CVE-2011-3941 in FFmpeg
Summary
by MITRE
The decode_mb function in libavcodec/error_resilience.c in FFmpeg before 0.10 allows remote attackers to have an unspecified impact via vectors related to an uninitialized block index, which triggers an out-of-bounds write.
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Analysis
by VulDB Data Team • 01/11/2022
The vulnerability identified as CVE-2011-3941 represents a critical memory safety issue within the FFmpeg multimedia framework, specifically affecting the error resilience component responsible for handling video decoding operations. This flaw exists in the decode_mb function located within the libavcodec/error_resilience.c source file, where the software fails to properly initialize a block index variable during the decoding process. The vulnerability is classified under CWE-457 as an use of uninitialized variable, which creates a dangerous condition that can be exploited by remote attackers to manipulate memory access patterns. The issue stems from the improper handling of video frame data where the uninitialized block index variable can take on arbitrary values that exceed the bounds of allocated memory structures, leading to potential memory corruption scenarios.
The technical execution of this vulnerability occurs when a maliciously crafted media file is processed by FFmpeg applications, particularly those utilizing the affected library version. During the decoding of macroblocks within video frames, the decode_mb function accesses a block index variable that has not been properly initialized, causing the variable to contain garbage data from memory. When this uninitialized value is used as an array index or memory offset, it can reference memory locations outside the intended boundaries of the allocated buffer structures. This out-of-bounds write condition allows attackers to overwrite adjacent memory locations, potentially corrupting critical data structures, function pointers, or stack variables. The vulnerability is particularly dangerous because it can be triggered through remote network-based attacks, making it exploitable in web browsers, media players, and other applications that rely on FFmpeg for video processing.
The operational impact of this vulnerability extends beyond simple memory corruption, as it creates opportunities for more sophisticated attack vectors within the broader ATT&CK framework. Attackers can leverage this memory corruption to achieve arbitrary code execution by carefully crafting media files that overwrite function pointers or return addresses on the stack. The vulnerability affects a wide range of applications that depend on FFmpeg for multimedia processing, including web browsers, media players, content management systems, and streaming platforms. The unspecified impact mentioned in the CVE description reflects the potential for various outcomes depending on the memory layout and the specific values of the uninitialized variable, ranging from application crashes to full system compromise. This vulnerability demonstrates the critical importance of proper memory initialization and bounds checking in multimedia processing libraries that handle untrusted input data.
Mitigation strategies for CVE-2011-3941 require immediate patching of affected FFmpeg installations to version 0.10 or later, where the uninitialized block index variable is properly initialized before use. Organizations should implement comprehensive vulnerability management processes that include regular updates to multimedia libraries and frameworks, particularly those handling user-provided content. Additional defensive measures include deploying input validation controls that sanitize media files before processing, implementing sandboxing mechanisms for multimedia applications, and utilizing address space layout randomization to make exploitation more difficult. Network-based protections such as firewalls and content filters can help prevent malicious media files from reaching vulnerable systems, while application-level security controls should enforce strict memory access patterns and validate all buffer operations. The vulnerability underscores the importance of following secure coding practices and adheres to standards such as the CERT C Secure Coding Standard, which emphasizes proper initialization of variables and bounds checking to prevent memory safety issues that can lead to remote code execution.