CVE-2026-40962: FFmpeg Buffer Overflow Vulnerability

CVE-2026-40962 Overview

CVE-2026-40962 is an integer overflow vulnerability in FFmpeg before version 8.1 that leads to an out-of-bounds write condition. The vulnerability exists in the CENC (Common Encryption) subsample data handling within libavformat/mov.c. When processing specially crafted media files with malformed CENC subsample data, an integer overflow can occur during size calculations, resulting in insufficient buffer allocation and subsequent memory corruption.

Critical Impact

Successful exploitation could allow an attacker to corrupt memory through out-of-bounds writes, potentially leading to arbitrary code execution or application crashes when processing malicious media files.

Affected Products

• FFmpeg versions prior to 8.1
• Applications and systems integrating vulnerable FFmpeg libraries
• Media processing pipelines utilizing affected FFmpeg components

Discovery Timeline

• 2026-04-16 - CVE CVE-2026-40962 published to NVD
• 2026-04-16 - Last updated in NVD database

Technical Details for CVE-2026-40962

Vulnerability Analysis

This vulnerability stems from improper handling of integer arithmetic when processing CENC (Common Encryption) subsample data in FFmpeg's MOV/MP4 demuxer. CENC is a standard encryption scheme used for encrypted media content, and subsample data defines which portions of a sample are encrypted versus clear.

The vulnerability is classified under CWE-190 (Integer Overflow or Wraparound). When parsing subsample metadata from a crafted media file, the affected code in libavformat/mov.c performs arithmetic operations on user-controlled values without adequate overflow checks. This can cause the computed buffer size to wrap around to a smaller value than intended, leading to an undersized memory allocation.

Subsequent write operations to this buffer then exceed its boundaries, resulting in an out-of-bounds write condition. This type of memory corruption can potentially be leveraged for code execution, though exploitation requires local access and user interaction to open a malicious file.

Root Cause

The root cause is insufficient validation of integer arithmetic operations when calculating buffer sizes for CENC subsample data. The code fails to properly check for integer overflow conditions before memory allocation, allowing attackers to trigger a scenario where the allocated buffer is smaller than the data being written to it.

Attack Vector

The attack requires local access to the target system. An attacker must craft a malicious media file containing specially constructed CENC subsample data designed to trigger the integer overflow. The victim must then open or process this file using FFmpeg or an application that incorporates the vulnerable FFmpeg library.

The attack scenario typically involves:

1. Attacker creates a malformed MOV/MP4 file with crafted CENC subsample entries
2. The malicious file is delivered to the victim through various means (email attachment, download, etc.)
3. When the victim processes the file with FFmpeg, the integer overflow occurs during subsample parsing
4. The resulting out-of-bounds write corrupts adjacent memory regions

Detection Methods for CVE-2026-40962

Indicators of Compromise

• Unexpected crashes or segmentation faults in FFmpeg or dependent applications when processing media files
• Abnormal memory consumption patterns during media file parsing
• Core dumps or crash reports indicating memory corruption in libavformat/mov.c
• Presence of suspicious media files with unusual CENC subsample structures

Detection Strategies

• Deploy file integrity monitoring on systems processing untrusted media content
• Implement memory sanitizer tools (ASan, MSan) in development and testing environments to detect out-of-bounds writes
• Monitor application logs for FFmpeg parsing errors or unexpected terminations
• Use static analysis tools to scan for potentially malicious media files before processing

Monitoring Recommendations

• Enable verbose logging for media processing pipelines to capture parsing anomalies
• Implement crash reporting and analysis for applications using FFmpeg libraries
• Monitor system resource usage for unusual patterns during media file operations
• Configure alerting for repeated application crashes associated with media processing

How to Mitigate CVE-2026-40962

Immediate Actions Required

• Upgrade FFmpeg to version 8.1 or later where the vulnerability has been patched
• Audit systems and applications to identify all instances of vulnerable FFmpeg versions
• Implement input validation and sandboxing for media processing workflows
• Restrict processing of untrusted media files until patches are applied

Patch Information

The FFmpeg development team has addressed this vulnerability through a fix that adds proper integer overflow checks during CENC subsample data processing. The patch ensures that size calculations are validated before memory allocation to prevent wraparound conditions.

For detailed patch information, refer to the FFmpeg Pull Request #22348 which contains the security fix.

Organizations should update to FFmpeg version 8.1 or later to receive this fix.

Workarounds

• Implement strict input validation to reject media files with suspicious CENC subsample configurations
• Run FFmpeg processes in sandboxed environments with limited privileges to contain potential exploitation
• Disable CENC decryption support if not required for your use case
• Use application-level controls to restrict processing of untrusted media sources

If immediate patching is not possible, consider implementing additional security controls around media processing:

# Run FFmpeg in a restricted environment using firejail (Linux)
firejail --private --net=none ffmpeg -i input.mp4 output.mp4

# Use seccomp-bpf filtering to limit system calls
# Configure resource limits to contain potential exploitation
ulimit -v 1048576  # Limit virtual memory to 1GB
ulimit -t 60       # Limit CPU time to 60 seconds