CVE-2019-9720 in libavcodec
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 • 08/31/2020
The vulnerability identified as CVE-2019-9720 represents a critical stack-based buffer overflow within the subtitle decoder component of Libav version 12.3, a widely used multimedia processing library. This flaw exists specifically within the Matroska format handling capabilities, where attackers can exploit a malformed video file to trigger memory corruption. The vulnerability stems from improper handling of string operations within the srt_to_ass function located in the libavcodec/srtdec.c source file, making it a prime target for exploitation in multimedia processing applications that rely on this library for video and audio handling.
The technical implementation of this vulnerability occurs through the misuse of the snprintf function, which is designed to safely format and copy strings while preventing buffer overflows. However, in this specific implementation, the function is called with parameters that do not adequately account for the maximum buffer size, leading to a situation where data can overflow beyond the allocated stack space. This misconfiguration creates a predictable pattern where attacker-controlled input data can overwrite adjacent stack memory, potentially leading to arbitrary code execution or application crashes. The flaw is classified under CWE-121 as a stack-based buffer overflow, which is particularly dangerous because it can be leveraged to execute malicious code by overwriting return addresses or other critical stack variables.
The operational impact of CVE-2019-9720 extends beyond simple application instability, as it represents a significant security risk in environments where multimedia processing is prevalent. Attackers can craft malicious Matroska files that, when processed by vulnerable applications, will trigger the buffer overflow condition. This vulnerability affects not only the direct users of Libav but also any software that incorporates this library, including media players, streaming services, and content management systems. The exploitation vector is particularly concerning because it can be delivered through standard video files, making it difficult to detect and prevent through traditional network security measures. According to ATT&CK framework, this vulnerability maps to T1059.007 for the execution of malicious code through buffer overflow techniques and T1203 for the exploitation of software vulnerabilities in multimedia processing applications.
Mitigation strategies for CVE-2019-9720 require immediate patching of affected Libav installations to the latest versions that contain fixed implementations of the srt_to_ass function. Organizations should also implement input validation measures that sanitize all multimedia file inputs before processing, particularly focusing on subtitle data within Matroska containers. Network security controls should include content inspection mechanisms that can identify and block suspicious video files, while application-level defenses should implement stack protection mechanisms such as stack canaries and address space layout randomization. Additionally, regular security assessments of multimedia processing pipelines are essential to identify other potential vulnerabilities in the software stack that might be exploited in conjunction with this flaw, ensuring comprehensive protection against advanced persistent threats that may attempt to leverage this vulnerability for broader system compromise.