CVE-2021-38091 in FFmpeg
Summary
by MITRE • 09/20/2021
Integer Overflow vulnerability in function filter16_sobel in libavfilter/vf_convolution.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 integer overflow vulnerability identified as CVE-2021-38091 resides within the ffmpeg media processing library, specifically in the filter16_sobel function located in libavfilter/vf_convolution.c. This flaw manifests when processing video frames through convolution filters that utilize 16-bit signed integers for calculations. The vulnerability stems from insufficient input validation and boundary checking during the computation of convolution kernel operations that are critical for image processing tasks such as edge detection and noise reduction. The flaw allows attackers to craft specially malformed video streams or filter parameters that trigger integer overflow conditions during the execution of convolution operations.
The technical exploitation of this vulnerability occurs when the filter16_sobel function processes convolution kernels with dimensions or coefficients that exceed the maximum representable value for 16-bit signed integers, which is 32767. When arithmetic operations involving these values exceed this boundary, the integer overflow results in unexpected behavior where large positive values wrap around to negative values or zero, disrupting the normal flow of the convolution algorithm. This condition can occur during the initialization or execution phases of filter processing when kernel sizes, weights, or other parameters are improperly validated. The vulnerability is particularly concerning because it can be triggered through legitimate media processing operations, making it difficult to distinguish between normal usage and malicious input.
The operational impact of CVE-2021-38091 extends beyond simple denial of service to potentially enable more sophisticated attack vectors. While the primary effect is a denial of service condition that causes ffmpeg processes to crash or become unresponsive, the integer overflow can also lead to memory corruption or arbitrary code execution in certain scenarios. Attackers can leverage this vulnerability by submitting crafted video files or filter parameters that cause the convolution filter to overflow, potentially crashing the application or consuming excessive system resources. The vulnerability affects all versions of ffmpeg up to and including 4.2.1, making it a significant concern for systems that process user-uploaded media content or utilize ffmpeg as part of their media processing pipeline. This issue is particularly relevant in web applications, content management systems, and media processing services where ffmpeg is commonly deployed.
Mitigation strategies for CVE-2021-38091 should focus on both immediate patching and operational hardening measures. The most effective solution is to upgrade to ffmpeg version 4.3 or later where the integer overflow has been addressed through proper input validation and boundary checking in the convolution filter implementation. Organizations should also implement input sanitization measures that validate all convolution kernel parameters before processing, including checking for reasonable kernel sizes, coefficients, and dimensions that cannot cause integer overflow conditions. Additionally, deployment configurations should include resource limits and process isolation to prevent a single vulnerable operation from consuming all available system resources or causing cascading failures. The vulnerability aligns with CWE-190, which specifically addresses integer overflow conditions, and can be mapped to ATT&CK technique T1203, which covers execution through manipulation of system resources. Organizations should also consider implementing automated monitoring and alerting systems to detect unusual resource consumption patterns that may indicate exploitation attempts against this vulnerability.