Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Date: Tue, 22 Jul 2014 16:27:16 -0600
From: "Don A. Bailey" <>
Subject: Re: LMS-2014-06-16-3: Libav LZO

Hello All,

For fun, Lab Mouse has released a web application that allows users to
encode their own videos with an LZO exploit payload. Upload the video file
and shellcode of your choice to the app, choose the target, and it will
auto-encode your shellcode into the exploit video. Videos are then
presented for download within a zip file.

Source code for the payload generator and the ROP chains is included on the

Don A. Bailey
Founder / CEO
Lab Mouse Security
@InfoSecMouse <>

On Thu, Jun 26, 2014 at 12:54 PM, Don A. Bailey <>

> Hello All,
> A vulnerability has been identified in the Libav LZO implementation.
> Please find the bug report attached inline.
> Best,
> Don A. Bailey
> Founder / CEO
> Lab Mouse Security
> #############################################################################
> #
> # Lab Mouse Security Report
> # LMS-2014-06-16-3
> #
> Report ID: LMS-2014-06-16-3
> CVE ID: CVE-2014-4609
> Researcher Name: Don A. Bailey
> Researcher Organization: Lab Mouse Security
> Researcher Email: donb at
> Researcher Website:
> Vulnerability Status: Patched
> Vulnerability Embargo: Broken
> Vulnerability Class: Integer Overflow
> Vulnerability Effect: Memory Corruption
> Vulnerability Impact: DoS, OOW, RCE
> Vulnerability DoS Practicality: Practical
> Vulnerability OOW Practicality: Practical
> Vulnerability RCE Practicality: Practical
> Vulnerability Criticality: Critical
> Vulnerability Scope:
> All versions of libav are affected.
> All architectures supported by libav are affected.
> Vulnerability Tested:
> Yes. RCE proven on 10 separate platforms including but not limited to:
>  - Ubuntu and Mint x86, x86_64
>  - Debian x86_64, x86
>  - FreeBSD x86_64, x86
> Functions Affected:
> 	libavutil/lzo.c:av_lzo1x_decode
> Criticality Reasoning
> ---------------------
> This vulnerability can be triggered through a compression payload embedded
> in a video file. Due to the nature of this memory corruption vulnerability,
> exploitation of the bug can be seamless and work in the background during
> normal video playback. A user will never notice that playback has been
> compromised.
> Testing was successfully performed on all variants of mplayer2, including
> gecko-mplayer2 embedded in Firefox, Iceweasel, Opera, Chromium, and Konqueror
> on Linux.
> Ease of compromise is partly due to libav's use of tmalloc, which places
> a header containing function pointers at the beginning of allocated heap
> regions. Exploitation of the compression vulnerability overwrites these
> function pointers, which then point to ROP payloads that allow for the
> bypassing of ASLR and NX security enhancements.
> Vulnerability Description
> -------------------------
> An integer overflow can occur when processing any variant of a "literal run"
> in the av_lzo1x_decode function. Each of these three locations is
> subject to an integer overflow when processing zero bytes. The following code
> depicts how the size of the literal array is generated:
> static inline int get_len(LZOContext *c, int x, int mask)
> {
>     int cnt = x & mask;
>     if (!cnt) {
>         while (!(x = get_byte(c)))
>             cnt += 255;
>         cnt += mask + x;
>     }
>     return cnt;
> }
> As long as a zero byte (0x00) is encountered, the variable 'cnt' will be
> incremented by 255. Using approximately sixteen megabytes of zeros, 'cnt' will
> accumulate to a maximum unsigned integer value in the 32bit variable.
> Therefore, get_len() will return a negative 'cnt' value to its caller. The
> checks in copy_backptr() will fail to properly test for negative 'cnt' values
> resulting in the following test never catching an error:
>     if (cnt > c->out_end - dst) {
>         cnt       = FFMAX(c->out_end - dst, 0);
>         c->error |= AV_LZO_OUTPUT_FULL;
>     }
> av_memcpy_backptr does not check for negative 'cnt' values, which results in
> a copy of one byte from 'src' to 'dst', evading a crash do to excessive
> copying.
> Finally, the copy function will never crash by calling memcpy with a negative
> value because it only calls memcpy when the signed 'cnt' variable is greater
> than zero. However, the pointers 'c->in' and 'c->out' will still be adjusted
> by a negative value, causing 'c->out' to point to an area of memory prior to
> the actual output buffer. This is how Lab Mouse Security was able to
> instrument this vulnerability to overwrite tmalloc function pointers with
> ROP payloads.
> It is notable that since the count value 'cnt' is passed around as an 'int',
> it will always be interpreted as a signed 32bit integer regardless of the
> underlying architecture. This means that this vulnerability affects all
> platforms and architectures regardless of whether they are 32bit or 64bit
> in nature.
> Vulnerability Resolution
> ------------------------
> Resolving this issue requires several separate fixes.
> 1) lzo.c:get_len()
> The return value of get_len must be evaluated for negative count values.
> A negative value should never be allowed in this context. Always error
> when a negative or zero value is returned.
> 2) lzo.c:copy()
> A negative value should not be allowed as a parameter to copy(). In
> addition, the pointers 'c->in' and 'c->out' should be tested after they
> are changed by the count value. Verify that the new offset does not land
> outside of the bounds of the 'out' buffer.
> 3) lzo:copy_backptr()
> Do not allow a negative 'cnt' value to be passed to copy_backptr. Augment
> the test cases to ensure that a negative value cannot be used to adjust
> the 'c->out' pointer.
> 4) libavutil/mem.c:av_memcpy_backptr
> Return an error value.
> Do not allow a negative 'cnt' or 'back' value to be used.
> 5) Always use a size_t for any size variable.
> Size variables should always represent the underlying architecture's largest
> natural unsigned integer. Use size_t, or a variant, to automatically scale
> the value to the underlying architecture.

Powered by blists - more mailing lists

Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.