CVE-2020-20895 in FFmpeg
Summary
by MITRE • 09/20/2021
Buffer Overflow vulnerability in function filter_vertically_##name in libavfilter/vf_avgblur.c in Ffmpeg 4.2.1, allows attackers to cause a Denial of Service or other unspecified impacts.
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Analysis
by VulDB Data Team • 09/29/2021
The CVE-2020-20895 vulnerability represents a critical buffer overflow condition within the FFmpeg multimedia framework version 4.2.1, specifically within the vertical filtering function in the libavfilter/vf_avgblur.c module. This flaw manifests when processing video content through the average blur filter, where improper bounds checking allows maliciously crafted input data to exceed allocated memory buffers. The vulnerability stems from inadequate validation of input parameters that control the vertical filtering operations, creating a scenario where attacker-controlled data can overwrite adjacent memory regions. Such buffer overflow conditions typically arise when developers fail to properly validate the size or range of input values before using them in memory allocation or manipulation operations, a pattern commonly classified under CWE-121. The affected function filter_vertically_##name processes video frames through a vertical blur algorithm that requires precise handling of pixel data and filter coefficients, making it particularly susceptible to memory corruption when input parameters are manipulated.
The operational impact of this vulnerability extends beyond simple denial of service, as buffer overflows in multimedia processing libraries can potentially enable more sophisticated attack vectors. When exploited, the vulnerability can cause the FFmpeg application to crash or behave unpredictably, leading to service disruption for applications relying on this library for video processing. However, the potential for remote code execution cannot be entirely ruled out, particularly if the overflow affects critical memory structures or if the application continues execution after the overflow occurs. The attack surface is broad since FFmpeg is widely integrated into numerous applications including media servers, content management systems, and streaming platforms. The vulnerability demonstrates a classic example of how multimedia processing libraries can become attack vectors when input validation is insufficient, as the filter operations are commonly used in video editing, transcoding, and streaming applications. This makes the impact particularly severe in environments where FFmpeg is used to process untrusted video content from external sources.
Mitigation strategies for CVE-2020-20895 must address both immediate remediation and long-term architectural improvements to prevent similar vulnerabilities. The primary solution involves upgrading to FFmpeg version 4.3 or later, where the buffer overflow has been corrected through proper bounds checking and input validation mechanisms. System administrators should prioritize patching affected installations, particularly those processing untrusted video content, as the vulnerability can be exploited remotely through media processing pipelines. Additionally, implementing input sanitization measures at the application level can provide defense-in-depth protection, including validating video file parameters and limiting the size of processed media files. Network segmentation and access controls should be enforced to limit exposure of systems running FFmpeg to untrusted inputs, aligning with ATT&CK technique T1059.007 for command and scripting interpreter usage. Organizations should also consider implementing memory protection mechanisms such as stack canaries and address space layout randomization to mitigate potential exploitation. The vulnerability highlights the importance of comprehensive security testing for multimedia libraries and demonstrates how seemingly benign filter operations can become critical attack surfaces when proper input validation is absent. Regular security assessments of multimedia processing pipelines and adherence to secure coding practices, particularly regarding buffer management and input validation, are essential for preventing similar issues in future implementations.