curl is no POODLE

Friday, October 17th, 2014

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.

FOSS them students

Thursday, October 16th, 2014

On October 16th, I visited DSV at Stockholm University where I had the pleasure of holding a talk and discussion with students (and a few teachers) under the topic Contribute to Open Source. Around 30 persons attended.

Here are the slides I use, as usual possibly not perfectly telling stand-alone without the talk but there was no recording made and I talked in Swedish anyway…

Contribute to Open Source from Daniel Stenberg

internal timers and timeouts of libcurl

Friday, October 10th, 2014

wall clockBear with me. It is time to take a deep dive into the libcurl internals and see how it handles timeouts and timers. This is meant as useful information to libcurl users but even more as insights for people who’d like to fiddle with libcurl internals and work on its source code and architecture.

socket activity or timeout

Everything internally in libcurl is using the multi, asynchronous, interface. We avoid blocking calls as far as we can. This means that libcurl always either waits for activity on a socket/file descriptor or for the time to come to do something. If there’s no socket activity and no timeout, there’s nothing to do and it just returns back out.

It is important to remember here that the API for libcurl doesn’t force the user to call it again within or at the specific time and it also allows users to call it again “too soon” if they like. Some users will even busy-loop like crazy and keep hammering the API like a machine-gun and we must deal with that. So, the timeouts are mostly to be considered advisory.

many timeouts

A single transfer can have multiple timeouts. For example one maximum time for the entire transfer, one for the connection phase and perhaps even more timers that handle for example speed caps (that makes libcurl not transfer data faster than a set limit) or detecting transfers speeds below a certain threshold within a given time period.

A single transfer is done with a single easy handle, which holds a list of all its timeouts in a sorted list. It allows libcurl to return a single time left until the nearest timeout expires without having to bother with the remainder of the timeouts (yet).


… is the internal function to set a timeout to expire a certain number of milliseconds into the future. It adds a timeout entry to the list of timeouts. Expiring a timeout just means that it’ll signal the application to call libcurl again. Internally we don’t have any identifiers to the timeouts, they’re just a time in the future we ask to be called again at. If the code needs that specific time to really have passed before doing something, the code needs to make sure the time has elapsed.


A newcomer in the timeout team. I figured out we need this function since if we are in a state where we need to be called no later than a certain specific future time this is useful. It will not add a new timeout entry in the timeout list in case there’s a timeout that expires earlier than the specified time limit.

This function is useful for example when there’s a state in libcurl that varies over time but has no specific time limit to check for. Like transfer speed limits and the like. If Curl_expire() is used in this situation instead of Curl_expire_latest() it would mean adding a new timeout entry every time, and for the busy-loop API usage cases it could mean adding an excessive amount of timeout entries. (And there was a scary bug reported that got “tens of thousands of entries” which motivated this function to get added.)

timeout removals

We don’t remove timeouts from the list until they expire. Like for example if we have a condition that is timing dependent, then we set a timeout with Curl_expire() and we know we should be called again at the end of that time.

If we wouldn’t add the timeout and there’s no socket activity on the socket then we may not be called again – ever.

When an internal state transition into something else and we therefore don’t need a previously set timeout anymore, we have no handle or identifier to the timeout so it cannot be removed. It will instead lead to us getting called again when the timeout triggers even though we didn’t really need it any longer. As we’re having an API that allows this anyway, this is already handled by the logic and getting called an extra time is usually very cheap and is not considered a problem worth addressing.

Timeouts are removed automatically from the list of timers when they expire. Timeouts that are in passed time are removed from the list and the timers following will then get moved to the front of the queue and be used to calculate how long the single timeout should be next.

The only internal API to remove timeouts that we have removes all timeouts, used when cleaning up a handle.

many easy handles

I’ve mentioned how each easy handle treats their timeouts above. With the multi interface, we can have any amount of easy handles added to a single multi handle. This means one list of timeouts for each easy handle.

To handle many thousands of easy handles added to the same multi handle, all with their own timeout (as each easy handle only show their closest timeout), it builds a splay tree of easy handles sorted on the timeout time. It is a splay tree rather than a sorted list to allow really fast insertions and removals.

As soon as a timeout expires from one of the easy handles and it moves to the next timeout in its list, it means removing one node (easy handle) from the splay tree and inserting it again with the new timeout timer.

Coverity scan defect density: 0.00

Thursday, October 9th, 2014

A couple of days ago I decided to stop slacking and grab this long dangling item in my TODO list: run the coverity scan on a recent curl build again.

Among the static analyzers, coverity does in fact stand out as the very best one I can use. We run clang-analyzer against curl every night and it hasn’t report any problems at all in a while. This time I got almost 50 new issues reported by Coverity.

