CVE-2021-38291: FFmpeg DOS Vulnerability

CVE-2021-38291 Overview

CVE-2021-38291 is a Denial of Service vulnerability affecting FFmpeg, the widely-used multimedia processing framework. The vulnerability exists in src/libavutil/mathematics.c where an assertion failure can be triggered by a remote attacker, causing the application to crash. This flaw allows attackers to disrupt services that rely on FFmpeg for media processing without requiring authentication or user interaction.

Critical Impact

Remote attackers can cause a complete denial of service condition in FFmpeg-based applications by triggering an assertion failure, potentially disrupting media processing pipelines and services.

Affected Products

• FFmpeg (multiple versions)
• Debian Linux 9.0
• Debian Linux 10.0
• Debian Linux 11.0

Discovery Timeline

• 2021-08-12 - CVE-2021-38291 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-38291

Vulnerability Analysis

This vulnerability is classified as CWE-617 (Reachable Assertion), where an assertion failure in the mathematics.c source file can be triggered during media processing operations. The flaw allows network-based attacks without requiring privileges or user interaction, making it particularly dangerous for internet-facing media processing services. When exploited, the assertion failure causes immediate application termination, resulting in complete service disruption.

The vulnerability affects the mathematical utility functions within FFmpeg's libavutil library, which handles core arithmetic operations used throughout the multimedia framework. When specific input conditions are met during media processing, the assertion check fails, causing an unrecoverable crash.

Root Cause

The root cause is an improperly handled assertion in src/libavutil/mathematics.c. The code contains a reachable assertion that can be triggered through crafted input, violating the assumption that certain conditions will always be true during normal execution. This assertion was intended for debugging purposes but remains active in production builds, creating an exploitable denial of service vector.

Attack Vector

The attack can be executed remotely over the network without authentication. An attacker can craft malicious media files or streams that, when processed by FFmpeg, trigger the assertion failure in the mathematics library. This attack requires no privileges and no user interaction beyond the target system processing the malicious input.

The exploitation path typically involves:

1. Crafting a media file with specific parameters that trigger the vulnerable code path
2. Submitting the malicious file to an FFmpeg-based service
3. The assertion failure occurs during processing, crashing the application

Since no verified code examples are available, readers should refer to the FFmpeg Ticket #9312 for detailed technical information about the assertion failure conditions.

Detection Methods for CVE-2021-38291

Indicators of Compromise

• FFmpeg process crashes with assertion failure messages referencing libavutil/mathematics.c
• Repeated crashes in media processing services handling user-supplied content
• Log entries showing abnormal termination of FFmpeg processes
• Core dumps generated from FFmpeg or dependent applications

Detection Strategies

• Monitor for process termination events involving FFmpeg with abnormal exit codes
• Implement application crash monitoring for services using FFmpeg libraries
• Deploy file integrity monitoring to detect unusual media files in processing queues
• Use SentinelOne's behavioral detection to identify repeated crash patterns indicative of exploitation attempts

Monitoring Recommendations

• Enable detailed logging for FFmpeg-based applications to capture assertion failure messages
• Configure alerting for abnormal FFmpeg process terminations
• Monitor system stability metrics for services processing user-supplied media
• Implement rate limiting on media processing endpoints to mitigate rapid exploitation attempts

How to Mitigate CVE-2021-38291

Immediate Actions Required

• Update FFmpeg to the latest patched version available for your distribution
• Apply security updates from Debian if running affected Debian Linux versions (9.0, 10.0, 11.0)
• Review and restrict access to media processing services that accept untrusted input
• Implement input validation and sandboxing for FFmpeg processing operations

Patch Information

Security patches are available through multiple distribution channels. Organizations should prioritize applying these updates:

• Debian Security Advisory DSA-4990 - Provides patched FFmpeg packages for Debian
• Debian Security Advisory DSA-4998 - Additional security updates
• Debian LTS Announcement - Long-term support updates
• Gentoo GLSA 202312-14 - Gentoo Linux security advisory

The original issue is tracked in FFmpeg Ticket #9312.

Workarounds

• Isolate FFmpeg processing in sandboxed environments to limit crash impact
• Implement process supervision to automatically restart crashed FFmpeg instances
• Limit input sources to trusted origins until patches can be applied
• Consider using alternative processing pipelines for high-availability requirements

# Example: Configure process supervision for FFmpeg services
# Using systemd to automatically restart crashed services
[Service]
Restart=always
RestartSec=5
# Limit restart attempts to prevent rapid crash loops
StartLimitBurst=10
StartLimitIntervalSec=60