met all of them. I have not even seen the faces of all of them. They have all proven themselves worthy of their administrative rights based on their track record. I cannot know if anyone of them is using a false identity and I do not ask nor keep track in which country they reside. A former top maintainer in the curl projected even landed a large amount of changes under a presumed/false name during several years.

If a curl maintainer suddenly goes rogue and attempts to land malicious content, our only effective remedy is review. To rely on the curl community to have eyes on the changes and to test things.

Can curl be targeted?

I think it would be very hard but I can of course not rule it out. Possibly I am also blind for our weaknesses because I have lived with them for so long.

Everyone can help the greater ecosystem by verifying a package or two. If we all tighten all screws just a little bit more, things will get better.

Vulnerabilities

I maintain that planting a backdoor in curl code is so infuriatingly hard to achieve that efforts and energy are probably much rather spent on finding security vulnerabilities and ways to exploit them. After all, we have 155 published past vulnerabilities in curl so far, out of which 42 have been at severity high or critical.

I can be fairly sure that none of those 42 somewhat serious issues were deliberately planted, because just about every one of them were found in code that I personally authored…

People often ask. But I have never seen a backdoor attempt in curl. Maybe that is just me being naive.

Credits

Top Image by Gerd Altmann from Pixabay. Fake book cover by Daniel Stenberg.