To put it shortly, a little less than half of them were issues done on purpose: for example we got several reports on ignored return codes we really don’t care about and there were several reports on dead code for code that are conditionally built on other platforms than the one I used to do this with.

But there were a whole range of legitimate issues. Nothing really major popped up but a range of tiny flaws that were good to polish away and smooth out. Clearly this is an exercise worth repeating every now and then.

End result

21 new curl commits that mention Coverity. Coverity now says “defect density: 0.00” for curl and libcurl since it doesn’t report any more flaws. (That’s the number of flaws found per thousand lines of source code.)

Want to see?

I can’t seem to make all the issues publicly accessible, but if you do want to check them out in person just click over to the curl project page at coverity and “request more access” and I’ll grant you view access, no questions asked.

A day in the curl project

Monday, September 29th, 2014

cURLI maintain curl and lead the development there. This is how I spend my time an ordinary day in the project. Maybe I don’t do all of these things every single day, but sometimes I do and sometimes I just do a subset of them. I just want to give you a look into what I do and why I don’t add new stuff more often or faster… I spend about one to three hours on the project every day. Let me also stress that curl is a tiny little project in comparison with many other open source projects. I’m certainly not saying otherwise.

the new bug

Someone submits a new bug in the bug tracker or on one of the mailing lists. Most initial bug reports lack sufficient details so the first thing I do is ask for more info and possibly ask the submitter to try a recent version as very often we get bug reported on very old versions. Many bug reports take several demands for more info before the necessary details have been provided. I don’t really start to investigate a problem until I feel I have a sufficient amount of details. We’re a very small core team that acts on other people’s bugs.

the question by a newbie in the project

A new person shows up with a question. The question is usually similar to a FAQ entry or an example but not exactly. It deserves a proper response. This kind of question can often be answered by anyone, but also most people involved in the project don’t feel the need or “familiarity” to respond to such questions and therefore remain quiet.

the old mail I haven’t responded to yet

I want every serious email that reaches the mailing lists to get a response, so all mails that neither I nor anyone else responds to I keep around in my inbox and when I have idle time over I go back and catch up on old mails. Some of them can then of course result in a new bug or patch or whatever. Occasionally I have to resort to simply saving away the old mail without responding in order to catch up, just to cut the list of outstanding things to do a little.

the TODO list for my own sake, things I’d like to get working on

There are always things I really want to see done in the project, and I work on them far too little really. But every once in a while I ignore everything else in my life for a couple of hours and spend them on adding a new feature or fixing something I’ve been missing. Actual development of new features is a very small fraction of all time I spend on this project.

the list of open bug reports

I regularly revisit this list to see what I can do to push the open ones forward. Follow-up questions, deep dives into source code and specifications or just the sad realization that a particular issue won’t be fixed within the nearest time (year?) so that I close it as “future” and add the problem to our KNOWN_BUGS document. I strive to keep the bug list clean and only keep relevant bugs open. Those issues that are not reproducible, are left without the proper attention from the reporter or otherwise stall will get closed. In general I feel quite lonely as responder in the bug tracker…

the mailing list threads that are sort of dying but I do want some progress or feedback on

In my primary email inbox I usually keep ongoing threads around. Lots of discussions just silently stop getting more posts and thus slowly wither away further up the list to become forgotten and ignored. With some interval I go back to see if the posters are still around, if there’s any more feedback or whatever in order to figure out how to proceed with the subject. Very often this makes me get nothing at all back and instead I just save away the entire conversation thread, forget about it and move on.

the blog post I want to do about a recent change or fix I did I’d like to highlight

I try to explain some changes to the world in blog posts. Not all changes but the ones that are somehow noteworthy as they perhaps change the way things have been or introduce new fun features perhaps not that easily spotted. Of course all features are always documented etc, but sometimes I feel I need to put some extra attention on focus on things in a more free-form style. Or I just write about meta stuff, like this very posting.

the reviewing and merging of patches

One of the most important tasks I have is to review patches. I’m basically the only person in the project who volunteers to review patches against any angle or corner of the project. When people have spent time and effort and gallantly send the results of their labor our way in the best possible format (a patch!), the submitter deserves a good review and proper feedback. Also, paving the road for more patches is one of the best way to scale the project. Helping newcomers become productive is important.

Patches are preferably posted on the mailing lists but there’s also some coming in via pull requests on github and while I strongly discourage that (due to them not getting the same attention and possible scrutiny on the list like the others) I sometimes let them through anyway just to be smooth.

