CVE-2019-9718 in FFmpeg
Summary
by MITRE
In FFmpeg 4.1, a denial of service in the subtitle decoder allows attackers to hog the CPU via a crafted video file in Matroska format, because ff_htmlmarkup_to_ass in libavcodec/htmlsubtitles.c has a complex format argument to sscanf.
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Analysis
by VulDB Data Team • 07/31/2023
The vulnerability CVE-2019-9718 represents a critical denial of service flaw affecting FFmpeg version 4.1, specifically within the subtitle decoding functionality of Matroska format files. This issue stems from the ff_htmlmarkup_to_ass function located in the libavcodec/htmlsubtitles.c source file, where the implementation suffers from improper handling of complex format arguments passed to the sscanf function. The flaw manifests when attackers craft malicious video files that contain specially constructed subtitle data, enabling them to exploit a computational complexity issue that results in excessive cpu resource consumption.
The technical root cause of this vulnerability lies in the insecure use of sscanf function with complex format strings that can trigger excessive computational overhead during subtitle processing. When FFmpeg encounters a crafted Matroska file containing malicious HTML markup in subtitles, the ff_htmlmarkup_to_ass function processes these inputs without adequate input validation or resource limiting measures. This leads to a scenario where the parsing routine becomes trapped in computationally expensive operations that consume disproportionate cpu cycles relative to the input size, effectively creating a resource exhaustion condition that can be exploited to cause system instability or complete denial of service.
From an operational perspective, this vulnerability presents significant risks to systems that process multimedia content, particularly those handling user-uploaded videos or streaming platforms that rely on FFmpeg for content transcoding and subtitle processing. The attack vector requires minimal privileges as it operates through normal file processing workflows, making it particularly dangerous for automated systems or services that continuously process media files. The impact extends beyond simple service disruption to potentially enable more sophisticated attacks if combined with other vulnerabilities, as the sustained cpu consumption can be leveraged to create resource exhaustion conditions that affect system availability.
The vulnerability maps to CWE-770, which describes allocation of resources without limits or throttling, and aligns with ATT&CK technique T1499.004 for resource exhaustion attacks. Organizations using FFmpeg for media processing should prioritize immediate patching to version 4.1.1 or later, where the issue has been addressed through improved input validation and computational complexity limiting in the subtitle parsing routines. Additional mitigations include implementing resource limits on processing time and cpu usage for media transcoding operations, employing sandboxing techniques for untrusted media files, and deploying input validation mechanisms that can detect and reject malformed subtitle data before it reaches the vulnerable parsing functions. Network-based solutions should also consider implementing content filtering to prevent the processing of known malicious media files that exploit this vulnerability.