Another wget reference was Bourne

wget-is-not-a-crimeBack in 2013, it came to light that Wget was used to to copy the files private Manning was convicted for having leaked. Around that time, EFF made and distributed stickers saying wget is not a crime.

Weirdly enough, it was hard to find a high resolution version of that image today but I’m showing you a version of it on the right side here.

In the 2016 movie Jason Bourne, Swedish actress Alicia Vikander is seen working on her laptop at around 1:16:30 into the movie and there’s a single visible sticker on that laptop. Yeps, it is for sure the same EFF sticker. There’s even a very brief glimpse of the top of the red EFF dot below the “crime” word.


Also recall the wget occurance in The Social Network.

curl up in Nuremberg!

I’m very happy to announce that the curl project is about to run our first ever curl meeting and developers conference.

March 18-19, Nuremberg Germany

Everyone interested in curl, libcurl and related matters is invited to participate. We only ask of you to register and pay the small fee. The fee will be used for food and more at the event.

You’ll find the full and detailed description of the event and the specific location in the curl wiki.

The agenda for the weekend is purposely kept loose to allow for flexibility and unconference-style adding things and topics while there. You will thus have the chance to present what you like and affect what others present. Do tell us what you’d like to talk about or hear others talk about! The sign-up for the event isn’t open yet, as we first need to work out some more details.

We have a dedicated mailing list for discussing the meeting, called curl-meet, so please consider yourself invited to join in there as well!

Thanks a lot to SUSE for hosting!

Feel free to help us make a cool logo for the event!


(The 19th birthday of curl is suitably enough the day after, on March 20.)

a single byte write opened a root execution exploit

Thursday, September 22nd 2016. An email popped up in my inbox.

Subject: ares_create_query OOB write

As one of the maintainers of the c-ares project I’m receiving mails for suspected security problems in c-ares and this was such a one. In this case, the email with said subject came from an individual who had reported a ChromeOS exploit to Google.

It turned out that this particular c-ares flaw was one important step in a sequence of necessary procedures that when followed could let the user execute code on ChromeOS from JavaScript – as the root user. I suspect that is pretty much the worst possible exploit of ChromeOS that can be done. I presume the reporter will get a fair amount of bug bounty reward for this.

The setup and explanation on how this was accomplished is very complicated and I am deeply impressed by how this was figured out, tracked down and eventually exploited in a repeatable fashion. But bear with me. Here comes a very simplified explanation on how a single byte buffer overwrite with a fixed value could end up aiding running exploit code as root.

The main Google bug for this problem is still not open since they still have pending mitigations to perform, but since the c-ares issue has been fixed I’ve been told that it is fine to talk about this publicly.

c-ares writes a 1 outside its buffer

c-ares has a function called ares_create_query. It was added in 1.10 (released in May 2013) as an updated version of the older function ares_mkquery. This detail is mostly interesting because Google uses an older version than 1.10 of c-ares so in their case the flaw is in the old function. This is the two functions that contain the problem we’re discussing today. It used to be in the ares_mkquery function but was moved over to ares_create_query a few years ago (and the new function got an additional argument). The code was mostly unchanged in the move so the bug was just carried over. This bug was actually already present in the original ares project that I forked and created c-ares from, back in October 2003. It just took this long for someone to figure it out and report it!

I won’t bore you with exactly what these functions do, but we can stick to the simple fact that they take a name string as input, allocate a memory area for the outgoing packet with DNS protocol data and return that newly allocated memory area and its length.

Due to a logic mistake in the function, you could trick the function to allocate a too short buffer by passing in a string with an escaped trailing dot. An input string like “one.two.three\.” would then cause the allocated memory area to be one byte too small and the last byte would be written outside of the allocated memory area. A buffer overflow if you want. The single byte written outside of the memory area is most commonly a 1 due to how the DNS protocol data is laid out in that packet.

This flaw was given the name CVE-2016-5180 and was fixed and announced to the world in the end of September 2016 when c-ares 1.12.0 shipped. The actual commit that fixed it is here.

What to do with a 1?

Ok, so a function can be made to write a single byte to the value of 1 outside of its allocated buffer. How do you turn that into your advantage?

The Redhat security team deemed this problem to be of “Moderate security impact” so they clearly do not think you can do a lot of harm with it. But behold, with the right amount of imagination and luck you certainly can!

