CVE-2026-95127 in libass
Summary
by MITRE • 10/13/2026
An issue in libass v.0.17.5 allows a remote attacker to cause a denial of service via the ass_outline_construct() in libass/ass_render.c
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Analysis
by VulDB Data Team • 10/13/2026
The vulnerability identified in libass version 0.17.5 represents a critical security flaw within the subtitle rendering engine, specifically located within the ass_outline_construct function found in the ass_render.c source file. Libass is widely utilized by various media players and video processing applications to render Advanced SubStation Alpha (ASS) subtitles, which are complex text-based formats capable of defining detailed styling, positioning, and effects for on-screen text. The core issue stems from improper handling of input data during the construction of subtitle outlines, a process essential for rendering styled or outlined text correctly. When an attacker provides specially crafted ASS subtitle files containing malformed or excessively large outline parameters, the function fails to validate these inputs against expected bounds or resource limits before proceeding with memory allocation and processing operations. This lack of rigorous validation allows the application to enter an unstable state during the rendering pipeline.
From a technical perspective, this flaw is classified as a denial of service vulnerability resulting from improper input validation. The specific mechanism involves the ass_outline_construct function failing to adequately check for integer overflows or invalid array indices when processing outline data structures. In many implementations of such functions, if an attacker can control values that determine buffer sizes or loop iterations without proper boundary checks, it may lead to out-of-bounds memory access or excessive resource consumption. While some reports indicate this primarily leads to application crashes rather than arbitrary code execution, the instability introduced by processing maliciously crafted subtitle files disrupts the normal flow of the media player. The vulnerability exploits the trust that applications place in the structural integrity of subtitle data, assuming it adheres to standard specifications when, in reality, attackers can deviate significantly from these norms to trigger the fault condition within the rendering engine.
The operational impact of this vulnerability is significant for any software ecosystem relying on libass for video playback or processing. Media players such as VLC, MPV, and various web browsers that integrate subtitle rendering capabilities are potentially affected if they utilize vulnerable versions of the library. A successful exploitation allows a remote attacker to cause a denial of service by forcing the application to crash or become unresponsive when a victim opens or plays a maliciously crafted video file containing embedded ASS subtitles. This disrupts availability, which is one of the three pillars of information security alongside confidentiality and integrity. In scenarios where media players are used in public-facing environments or automated processing pipelines, such crashes can lead to service outages, requiring manual intervention to restart services or clear corrupted cache states, thereby increasing operational overhead and reducing system reliability.
To mitigate this risk, organizations and developers must ensure that all instances of libass are updated to version 0.17.6 or later, where the issue in ass_outline_construct has been addressed through improved input validation and boundary checks. For applications unable to update immediately, implementing a sandboxing strategy can help contain potential crashes within isolated processes rather than affecting the entire application suite. Additionally, integrating static analysis tools that detect improper memory handling patterns during development phases can prevent similar vulnerabilities from being introduced in future updates. Security teams should also monitor for patches related to subtitle parsing libraries and apply them promptly as part of routine vulnerability management procedures aligned with industry standards such as CWE-20 Improper Input Validation and ATT&CK technique T1496 Resource Hijacking, which covers denial-of-service attacks that consume system resources to degrade service availability.