CVE-2021-30473: AOMedia libaom Use-After-Free Vulnerability

CVE-2021-30473 Overview

CVE-2021-30473 is a memory corruption vulnerability discovered in aom_image.c within the libaom library, which is part of the AOMedia Video 1 (AV1) codec implementation. The vulnerability occurs when the application attempts to free memory that is not located on the heap, leading to undefined behavior and potential security implications. This improper memory management flaw (CWE-763: Release of Invalid Pointer or Reference) affects AOMedia versions before the April 7, 2021 security fix.

The libaom library is widely used for AV1 video encoding and decoding across various platforms and applications, making this vulnerability particularly significant for media processing software and web browsers that incorporate the AV1 codec.

Critical Impact

This vulnerability allows remote attackers to potentially achieve code execution or cause denial of service through network-accessible vectors without requiring authentication or user interaction.

Affected Products

• AOMedia libaom (versions before 2021-04-07 fix)
• Fedora 34 (via bundled libaom packages)
• Debian-based distributions with vulnerable libaom versions

Discovery Timeline

• 2021-05-06 - CVE-2021-30473 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-30473

Vulnerability Analysis

The vulnerability resides in the aom_image.c source file within the libaom library. The core issue involves improper memory management where the code attempts to release memory through a pointer that does not reference heap-allocated memory. This type of flaw, classified as CWE-763 (Release of Invalid Pointer or Reference), occurs when a program calls free() or similar deallocation functions on a pointer that either points to stack memory, static memory, or has already been freed.

When exploited, this vulnerability can lead to memory corruption, potentially allowing an attacker to hijack program execution flow or cause the application to crash. The impact is amplified by the fact that libaom processes untrusted media content, meaning a specially crafted AV1 video file could trigger this vulnerability.

Root Cause

The root cause of CVE-2021-30473 is an invalid memory deallocation operation in the image handling code of libaom. The aom_image.c file contains logic that incorrectly attempts to free memory that was not dynamically allocated on the heap. This could occur due to:

• Incorrect pointer arithmetic leading to invalid memory addresses being passed to deallocation functions
• Logic errors that result in stack-allocated or statically-allocated memory being treated as heap memory
• Missing validation checks before memory release operations

The fix, committed as 4efe20e99dcd9b6f8eadc8de8acc825be7416578, addresses this by ensuring proper validation of memory pointers before deallocation.

Attack Vector

The vulnerability can be exploited remotely over a network without requiring privileges or user interaction. An attacker could craft a malicious AV1 media file that, when processed by an application using the vulnerable libaom library, triggers the invalid memory free operation.

Exploitation scenarios include:

• Hosting a malicious AV1 video on a website that is rendered by vulnerable browsers or media players
• Sending malicious media files through messaging platforms or email attachments
• Embedding malicious AV1 content in streaming services or video conferencing applications

The vulnerability mechanism involves the improper handling of image data structures in libaom's aom_image.c file. When processing certain malformed or specially crafted input, the code path leads to a free() call on memory that was not heap-allocated. For detailed technical analysis, refer to the AOMedia Bug Report #2998.

Detection Methods for CVE-2021-30473

Indicators of Compromise

• Unexpected crashes in applications using libaom when processing AV1 media content
• Memory corruption errors or segmentation faults in media processing workflows
• Unusual behavior in web browsers or media players when rendering AV1 video streams
• Core dumps indicating invalid memory access in aom_image.c related functions

Detection Strategies

• Monitor application crash reports for patterns indicating memory corruption in libaom or AV1 codec components
• Implement runtime memory sanitizers (ASAN, MSAN) in development and testing environments to detect invalid free operations
• Deploy endpoint detection solutions capable of identifying exploitation attempts targeting media processing libraries
• Utilize SentinelOne's behavioral AI to detect anomalous process behavior following media file processing

Monitoring Recommendations

• Enable detailed logging for media processing applications and monitor for unexpected terminations
• Implement network-level inspection for potentially malicious AV1 content delivery
• Track version inventories of libaom across the organization to identify vulnerable deployments
• Monitor security advisories from Debian, Fedora, and Gentoo for downstream patch availability

How to Mitigate CVE-2021-30473

Immediate Actions Required

• Update libaom to a version that includes commit 4efe20e99dcd9b6f8eadc8de8acc825be7416578 or later
• Apply security updates from your Linux distribution (Debian DSA-5490, Fedora, Gentoo GLSA 202401-32)
• Audit systems for applications that bundle or depend on vulnerable libaom versions
• Consider temporarily restricting processing of untrusted AV1 media content in high-risk environments

Patch Information

The vulnerability was fixed in the AOMedia libaom repository with the commit referenced in the AOMedia Code Change. This patch ensures proper memory management in aom_image.c by validating pointer references before deallocation operations.

Distribution-specific patches are available:

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

Workarounds

• Disable AV1 codec support in applications where it is not essential until patching is complete
• Implement input validation and sandboxing for media processing workflows
• Use application-level firewalls or content filters to restrict untrusted media file processing
• Deploy containerization to isolate media processing components and limit the impact of potential exploitation

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

# Update libaom on Debian/Ubuntu
sudo apt update && sudo apt upgrade libaom3

# Update libaom on Fedora
sudo dnf update libaom