CVE-2016-7972 in libass
Summary
by MITRE
The check_allocations function in libass/ass_shaper.c in libass before 0.13.4 allows remote attackers to cause a denial of service (memory allocation failure) via unspecified vectors.
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Analysis
by VulDB Data Team • 09/03/2020
The vulnerability identified as CVE-2016-7972 resides within the libass library, a critical component for subtitle rendering in multimedia applications including video players and streaming platforms. This library serves as the backbone for Advanced Substation Alpha subtitle processing and is widely integrated into popular media frameworks such as VLC, MPlayer, and various web-based video players. The specific flaw exists in the check_allocations function located in the ass_shaper.c source file, which governs memory allocation operations during subtitle text shaping and rendering processes. The vulnerability represents a denial of service condition that can be triggered remotely, making it particularly dangerous in networked environments where users might encounter maliciously crafted subtitle files from untrusted sources.
The technical nature of this vulnerability stems from inadequate input validation and memory management within the subtitle processing pipeline. When libass encounters subtitle data containing malformed or excessively complex allocation requests, the check_allocations function fails to properly handle memory allocation failures or excessive memory demands. This allows attackers to craft specially designed subtitle files that trigger cascading memory allocation requests, potentially exhausting system resources or causing allocation failures that result in application crashes or complete system hang conditions. The unspecified vectors suggest that multiple input conditions can trigger this behavior, making the attack surface broader than typical memory corruption vulnerabilities. This flaw operates at the intersection of memory management and input processing, creating a scenario where legitimate subtitle rendering operations can be disrupted through carefully crafted malicious inputs.
The operational impact of CVE-2016-7972 extends beyond simple application crashes to potentially affect entire media streaming ecosystems. Media players that rely on libass for subtitle rendering become vulnerable to remote exploitation, allowing attackers to disrupt service availability for legitimate users. This vulnerability is particularly concerning in environments where automated media processing or streaming services are deployed, as it could enable attackers to create widespread disruption by simply providing malicious subtitle files to unsuspecting users. The memory allocation failure can manifest in various ways including complete application termination, unresponsive interfaces, or resource exhaustion that impacts system performance. The vulnerability affects systems where libass versions prior to 0.13.4 are deployed, which includes numerous legacy systems and applications that may not have received timely updates.
Mitigation strategies for this vulnerability primarily focus on immediate version updates to libass 0.13.4 or later, which contain the necessary patches to address the memory allocation handling issues. System administrators should prioritize updating all affected media applications that utilize libass, particularly those handling user-provided content or streaming from untrusted sources. Additional protective measures include implementing strict input validation for subtitle files, deploying sandboxing mechanisms for subtitle processing, and monitoring for unusual memory allocation patterns that might indicate exploitation attempts. Organizations should also consider implementing network segmentation to limit exposure and ensure that media processing systems are properly isolated from critical network infrastructure. The vulnerability aligns with CWE-400, which addresses unchecked resource allocation, and represents a typical example of how improper resource management in multimedia libraries can create denial of service conditions that impact availability of critical services. From an ATT&CK perspective, this vulnerability maps to the T1499.004 technique related to network denial of service, where the attack vector leverages legitimate application functionality to create service disruption through resource exhaustion.