CVE-2019-9721 in FFmpeg
Summary
by MITRE
A denial of service in the subtitle decoder in FFmpeg 4.1 allows attackers to hog the CPU via a crafted video file in Matroska format, because handle_open_brace 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 identified as CVE-2019-9721 represents a critical denial of service flaw within the FFmpeg multimedia framework version 4.1. This issue specifically targets the subtitle decoding functionality, particularly affecting Matroska format video files. The vulnerability stems from an insecure parsing mechanism in the htmlsubtitles.c component where the handle_open_brace function processes input through sscanf with a complex format argument. This design flaw creates an opportunity for attackers to craft malicious video files that can trigger excessive CPU consumption during subtitle processing. The attack vector is particularly concerning because it leverages legitimate video file formats that are commonly processed by multimedia applications, making it difficult to distinguish between benign and malicious content at runtime.
The technical implementation of this vulnerability resides in the improper handling of format strings within the sscanf function call, which creates a path for attackers to manipulate the parsing logic through carefully crafted input data. When FFmpeg encounters a maliciously formatted Matroska file containing specially constructed subtitle data, the handle_open_brace function executes with a format string that can cause the parsing routine to enter an infinite loop or consume excessive computational resources. This behavior directly maps to CWE-121, which describes heap-based buffer overflow conditions, and CWE-122, which covers buffer overflow vulnerabilities in heap-based data structures. The vulnerability essentially allows an attacker to craft input that causes the application to consume disproportionate CPU cycles, leading to system performance degradation or complete system unresponsiveness.
From an operational perspective, this vulnerability poses significant risks to multimedia processing systems, streaming platforms, and content management solutions that rely on FFmpeg for video processing. The impact extends beyond simple resource exhaustion, as it can affect service availability for legitimate users and potentially enable more sophisticated attacks when combined with other vulnerabilities. The attack requires minimal privileges and can be executed through standard video file manipulation, making it particularly dangerous in environments where automated processing of user-uploaded content occurs. Organizations using FFmpeg for video transcoding, streaming, or content delivery services face potential disruption of their services, as attackers can cause sustained CPU utilization that may lead to denial of service conditions affecting entire systems or applications.
The mitigation strategy for CVE-2019-9721 involves immediate patching of FFmpeg installations to version 4.1.1 or later, which contains the necessary fixes to address the vulnerable sscanf implementation. System administrators should also implement input validation and sanitization measures for video files, particularly those in Matroska format, to detect and reject potentially malicious content before processing. Additionally, deploying rate limiting and resource monitoring mechanisms can help detect and prevent exploitation attempts. The vulnerability aligns with ATT&CK technique T1499.001, which covers network denial of service attacks, and T1059.007, which addresses command and scripting interpreters through the potential for attackers to leverage the processing capabilities of multimedia frameworks. Organizations should also consider implementing sandboxing mechanisms for video processing to isolate potential exploitation attempts and prevent escalation to broader system compromise.