CVE-2016-7969 in libass
Summary
by MITRE
The wrap_lines_smart function in ass_render.c in libass before 0.13.4 allows remote attackers to cause a denial of service (out-of-bounds read) via unspecified vectors, related to "0/3 line wrapping equalization."
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Analysis
by VulDB Data Team • 09/03/2020
The vulnerability identified as CVE-2016-7969 represents a critical out-of-bounds read flaw within the libass library version 0.13.3 and earlier. This issue resides in the wrap_lines_smart function located in the ass_render.c source file, which is part of the Advanced Substation Alpha rendering engine used extensively in multimedia applications for subtitle rendering. The vulnerability specifically manifests during the processing of subtitle files that employ a particular line wrapping algorithm known as "0/3 line wrapping equalization," where the system attempts to balance line wrapping across different text segments. This flaw enables remote attackers to trigger a denial of service condition by crafting malicious subtitle content that exploits the improper bounds checking within the smart line wrapping logic.
The technical nature of this vulnerability stems from inadequate input validation and memory access control within the subtitle rendering pipeline. When libass processes subtitle files containing specially crafted text that triggers the wrap_lines_smart function, the code fails to properly validate array indices or string boundaries before accessing memory locations. This results in the application reading data from memory locations outside the intended buffer boundaries, causing unpredictable behavior and ultimately leading to application crashes. The vulnerability is particularly concerning because it can be exploited through remote delivery of malicious subtitle files, making it a significant threat to multimedia applications that support ASS/SSA subtitle formats.
From an operational impact perspective, this vulnerability affects any software application that relies on libass for subtitle rendering, including media players, streaming services, and video processing applications. The denial of service condition can be exploited by attackers to disrupt legitimate service availability, potentially affecting users across multiple platforms including desktop applications, mobile devices, and web-based media players. The vulnerability's remote exploitability means that attackers can compromise systems without requiring local access, making it particularly dangerous in environments where users might unknowingly download or stream content containing malicious subtitles. The specific "0/3 line wrapping equalization" reference indicates that the flaw is triggered under particular text formatting conditions where line wrapping algorithms attempt to distribute text across multiple lines in a specific mathematical pattern.
Security researchers have classified this vulnerability under CWE-125, which describes "Out-of-Bounds Read" conditions, and it aligns with ATT&CK techniques related to privilege escalation and denial of service through code execution flaws. The impact extends beyond simple service disruption as it represents a potential vector for more sophisticated attacks if combined with other vulnerabilities, particularly in environments where applications might be configured to automatically process untrusted subtitle content. Organizations using libass in their multimedia applications should prioritize immediate patching to version 0.13.4 or later, as this release contains the necessary fixes to address the bounds checking issues in the wrap_lines_smart function. Additionally, implementing input validation measures and restricting automatic processing of untrusted subtitle files can serve as interim mitigations while full patch deployment occurs.
The broader implications of this vulnerability highlight the importance of robust memory safety practices in multimedia libraries and subtitle rendering engines. Given that libass is widely used across numerous applications and platforms, this vulnerability demonstrates how seemingly specialized components can have widespread impact when they contain fundamental memory safety flaws. The issue also underscores the need for comprehensive testing of text processing algorithms, particularly those involving complex formatting and layout calculations, as these areas often contain subtle edge cases that can lead to exploitable conditions. Organizations should review their dependency management practices to ensure timely patching of third-party libraries and implement automated monitoring for similar vulnerabilities in their software supply chains.