Heap-overflow in http.c results Remote Code Execution

[Paul Ch]

Summary of the bug:

We managed to find and successfully exploit critical security bug in libavformat/http.c that results Remote Code Execution in latest version of FFmpeg.

Prerequisites:

Attacker has to reproduce SSRF bug and send HTTP request to his remote host. There are multiple ways of doing this already described all over the Internet. (For ex. you can trigger SSRF using HLS playlists or the way I described earlier in this ​ticket ).

Overview:

libavformat/http.c supports different types of HTTP responses, one of them is "Transfer-Encoding: chunked". Because ​int64_t type is used to store size of the chunk and ​strtoll function to parse the value of next chunk size it is possible to pass negative value and it will be successfully stored in chunksize variable. Later FFMIN ​function is used to determine final size of chunk and now it turns negative and it is passed to ​http_buf_read function. Inside http_buf_read function our negative value is ​assigned to len variable and passed as argument to ​memcpy which results a crash.

Steps to reproduce:

I am attaching PoC file that reproduces the simple crash. Steps to reproduce bug:

$ python http_poc.py &
$ ffmpeg -v trace -i "http://localhost:12345/1.avi" 1.avi
$ gdb -q ffmpeg_g 
(gdb) r -v trace -i "http://localhost:12345/1.avi" 1.avi

I am also attaching gdb stack-trace.

Recommended fix:

The best fix for that should be completely rewrite http parser, because some sneaky bugs still can be found there. Good example of http parser can be found ​here, it is implemented with finite-state machine.

But for now simple quick fix could be making chunksize unsigned long long.

Final words:

Our team is planning to release full exploit and blogpost with full details on exploitation in 30 days or as soon as you patch vulnerability.

Contact me if you need more details on vulnerability.

[Paul Ch]

PoC

[Paul Ch]

gdb stack trace

[James]

For future reference, please report vulnerabilities you find to ​ffmpeg-security@ffmpeg.org as mentioned in https://ffmpeg.org/security.html instead of posting them on this bug tracker.

[James]

Replying to paulch:

Contact me if you need more details on vulnerability.

Sending this information to the above email address nonetheless would also be appreciated.

[Carl Eugen Hoyos]

Replying to paulch:

But for now simple quick fix could be making chunksize unsigned long long.

I believe all valid bug reports on this bug tracker are very much appreciated, no matter if security relevant or not but I don't understand why you didn't send a patch with this change to the development mailing list.

[Paul Ch]

Thanks for your feedback. All future bug reports will be submitted to ​ffmpeg-security@ffmpeg.org .

Patches have been already proposed by FFmpeg developers and reviewed by our team.
Thank you.

[Carl Eugen Hoyos]

Fixed by Ronald in 2a05c8f813de6f2278827734bf8102291e7484aa.