When the patch looks good (or sometimes good enough and I just edit some minor detail), I merge it.

the non-disclosed discussions about a potential security problem

We’re a small project with a wide reach and security problems can potentially have grave impact on users. We take security seriously, and we very often have at least one non-public discussion going on about a problem in curl that may have security implications. We then often work on phrasing security advisories, working down exactly which versions that are vulnerable, producing patches for at least the most recent ones of those affected versions and so on.

tame stackoverflow has become almost like a wikipedia for source code and programming related issues (although it isn’t wiki), and that site is one of the primary referrers to curl’s web site these days. I tend to glance over the curl and libcurl related questions and offer my answers at times. If nothing else, it is good to help keeping the amount of disinformation at low levels.

I strongly disapprove of people filing bug reports on such places or even very detailed (lib)curl core questions that should’ve been asked on the curl-library list.

there are idle times too

Yeah. Not very often, but sometimes I actually just need a day off all this. Sometimes I just don’t find motivation or energy enough to dig into that terrible seldom-happening bug on a platform I’ve never seen personally. A project like this never ends. The same day we release a new release, we just reset our clocks and we’re back on improving curl, fixing bugs and cleaning up things for the next release. Forever and ever until the end of time.

keep-calm-and-improve-curl week #3

Monday, September 22nd, 2014

I won’t keep posting every video update here, but I mostly wanted to mention that I’ve kept posting a weekly video over at youtube basically explaining what’s going on right now within my dearest projects. Mostly curl and some Firefox stuff.

This week: libcurl server cert verification API got a bashing at SEC-T, is HTTP for UDP a good idea? How about adding HTTP cache support to libcurl? HTTP/2 is getting deployed as we speak. Interesting curl bug when used by XBMC. The patch series for Firefox bug 939318 is improving slowly – will it ever land?

Using APIs without reading docs

Thursday, September 18th, 2014

This morning, my debug session was interrupted for a brief moment when two friends independently of each other pinged me to inform me about a talk at the current SEC-T conference going on here in Stockholm right now. It was yet again time to bring up the good old fun called libcurl API bashing. Again from the angle that users who don’t read the API docs might end up using it wrong.

Updated: You can see Meredith Patterson’s talk here, and the libcurl parts start at 24:15.

The specific libcurl topic at hand once again mostly had the CURLOPT_VERIFYHOST option in focus, with basically is the same argument that was thrown at us two years ago when libcurl was said to be dangerous. It is not a boolean. It is an option that takes (or took) three different values, where 2 is the secure level and 0 is disabled.

SEC-T on curl API

(This picture is a screengrab from the live stream off youtube, I don’t have any link to a stored version of it yet. Click it for slightly higher resolution.)

Speaker Meredith L. Patterson actually spoke for quite a long time about curl and its options to verify server certificates. While I will agree that she has a few good points, it was still riddled with errors and I think she deliberately phrased things in a manner to make the talk good and snappy rather than to be factually correct and trying to understand why things are like they are.

The VERIFYHOST option apparently sounds as if it takes a boolean (accordingly), but it doesn’t. She says verifying a certificate has to be a Yes/No question so obviously it is a boolean. First, let’s be really technical: the libcurl options that take numerical values always accept a ‘long’ and all documentation specify which values you can pass in. None of them are boolean, not by actual type in the C language and not described like that in the man pages. There are however language bindings running on top of libcurl that may use booleans for the values that take 0 or 1, but there’s no guarantee we won’t add more values in a future to numerical options.

I wrote down a few quotes from her that I’d like to address.

“In order for it to do anything useful, the value actually has to be set to two”

I get it, she wants a fun presentation that makes the audience listen and grin cheerfully. But this is highly inaccurate. libcurl has it set to verify by default. An application doesn’t have to set it to anything. The only reason to set this value is if you’re not happy with checking the cert unconditionally, and then you’ve already wondered off the secure route.

“All it does when set to to two is to check that the common name in the cert matches the host name in the URL. That’s literally all it does.”

No, it’s not. It “only” verifies the host name curl connects to against the name hints in the server cert, yes, but that’s a lot more than just the common name field.

“there’s been 10 versions and they haven’t fixed this yet [...] the docs still say they’re gonna fix this eventually [...] I wanna know when eventually is”

Qualified BS and ignorance of details. Let’s see the actual code first: it ignores the 1 value and returns an error and thus leaves the internal default 2, Alas, code that sets 1 or 2 gets the same effect == verified certificate. Why is this a problem?

Then, she says she really wants to know when “eventually” is. (The docs say “Future versions will…”) So if she was so curious you’d think she would’ve tried to ask us? We’re an accessible bunch, on mailing lists, on IRC and on twitter. No she didn’t ask.

