CVE-2021-30123: FFmpeg Buffer Overflow RCE Vulnerability

CVE-2021-30123 Overview

CVE-2021-30123 is a buffer overflow vulnerability affecting FFmpeg versions 4.3 and earlier. The flaw exists within libavcodec, FFmpeg's core audio/video codec library. When processing a specially crafted media file, the vulnerable code fails to properly validate input boundaries, resulting in a classic buffer overflow condition. Successful exploitation of this vulnerability could allow an attacker to execute arbitrary code on the target system with the privileges of the user running FFmpeg.

Critical Impact

This buffer overflow vulnerability in FFmpeg's libavcodec library can be exploited remotely through malicious media files, potentially leading to complete system compromise via remote code execution.

Affected Products

• FFmpeg version 4.3 and earlier
• FFmpeg version 4.4 (based on CPE data)
• Applications and services utilizing vulnerable FFmpeg/libavcodec libraries

Discovery Timeline

• April 7, 2021 - CVE-2021-30123 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-30123

Vulnerability Analysis

This vulnerability is classified as CWE-120 (Buffer Copy without Checking Size of Input), commonly known as a classic buffer overflow. The flaw resides in FFmpeg's libavcodec component, which is responsible for encoding and decoding audio and video streams. When the library processes a maliciously crafted media file, insufficient bounds checking allows data to be written beyond the allocated buffer boundaries.

The attack requires user interaction—specifically, a victim must process a malicious file using an application that leverages the vulnerable FFmpeg library. This could occur through various scenarios including opening a video file, streaming media content, or processing multimedia through automated pipelines.

Root Cause

The root cause of CVE-2021-30123 is improper input validation in the libavcodec component. When handling certain media file structures, the codec library fails to verify that input data fits within the expected buffer size before performing copy operations. This allows an attacker to craft a media file with oversized or malformed data fields that overflow the destination buffer when processed.

Attack Vector

The attack vector for this vulnerability is network-based, requiring user interaction. An attacker can exploit this vulnerability by:

1. Crafting a malicious media file containing specially formatted data designed to trigger the buffer overflow
2. Distributing the malicious file through various channels (websites, email attachments, file sharing)
3. Waiting for a victim to process the file using an application built with the vulnerable FFmpeg library
4. Upon processing, the overflow occurs, potentially allowing the attacker to overwrite adjacent memory and execute arbitrary code

The vulnerability is particularly concerning for web services that process user-uploaded media, video transcoding pipelines, and multimedia applications that automatically process media files.

For technical details on the specific fix, refer to the FFmpeg Commit Diff which addresses the bounds checking issue.

Detection Methods for CVE-2021-30123

Indicators of Compromise

• Unexpected crashes or segmentation faults in FFmpeg or applications using libavcodec
• Abnormal memory consumption patterns during media file processing
• Process execution anomalies following media file operations
• Unusual child processes spawned from FFmpeg-dependent applications

Detection Strategies

• Monitor application logs for segmentation faults or memory access violations related to FFmpeg processes
• Implement file integrity monitoring for known malicious media file signatures
• Deploy endpoint detection and response (EDR) solutions capable of detecting memory corruption exploitation
• Utilize behavioral analysis to identify anomalous code execution patterns following media file processing

Monitoring Recommendations

• Enable detailed logging for applications utilizing FFmpeg libraries
• Monitor network traffic for suspicious media file downloads, particularly from untrusted sources
• Implement application-level monitoring for FFmpeg processes and their resource utilization
• Track version information of FFmpeg installations across the environment to identify vulnerable instances

How to Mitigate CVE-2021-30123

Immediate Actions Required

• Update FFmpeg to a patched version that addresses CVE-2021-30123
• Audit all applications and services utilizing FFmpeg/libavcodec for vulnerable versions
• Restrict processing of media files from untrusted sources until patching is complete
• Implement network segmentation to limit the impact of potential exploitation

Patch Information

FFmpeg has released patches to address this vulnerability. Organizations should update to the latest stable release that includes the security fix. The specific commit addressing this issue is referenced in the FFmpeg Commit Diff. Additionally, Gentoo Linux users should apply the security update detailed in Gentoo GLSA 2021-24.

For detailed bug information, refer to FFmpeg Ticket #8845 and FFmpeg Ticket #8863.

Workarounds

• Implement strict input validation and file type verification before processing media files
• Run FFmpeg processes in sandboxed environments with limited privileges
• Disable or restrict unused codec support in FFmpeg configurations to reduce attack surface
• Deploy application-level firewalls to filter potentially malicious media files before they reach FFmpeg

# Configuration example - Running FFmpeg with reduced privileges
# Create a dedicated low-privilege user for FFmpeg processing
useradd -r -s /bin/false ffmpeg-processor

# Run FFmpeg with the restricted user
sudo -u ffmpeg-processor ffmpeg -i input.mp4 -c:v libx264 output.mp4

# Consider using containerization for additional isolation
docker run --rm -v /path/to/files:/data:ro ffmpeg-container -i /data/input.mp4 output.mp4