CVE-2015-6761 in Chrome
Summary
by MITRE
The update_dimensions function in libavcodec/vp8.c in FFmpeg through 2.8.1, as used in Google Chrome before 46.0.2490.71 and other products, relies on a coefficient-partition count during multi-threaded operation, which allows remote attackers to cause a denial of service (race condition and memory corruption) or possibly have unspecified other impact via a crafted WebM file.
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Analysis
by VulDB Data Team • 06/22/2022
The vulnerability identified as CVE-2015-6761 represents a critical race condition flaw within the FFmpeg multimedia framework's VP8 video codec implementation. This issue specifically affects the update_dimensions function located in libavcodec/vp8.c and impacts FFmpeg versions through 2.8.1. The vulnerability's significance stems from its exploitation potential in multi-threaded environments where concurrent access to shared resources creates dangerous conditions. The flaw manifests when processing WebM video files, which utilize the VP8 codec, making it particularly dangerous in web browsers and multimedia applications that rely on FFmpeg for video decoding operations. This vulnerability affects Google Chrome versions prior to 46.0.2490.71, demonstrating how widely spread this issue was across major software platforms that depend on FFmpeg for multimedia processing.
The technical root cause of this vulnerability lies in the improper handling of coefficient-partition counts during multi-threaded VP8 decoding operations. When FFmpeg processes WebM files containing VP8 video streams, the update_dimensions function attempts to modify dimension parameters while multiple threads may be simultaneously accessing or modifying the same memory locations. This race condition creates a scenario where thread synchronization mechanisms fail to properly protect shared data structures, leading to inconsistent state management and memory corruption. The coefficient-partition count mechanism, which is crucial for VP8's entropy decoding process, becomes corrupted when accessed concurrently by multiple threads without proper locking mechanisms. This fundamental flaw in concurrent programming directly violates established software engineering principles for multi-threaded applications and creates a pathway for attackers to manipulate memory contents through carefully crafted input files.
The operational impact of CVE-2015-6761 extends beyond simple denial of service to potentially enable more severe consequences including arbitrary code execution. Remote attackers can craft malicious WebM files that trigger the race condition during video decoding, causing the application to crash or behave unpredictably. In some cases, the memory corruption could be leveraged to execute arbitrary code with the privileges of the affected application, potentially leading to full system compromise. The vulnerability's exploitation requires minimal user interaction since it can be triggered through automatic playback of malicious media content in web browsers or multimedia players. The widespread adoption of FFmpeg in various software products means that numerous applications and platforms were potentially vulnerable to this attack vector, making it particularly dangerous in enterprise and consumer environments where media playback is common.
Mitigation strategies for CVE-2015-6761 require immediate software updates and patches to address the underlying race condition in FFmpeg's VP8 implementation. Organizations should prioritize updating their FFmpeg installations to versions that contain proper thread synchronization mechanisms and fix the coefficient-partition count handling during multi-threaded operations. Browser vendors like Google addressed this issue in Chrome version 46.0.2490.71, and similar updates were implemented across other affected software products. Additionally, implementing input validation and sanitization measures for WebM files can provide an additional layer of protection. Security professionals should consider deploying network-based intrusion detection systems that can identify and block malicious WebM files, while also implementing strict content filtering policies for multimedia content in enterprise environments. The vulnerability serves as a reminder of the critical importance of proper concurrent programming practices and the need for thorough testing of multi-threaded applications in security-sensitive contexts. This issue aligns with CWE-362, which specifically addresses race conditions in concurrent programming, and represents a classic example of how improper synchronization can lead to severe security implications. The ATT&CK framework categorizes this vulnerability under the T1059.007 technique for exploitation of multimedia processing components, highlighting its relevance in modern cyber attack vectors targeting application vulnerabilities in media processing frameworks.