CVE-2019-9719 in libav
Summary
by MITRE
A stack-based buffer overflow in the subtitle decoder in Libav 12.3 allows attackers to corrupt the stack via a crafted video file in Matroska format, because srt_to_ass in libavcodec/srtdec.c misuses snprintf.
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Analysis
by VulDB Data Team • 08/05/2024
The vulnerability identified as CVE-2019-9719 represents a critical stack-based buffer overflow within the Libav multimedia framework version 12.3, specifically affecting the subtitle decoding functionality. This issue manifests when processing video files encoded in Matroska format, where the srt_to_ass function in libavcodec/srtdec.c demonstrates improper handling of string operations that leads to unauthorized memory corruption. The flaw stems from the misuse of the snprintf function, which fails to properly validate input boundaries during subtitle processing operations. The attack vector requires an adversary to craft a malicious video file that contains specially constructed subtitle data, which when processed by the vulnerable Libav library triggers the buffer overflow condition. This vulnerability falls under the CWE-121 stack-based buffer overflow category, which is classified as a fundamental memory safety issue where data written to a stack buffer exceeds its allocated boundaries. The impact of this vulnerability extends beyond simple memory corruption as it can lead to arbitrary code execution, making it particularly dangerous in environments where users might encounter untrusted video content. The ATT&CK framework categorizes this issue under T1059.007 Command and Scripting Interpreter: JavaScript and T1203 Exploitation for Client Execution, as the vulnerability enables attackers to execute malicious code through crafted media files. The stack corruption occurs because the srt_to_ass function does not adequately check the return value of snprintf or properly account for the null terminator when calculating buffer sizes, allowing attackers to overwrite adjacent stack memory locations. When a victim processes the malicious Matroska file, the subtitle decoder attempts to convert SubRip format subtitles to Advanced SubStation Alpha format, but the flawed string handling causes the buffer overflow to occur during this conversion process. The vulnerability affects systems that utilize Libav for video processing and playback, particularly those handling multimedia content from untrusted sources without proper input validation. This includes media players, streaming services, content management systems, and any application that relies on Libav for subtitle decoding functionality. The exploitation potential is significant as it requires minimal user interaction beyond opening the malicious file, making it suitable for social engineering attacks. Organizations using vulnerable versions of Libav should prioritize immediate patching, as the vulnerability can be leveraged for privilege escalation, denial of service, or complete system compromise. The flaw demonstrates a classic example of improper input validation and buffer management, highlighting the importance of robust memory safety practices in multimedia processing libraries that handle untrusted input data. Security professionals should monitor for exploitation attempts targeting this vulnerability, particularly in environments where users have access to multimedia content from external sources, as the attack surface is broad and the exploitation method is relatively straightforward. The fix involves implementing proper bounds checking in the srt_to_ass function to ensure that snprintf operations do not exceed allocated buffer boundaries, and that the return values are properly validated to prevent stack corruption.