Back to ChromeOS we go.

First, we need to know that ChromeOS runs an internal HTTP proxy which is very liberal in what it accepts – this is the software that uses c-ares. This proxy is a key component that the attacker needed to tickle really badly. So by figuring out how you can send the correctly crafted request to the proxy, it would send the right string to c-ares and write a 1 outside its heap buffer.

ChromeOS uses dlmalloc for managing the heap memory. Each time the program allocates memory, it will get a pointer back to the request memory region, and dlmalloc will put a small header of its own just before that memory region for its own purpose. If you ask for N bytes with malloc, dlmalloc will use ( header size + N ) and return the pointer to the N bytes the application asked for. Like this:


With a series of cleverly crafted HTTP requests of various sizes to the proxy, the attacker managed to create a hole of freed memory where he then reliably makes the c-ares allocated memory to end up. He knows exactly how the ChromeOS dlmalloc system works and its best-fit allocator, how big the c-ares malloc will be and thus where the overwritten 1 will end up. When the byte 1 is written after the memory, it is written into the header of the next memory chunk handled by dlmalloc:


The specific byte of that following dlmalloc header that it writes to, is used for flags and the lowest bits of size of that allocated chunk of memory.

Writing 1 to that byte clears 2 flags, sets one flag and clears the lowest bits of the chunk size. The important flag it sets is called prev_inuse and is used by dlmalloc to tell if it can merge adjacent areas on free. (so, if the value 1 simply had been a 2 instead, this flaw could not have been exploited this way!)

When the c-ares buffer that had overflowed is then freed again, dlmalloc gets fooled into consolidating that buffer with the subsequent one in memory (since it had toggled that bit) and thus the larger piece of assumed-to-be-free memory is partly still being in use. Open for manipulations!


Using that memory buffer mess

This freed memory area whose end part is actually still being used opened up the play-field for more “fun”. With doing another creative HTTP request, that memory block would be allocated and used to store new data into.

The attacker managed to insert the right data in that further end of the data block, the one that was still used by another part of the program, mostly since the proxy pretty much allowed anything to get crammed into the request. The attacker managed to put his own code to execute in there and after a few more steps he ran whatever he wanted as root. Well, the user would have to get tricked into running a particular JavaScript but still…

I cannot even imagine how long time it must have taken to make this exploit and how much work and sweat that were spent. The report I read on this was 37 very detailed pages. And it was one of the best things I’ve read in a long while! When this goes public in the future, I hope at least parts of that description will become available for you as well.

A lesson to take away from this?

No matter how limited or harmless a flaw may appear at a first glance, it can serve a malicious purpose and serve as one little step in a long chain of events to attack a system. And there are skilled people out there, ready to figure out all the necessary steps.

Update: A detailed write-up about this flaw (pretty much the report I refer to above) by the researcher who found it was posted on Google’s Project Zero blog on December 14:
Chrome OS exploit: one byte overflow and symlinks.

poll on mac 10.12 is broken

When Mac OS X first launched they did so without an existing poll function. They later added poll() in Mac OS X 10.3, but we quickly discovered that it was broken (it returned a non-zero value when asked to wait for nothing) so in the curl project we added a check in configure for that and subsequently avoided using poll() in all OS X versions to and including Mac OS 10.8 (Darwin 12). The code would instead switch to the alternative solution based on select() for these platforms.

With the release of Mac OS X 10.9 “Mavericks” in October 2013, Apple had fixed their poll() implementation and we’ve built libcurl to use it since with no issues at all. The configure script picks the correct underlying function to use.

Enter macOS 10.12 (yeah, its not called OS X anymore) “Sierra”, released in September 2016. Quickly we discovered that poll() once against did not act like it should and we are back to disabling the use of it in preference to the backup solution using select().

The new error looks similar to the old problem: when there’s nothing to wait for and we ask poll() to wait N milliseconds, the 10.12 version of poll() returns immediately without waiting. Causing busy-loops. The problem has been reported to Apple and its Radar number is 28372390. (There has been no news from them on how they plan to act on this.)

poll() is defined by POSIX and The Single Unix Specification it specifically says:

If none of the defined events have occurred on any selected file descriptor, poll() waits at least timeout milliseconds for an event to occur on any of the selected file descriptors.

