INTRODUCTION ============ According to QuickTime's specification, The sample description atom (STSD) stores information that allows QuickTime to decode samples in the media. It has the following structure: 0 DWORD Size 4 DWORD Type 8 BYTE Version 9 BYTE[3] FLAGS 12 DWORD Number of entries 16 DWORD Sample description table The structure of each entry in the sample description table varies by the media type, however the first four fields are the same for all media types: 0 DWORD Sample description size 4 DWORD Data format 6 BYTE[6] Reserved 12 WORD Data reference index These four fields may be followed by additional data specific to the media type and data format. For video media, the general sample description format is extended by the following structure: 14 WORD Version 16 WORD Revision level 18 DWORD Vendor 22 DWORD Temporal quality 26 DWORD Spatial quality 30 WORD Width 32 WORD Height 34 DWORD Horizontal resolution 38 DWORD Vertical resolution 42 DWORD Data size 46 WORD Frame count 48 BYTE[32] Compressor name 80 WORD Depth 82 WORD Color table ID VULNERABILITY DETAILS ==================== When the data format field (offset 4 of the sample description table extension) is 'RVZA' (Apple Video), it is possible to trigger a sign extension vulnerability which leads to a buffer underflow. The following is the faulty sign extended MOV: MOVSX ECX,WORD PTR SS:[ESP+4C] [ESP+4C] contains a user controlled input, which is equal to "((width+(4-width%4))*4 & 0xFFFF" where 'width' is taken from the RVZA sample description entry (offset 30). If width >= 0x5FFD, then [ESP+4C] >= 0x8000. Sign-extending such values results in very large unsigned values, as their most significant word becomes 0xFFFF (so 0x8000 is sign-extended to 0xFFFF8000). Deeper in the code, the user controllable sign-extended value is treated as the size of a structure. A vector of this structure is walked over: [1] At each iteration the base pointer is incremented by the user's controlled sign-extended value. This means that it is possible to force the pointer to reference memory regions below the vector's VA: ADD EAX,EDX ; EAX = vector, EDX = sign extended value [2] At each iteration values are written to an element in the vector (a single structure) which is referenced by the incremented pointer. This means that it is possible to write to memory regions below the buffer's VA. MOV DWORD PTR DS:[EAX],EBX MOV DWORD PTR DS:[EAX+4],EBX MOV DWORD PTR DS:[EAX+4],EBX MOV DWORD PTR DS:[EAX],EBX IMPACT ====== By writing to memory regions below the buffer's VA, An attacker may overwrite crucial data such as function pointers, flags, heap structures and so forth. Doing so may allow an attacker to alter the normal control flow of the application and execute arbitrary code. A simple attack vector would be to lure the victim to browse to a web site controlled by the attacker, which serves a malicious QuickTime file that exploits this vulnerability. TEST ENVIRONMENT ================ Windows XP Service Pack 3 QuickTime 7.6 (472) REMEDIATION =========== A new version of QuickTime (7.6.2) has been released in order to address this issue. IDENTIFIERS ========== 1. CVE-ID: CVE-2009-0955 2. BID: 35166 REFERENCES =========== 1. Apple's advisory: http://support.apple.com/kb/HT3591 2. The original blog post: http://roeehay.blogspot.com/2009/06/apple-quicktime-image-description-atom.html