But perhaps most importantly: did she really consider why it returns an error for 1? Since libcurl silently accepted 1 as a value for something like 10 years, there are a lot of old installations “out there” in the wild, and by returning an error for 1 we try to make applications notice and adjust. By silently accepting 1 without errors, there would be no notice and people will keep using 1 in new applications as well and thus when running such an newly written application with an older libcurl – you’d be back to having the security problem again. So, we have the error there to improve the situation.

“a peer is someone like you [...] a host is a server”

I’m a networking guy since 20+ years and I’m not used to people having a hard time to understand these terms. While perhaps there are rookies out in the world who don’t immediately understand some terms in the curl option names, should we really be criticized for that? I find that a hilarious critique. Also, these names were picked 13 years ago and we have them around for compatibility and API stability.

“why would you ever want to …”

Welcome to the real world. Why would an application author ever want to have these options to something else than just full check and no check? Because people and software development is a large world with many different desires and use case scenarios and curl is more widely used and abused than what many people consider. Lots of people have wanted something else than just a Yes/No to server cert verification. In fact, I’ve had many users ask for even more switches and fine-grained ways to fiddle with verification. Yes/No is a lay mans simplified view of certificate verification.

SEC-T curl slide

(This picture is the slide from the above picture, just zoomed and straightened out a bit.)

API age, stability and organic growth

We started working on libcurl in spring 1999, we added the CURLOPT_SSL_VERIFYPEER option in October 2000 and we added CURLOPT_SSL_VERIFYHOST in August 2001. All that quite a long time ago.

Then add thousands of hours, hundreds of hackers, thousands of applications, a user count that probably surpasses one billion users by now. Then also add the fact that option names are sticky in the way we write docs, examples pop up all over the internet and everyone who’s close to the project learns them by name and spirit and we quite simply grow attached to them and the way they work. Changing the name of an option is really painful and cause of a lot of confusion.

I’ve instead tried to more and more emphasize the functionality in the docs, to stress what the options do and how to do server cert verifications with curl the safe way.

I can’t force users to read docs. I can’t forbid users to blindly assume something and I’m not in control of, nor do I want to affect, the large population of third party bindings that exist for using on top of libcurl to cater for every imaginable programming language – and some of them may of course themselves have documentation problems and what not.

Would I change some of the APIs and names for options we have in libcurl if I would redo them today? Yes I would.

So what do we do about it?

I think this is the only really interesting question to take from all this. Everyone wants stable APIs. Everyone wants sensible and easy to understand APIs and as we can see they should also basically be possible to figure out without reading any documentation. And yet the API has to be powerful and flexible enough to be really useful for all those different applications.

At this point where we have these options that we do, when you’ve done your mud slinging and the finger of blame is firmly pointed at us. How exactly do you suggest we move forward to fix these claimed problems?

Taking it personally

Before anyone tells me to not take it personally: curl is my biggest hobby and a project I’ve spent many years and thousands of hours on. Of course I take it personally, otherwise I would’ve stopped working in the project a long time ago. This is personal to me. I give it my loving care and personal energy and then someone comes here and throw ill-founded and badly researched criticisms at me. I think criticizers of open source projects should learn to discuss the matters with the projects as their primary way instead of using it to make their conference presentations become more feisty.

Daladevelop hackathon

Monday, September 15th, 2014

On Saturday the 13th of September, I took part in a hackathon in Falun Sweden organized by Daladevelop.

20-something hacker enthusiasts gathered in a rather large and comfortable room in this place, an almost three hour drive from my home. A number of talks and lectures were held through the day and the difficulty level ranged from newbie to more advanced. My own contribution was a talk about curl followed by one about HTTP/2. Blabbermouth as I am, I exhausted the friendly audience by talking a good total of almost 90 minutes straight. I got a whole range of clever and educated questions and I think and hope we all had a good time as a result.

The organizers ran a quiz for two-person teams. I teamed up with Andreas Olsson in team Emacs, and after having identified x86 assembly, written binary, spotted perl, named Ada Lovelace, used the term lightfoot and provided about 15 more answers we managed to get first prize and the honor of having beaten the others. Great fun!

Video perhaps?

Monday, September 8th, 2014

I decided to try to do a short video about my current work right now and make it available for you all. I try to keep it short (5-7 minutes) and I’m certainly no pro at it, but I will try to make a weekly one for a while and see if it gets any fun. I’m going to read your comments and responses to this very eagerly and that will help me decide how I will proceed on this experiment.


Going to FOSDEM 2015

Wednesday, August 27th, 2014


I’m going there and I know several friends are going too, so this is just my way of pointing this out to the ones of you who still haven’t made up your mind! There’s still a lot of time left as this event is taking place late January next year.

I intend to try to get a talk to present this time and I would love to meet up with more curl contributors and fans.