We pushed a configure check for this in curl, to be part of the upcoming 7.51.0 release. I’ll also show you a small snippet you can use stand-alone below.

Apple is hardly alone in the broken-poll department. Remember how Windows’ WSApoll is broken?

Here’s a little code snippet that can detect the 10.12 breakage:

#include <poll.h>
#include <stdio.h>
#include <sys/time.h>

int main(void)
  struct timeval before, after;
  int rc;
  size_t us;

  gettimeofday(&before, NULL);
  rc = poll(NULL, 0, 500);
  gettimeofday(&after, NULL);

  us = (after.tv_sec - before.tv_sec) * 1000000 +
    (after.tv_usec - before.tv_usec);

  if(us < 400000) {
    puts("poll() is broken");
    return 1;
  else {
    puts("poll() works");
  return 0;

Follow-up, January 2017

This poll bug has been confirmed fixed in the macOS 10.12.2 update (released on December 13, 2016), but I’ve found no official mention or statement about this fact.

screenshotted curl credits

If you have more or better screenshots, please share!


This shot is taken from the ending sequence of the PC version of the game Grand Theft Auto V. 44 minutes in! See the youtube version.


Sky HD is a satellite TV box.


This is a Philips TV. The added use of c-ares I  consider a bonus!


The infotainment display of a BMW car.


Playstation 4 lists open source products it uses.


This is a screenshot from an Iphone open source license view. The iOS 10 screen however, looks like this:


curl in iOS 10 with an older year span than in the much older screenshot?

Instagram credits screenshot

Instagram on an Iphone.

Spotify credits screenshot

Spotify on an Iphone.


Virtualbox (thanks to Anders Nilsson)

curl-battle-net (thanks Anders Nilsson)


Freebox (thanks Alexis La Goutte)


The Youtube app on Android. (Thanks Ray Satiro)


The Youtube app on iOS (Thanks Anthony Bryan)


UBReader is an ebook reader app on Android.

MindMaple is using curl (Thanks to Peter Buyze)

license screen from a VW Sharan car (Thanks to Jonas Lejon)

Skype on Android

Skype on an iPad

Nissan Qashqai 2016 (thanks to Peteski)

The Mercedes Benz license agreement from 2015 listing which car models that include curl.

Nintendo Switch uses curl (Thanks to Anders Nilsson)

The Thermomix TM 5 kitchen/cooking appliance (Thanks to Sergio Conde)

Cisco Anyconnect (Thanks to Dane Knecht) – notice the age of the curl copyright string in comparison to the main one!

Sony Android TV (Thanks to Sajal Kayan)

The reMarkable E-paper tablet uses curl. (Thanks to Zakx)

BMW i3, snapshot from this video (Thanks to Terence Eden)

BMW i8. (Thanks to eeeebbbbrrrr)

Amazon Kindle Paperwhite 3 (thanks to M Hasbini)

Xiaomi Android uses both curl and libcurl. (Thanks to Björn Stenberg)

Verisure V box microenhet smart lock runs curl (Thanks to Jonas Lejon)

curl in a Subaru (Thanks to Jani Tarvainen)

Another VW (Thanks to Michael Topal)

Oppo Android uses curl (Thanks to Dio Oktarianos Putra)

Chevrolet Traverse 2018 uses curl according to its owners manual on page 403. It is mentioned almost identically in other Chevrolet model manuals such as for the Corvette, the 2018 Camaro, the 2018 TRAX, the 2013 VOLT, the 2014 Express and the 2017 BOLT.

The curl license is also in owner manuals for other brands and models such as in the GMC Savana, Cadillac CT6 2016, Opel Zafira, Opel Insignia, Opel Astra, Opel Karl, Opel Cascada, Opel Mokka, Opel Ampera, Vauxhall Astra … (See 100 million cars run curl).

The Onkyo TX-NR609 AV-Receiver uses libcurl as shown by the license in its manual. (Thanks to Marc Hörsken)

Fortnite uses libcurl. (Thanks to Neil McKeown)

Red Dead Redemption 2 uses libcurl. The ending sequence video. (Thanks to @nadbmal)

Philips Hue Lights uses libcurl (Thanks to Lorenzo Fontana)

Pioneer makes Blu-Ray players that use libcurl. (Thanks to Maarten Tijhof)