CVE-2021-30475: AOMedia libaom Buffer Overflow Flaw

CVE-2021-30475 Overview

CVE-2021-30475 is a critical buffer overflow vulnerability discovered in the aom_dsp/noise_model.c file within the libaom library, developed by the Alliance for Open Media (AOMedia). This vulnerability affects versions of libaom released before March 24, 2021, and could allow attackers to corrupt memory, potentially leading to remote code execution or denial of service conditions.

The libaom library is a widely-used codec implementation for the AV1 video format, employed in various applications including web browsers, video streaming services, and media players. Due to its broad adoption, this vulnerability has significant implications for systems processing untrusted video content.

Critical Impact

This buffer overflow vulnerability in libaom's noise model processing could allow attackers to execute arbitrary code or crash applications by providing maliciously crafted media content.

Affected Products

• AOMedia libaom (versions before 2021-03-24)
• Fedora 34
• Debian-based Linux distributions (addressed in DSA-5490)

Discovery Timeline

• 2021-06-04 - CVE-2021-30475 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-30475

Vulnerability Analysis

The vulnerability exists in the noise model component of the libaom library, specifically within the aom_dsp/noise_model.c source file. This component is responsible for processing noise models during video encoding and decoding operations.

The buffer overflow occurs when the code fails to properly validate buffer boundaries during noise model processing. When handling specially crafted input, the vulnerable code can write data beyond allocated buffer boundaries, corrupting adjacent memory regions.

This vulnerability is classified as CWE-120 (Buffer Copy without Checking Size of Input), which represents a classic buffer overflow pattern where data is copied to a buffer without adequate size verification.

Root Cause

The root cause of CVE-2021-30475 is insufficient bounds checking in the noise model processing code. The vulnerable code path does not properly validate the size of input data before copying it to a fixed-size buffer, allowing an attacker to supply data that exceeds the buffer's capacity.

Buffer overflows of this nature typically arise when:

• Array indices are not validated against array bounds
• Memory copy operations lack proper length checks
• Input data length is assumed rather than verified

Attack Vector

The attack vector for this vulnerability is network-based, meaning exploitation can occur when a victim processes malicious media content delivered over a network connection.

An attacker could exploit this vulnerability by:

1. Crafting a malicious AV1-encoded media file containing specially constructed noise model data
2. Delivering the malicious content to a target through various means (web pages, streaming services, file downloads)
3. When the target application uses libaom to decode the content, the buffer overflow is triggered
4. Successful exploitation could result in arbitrary code execution with the privileges of the vulnerable application, or cause the application to crash

The vulnerability requires no user interaction beyond processing the malicious content, and no privileges are required for exploitation, making it particularly dangerous in automated media processing pipelines.

Technical details and the specific code changes addressing this vulnerability can be found in the AOMedia Code Changes commit.

Detection Methods for CVE-2021-30475

Indicators of Compromise

• Unexpected crashes in applications using libaom when processing media files
• Memory corruption errors or segmentation faults during AV1 video decoding
• Abnormal process behavior following media file processing
• Core dumps indicating buffer overflows in libaom-related functions

Detection Strategies

• Monitor application logs for crash reports involving libaom or AV1 decoding components
• Implement memory protection mechanisms such as ASLR and stack canaries to detect exploitation attempts
• Deploy endpoint detection solutions capable of identifying memory corruption exploitation patterns
• Use static analysis tools to identify applications linking to vulnerable libaom versions

Monitoring Recommendations

• Enable crash reporting and analysis for applications processing media content
• Monitor for unusual network traffic patterns associated with media file delivery
• Implement file integrity monitoring on systems with libaom installations
• Review application logs for repeated crashes that may indicate exploitation attempts

How to Mitigate CVE-2021-30475

Immediate Actions Required

• Update libaom to a version released after March 24, 2021
• Apply vendor-provided security patches for affected Linux distributions
• Restrict processing of untrusted media content on vulnerable systems
• Consider disabling AV1 codec support in applications until patches are applied

Patch Information

AOMedia has addressed this vulnerability in commit 12adc723acf02633595a4d8da8345742729f46c0. The fix implements proper bounds checking in the noise model processing code to prevent buffer overflow conditions.

Distribution-specific patches are available:

• Debian DSA-5490 Security Update
• Debian LTS Security Notice
• Fedora Package Announcement
• Gentoo GLSA 202401-32

Workarounds

• Implement input validation to filter potentially malicious media files before processing
• Use sandboxing or containerization for applications that process untrusted media content
• Deploy application-level firewalls to inspect media content before it reaches vulnerable decoders
• Enable memory protection features (ASLR, DEP/NX) on systems running vulnerable applications

# Check installed libaom version on Debian/Ubuntu systems
dpkg -l | grep libaom

# Update libaom on Debian/Ubuntu
sudo apt update && sudo apt upgrade libaom0

# Check installed libaom version on Fedora
rpm -qa | grep libaom

# Update libaom on Fedora
sudo dnf update libaom