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NGS00068 Technical Advisory - LibAVCodec AMV Out of Array Write
From: "Research () NGSSecure" <research () ngssecure com>
Date: Mon, 1 Aug 2011 08:42:00 +0000

=======
Summary
=======
Name: LibAVCodec AMV Out of Array Write
Release Date:  31 July 2011
Reference: NGS00068
Discoverer: Dominic Chell <dominic.chell () ngssecure com>
Vendor: VideoLAN
Vendor Reference: CVE-2011-1931
Systems Affected: VLC media player 1.1.9 and earlier releases 
Risk: High
Status: Published

========
TimeLine
========
Discovered: 31 March 2011
Released: 31 March 2011
Approved: 31 March 2011
Reported: 21 April 2011
Fixed: 21 April 2011
Published: 31 July 2011

===========
Description
===========
Dominic Chell of NGS Secure has discovered a high risk vulnerability in LibAVCodec. Opening a malformed AMV file can 
result in an out of array write and potentially arbitrary code execution when using this library. Whilst the 
vulnerability may affect multiple applications that use this library, it was only tested on VLC media player.

=================
Technical Details
=================
(b80.d80): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception 
handling.
This exception may be expected and handled.
eax=ffffff60 ebx=11186110 ecx=0e420ec0 edx=fffffe20 esi=00000100
edi=a2a6c008
eip=0ad3e272 esp=0ef8fa08 ebp=0e607070 iopl=0         nv up ei pl nz na pe
nc
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000            
efl=00010206
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Program 
Files\VideoLAN\VLC\plugins\libavcodec_plugin.dll -
libavcodec_plugin!vlc_entry__1_1_0g+0x33cef2:
0ad3e272 0f7f2443        movq    mmword ptr [ebx+eax*2],mm4
ds:0023:11185fd0=????????????????
Missing image name, possible paged-out or corrupt data.
Missing image name, possible paged-out or corrupt data.
Missing image name, possible paged-out or corrupt data.
0:008> !exploitable
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Program 
Files\VideoLAN\VLC\libvlccore.dll -
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\WINDOWS\system32\msvcrt.dll - 


Exploitability Classification: EXPLOITABLE Recommended Bug Title: Exploitable - User Mode Write AV starting at
libavcodec_plugin!vlc_entry__1_1_0g+0x33cef2 (Hash=0x64744c60.0x724a4f4e)

User mode write access violations that are not near NULL are exploitable.

diff --git a/libavcodec/sp5xdec.c b/libavcodec/sp5xdec.c index 8bcdbe4..dd31eda 100644 (file)
--- a/libavcodec/sp5xdec.c
+++ b/libavcodec/sp5xdec.c
@@ -86,7 +86,6 @@ static int sp5x_decode_frame(AVCodecContext *avctx,
     recoded[j++] = 0xFF;
     recoded[j++] = 0xD9;
 
-    avctx->flags &= ~CODEC_FLAG_EMU_EDGE;
     av_init_packet(&avpkt_recoded);
     avpkt_recoded.data = recoded;
     avpkt_recoded.size = j;
@@ -121,6 +120,6 @@ AVCodec ff_amv_decoder = {
     NULL,
     ff_mjpeg_decode_end,
     sp5x_decode_frame,
-    CODEC_CAP_DR1,
+    0,
     .long_name = NULL_IF_CONFIG_SMALL("AMV Video"),  };

===============
Fix Information
===============
http://git.videolan.org/?p=ffmpeg.git;a=commit;h=89f903b3d5ec38c9c5d90fba7e626fa0eda61a32

NGS Secure Research
http://www.ngssecure.com


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