CVE-2020-21041 in FFmpeg
Summary
by MITRE • 05/25/2021
Buffer Overflow vulnerability exists in FFmpeg 4.1 via apng_do_inverse_blend in libavcodec/pngenc.c, which could let a remote malicious user cause a Denial of Service
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Analysis
by VulDB Data Team • 06/26/2026
The buffer overflow vulnerability identified as CVE-2020-21041 resides within FFmpeg version 4.1's implementation of the Animated Portable Network Graphics (APNG) encoding functionality. This flaw specifically manifests in the apng_do_inverse_blend function located in the libavcodec/pngenc.c source file, representing a critical security weakness that can be exploited by remote attackers to disrupt system operations. The vulnerability stems from insufficient input validation and memory boundary checking during the processing of APNG image sequences, where maliciously crafted APNG files can trigger unexpected memory access patterns that exceed allocated buffer boundaries.
The technical exploitation of this buffer overflow occurs when FFmpeg processes APNG files containing specially constructed image data that causes the apng_do_inverse_blend function to write beyond the intended memory allocation. This condition creates a situation where attacker-controlled data can overwrite adjacent memory locations, potentially leading to arbitrary code execution or system instability. The vulnerability operates at the intersection of multiple security domains including software security, memory management, and multimedia processing protocols, making it particularly dangerous in environments where FFmpeg is used to process untrusted media content. According to CWE classification, this represents a classic buffer overflow vulnerability categorized under CWE-121, which specifically addresses heap-based buffer overflow conditions where insufficient bounds checking allows memory corruption.
The operational impact of CVE-2020-21041 extends beyond simple denial of service scenarios to encompass potential system compromise and service disruption across various platforms where FFmpeg is deployed. Remote attackers can leverage this vulnerability through web applications, media processing services, or any system that utilizes FFmpeg for APNG handling without proper input sanitization. The attack surface is particularly broad given FFmpeg's widespread adoption in content management systems, streaming platforms, and multimedia applications that process user-uploaded media files. This vulnerability directly aligns with ATT&CK technique T1203, which covers "Exploitation for Client Execution" and T1499, addressing "Endpoint Denial of Service" through software exploitation methods. Systems utilizing FFmpeg for automated media processing or content ingestion workflows face heightened risk as the vulnerability can be triggered through legitimate file processing operations without explicit user interaction.
Mitigation strategies for CVE-2020-21041 require immediate implementation of FFmpeg version updates to 4.2 or later, where the buffer overflow has been addressed through proper memory boundary validation and input sanitization. Organizations should also implement input validation controls at application layers that process APNG files, including file format verification and size limitation mechanisms before FFmpeg processing occurs. Network-based defenses can include content filtering solutions that scan media files for known malicious patterns or suspicious APNG structures, though these measures provide only partial protection. Additionally, system administrators should conduct comprehensive vulnerability assessments to identify all systems running affected FFmpeg versions and establish monitoring protocols to detect potential exploitation attempts. The remediation approach aligns with security best practices outlined in NIST SP 800-128 and ISO/IEC 27001 frameworks, emphasizing the importance of timely patch management and proactive vulnerability remediation.