CVE-2016-7970 in libass
Summary
by MITRE
Buffer overflow in the calc_coeff function in libass/ass_blur.c in libass before 0.13.4 allows remote attackers to cause a denial of service via unspecified vectors.
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Analysis
by VulDB Data Team • 09/03/2020
The vulnerability identified as CVE-2016-7970 represents a critical buffer overflow condition within the libass library, specifically within the calc_coeff function located in the ass_blur.c source file. This library serves as a core component for subtitle rendering in multimedia applications, particularly those utilizing the Advanced SubStation Alpha subtitle format. The flaw exists in versions prior to 0.13.4 and manifests as an insufficient bounds checking mechanism that fails to properly validate input data when processing blur effects in subtitle rendering operations. Attackers can exploit this vulnerability through carefully crafted subtitle files or media content that triggers the problematic calculation path, leading to memory corruption that ultimately results in application termination.
The technical implementation of this buffer overflow stems from improper handling of coefficient calculations during the blur effect processing phase. When libass encounters subtitle data containing specific blur parameters, the calc_coeff function does not adequately verify the size or bounds of the data structures it operates on, allowing malicious input to overwrite adjacent memory locations. This condition aligns with CWE-121, which describes stack-based buffer overflow scenarios, and represents a classic example of insufficient input validation in multimedia processing libraries. The vulnerability operates at the intersection of multimedia processing and memory safety, where legitimate rendering operations become vectors for exploitation through malformed input data.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it can be leveraged by remote attackers to disrupt multimedia applications that depend on libass for subtitle rendering. This includes popular media players such as VLC, MPlayer, and various streaming applications that utilize the library for advanced subtitle effects. The exploitation requires minimal privileges and can be executed through standard media content delivery channels, making it particularly dangerous in environments where users may encounter untrusted media files. The vulnerability affects a wide range of applications across different platforms and operating systems, as libass is widely integrated into multimedia ecosystems and supports multiple audiovisual formats that commonly employ subtitle effects.
Mitigation strategies for CVE-2016-7970 primarily focus on immediate version updates to libass 0.13.4 or later, which contain the necessary patches to address the buffer overflow condition. System administrators should prioritize patching affected applications that utilize libass, particularly those handling untrusted media content or subtitles from external sources. Additional defensive measures include implementing strict input validation for subtitle files, deploying sandboxing mechanisms for media processing, and utilizing network-based filtering to restrict potentially malicious content. Organizations should also consider implementing monitoring solutions to detect unusual application behavior or termination patterns that may indicate exploitation attempts. From an ATT&CK framework perspective, this vulnerability maps to technique T1059.007 for execution through multimedia applications and T1499.004 for denial of service impacts, making it a significant concern for both enterprise security and consumer multimedia applications.