CVE-2021-21166 Overview
CVE-2021-21166 is a data race vulnerability in the audio component of Google Chrome prior to version 89.0.4389.72. This race condition allows a remote attacker to potentially exploit heap corruption via a crafted HTML page. The vulnerability has been confirmed as actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating significant real-world threat activity.
Critical Impact
This vulnerability enables remote attackers to achieve heap corruption through race conditions in Chrome's audio processing, potentially leading to arbitrary code execution when users visit malicious web pages.
Affected Products
- Google Chrome versions prior to 89.0.4389.72
- Fedora 32, 33, and 34
- Debian Linux 10.0
Discovery Timeline
- 2021-03-09 - CVE-2021-21166 published to NVD
- 2025-10-24 - Last updated in NVD database
Technical Details for CVE-2021-21166
Vulnerability Analysis
CVE-2021-21166 is classified as CWE-362 (Race Condition), specifically manifesting as a data race within Chrome's audio processing subsystem. Race conditions occur when multiple threads or processes access shared resources concurrently without proper synchronization, leading to unpredictable behavior.
In this case, the vulnerability exists in how Chrome's audio component handles concurrent operations. When two or more threads attempt to access or modify audio-related data structures simultaneously, the lack of proper locking mechanisms can result in heap corruption. This corruption can be leveraged by an attacker to achieve arbitrary code execution or cause a denial of service.
The attack is network-based and requires user interaction—specifically, the victim must navigate to a maliciously crafted HTML page. Once the page is loaded, the attacker-controlled content can trigger the race condition in the audio component, leading to exploitable heap corruption.
Root Cause
The root cause of this vulnerability is insufficient thread synchronization in Chrome's audio component. When audio data is processed, multiple threads may access shared memory regions without adequate locking mechanisms. This creates a Time-of-Check Time-of-Use (TOCTOU) scenario where the state of memory can change between when it is validated and when it is used, resulting in heap corruption.
The race condition specifically involves the audio rendering pipeline where buffer operations are not properly serialized, allowing concurrent access to corrupt heap metadata or audio buffer contents.
Attack Vector
The attack vector for CVE-2021-21166 is network-based with required user interaction. An attacker crafts a malicious HTML page containing specific audio elements or JavaScript code designed to trigger the race condition. The attack flow typically involves:
- Victim navigates to a malicious or compromised website
- The page loads audio content that triggers concurrent thread operations
- The race condition corrupts heap memory structures
- Attacker achieves code execution or denial of service
The vulnerability does not require authentication or elevated privileges, making it accessible to any attacker who can deliver a malicious webpage to potential victims. Given its inclusion in CISA's KEV catalog, threat actors have successfully weaponized this vulnerability in real-world attacks.
Detection Methods for CVE-2021-21166
Indicators of Compromise
- Unusual Chrome process crashes or abnormal audio subsystem behavior
- Unexpected memory access violations in Chrome's renderer process logs
- Network traffic to known malicious domains serving exploit payloads
- Anomalous JavaScript activity triggering rapid audio context creation and destruction
Detection Strategies
- Monitor Chrome crash reports for patterns indicating heap corruption in audio components
- Implement network-based detection for known exploit delivery patterns targeting CVE-2021-21166
- Deploy endpoint detection solutions capable of identifying race condition exploitation attempts
- Analyze browser telemetry for suspicious audio API usage patterns
Monitoring Recommendations
- Enable Chrome's built-in crash reporting and monitor for audio-related heap corruption signatures
- Implement centralized logging for browser security events across the enterprise
- Monitor for lateral movement following potential browser compromise
- Track Chrome version deployment across endpoints to identify unpatched systems
How to Mitigate CVE-2021-21166
Immediate Actions Required
- Update Google Chrome to version 89.0.4389.72 or later immediately
- Verify Chrome auto-update functionality is enabled and operational across all endpoints
- Review CISA KEV catalog requirements for federal agencies and compliance-bound organizations
- Apply Fedora and Debian security updates for affected Linux distributions
Patch Information
Google addressed this vulnerability in Chrome version 89.0.4389.72, released in March 2021. The fix implements proper synchronization mechanisms in the audio component to prevent the race condition. Organizations should reference the following resources for patching guidance:
For Fedora systems, security updates are available through the standard package management system as documented in the Fedora Package Announcements.
Workarounds
- Implement strict browsing policies to limit access to untrusted websites until patching is complete
- Consider disabling audio functionality in Chrome via enterprise policies as a temporary measure
- Deploy network-level filtering to block known exploit delivery infrastructure
- Use browser isolation solutions to contain potential exploitation attempts
# Verify Chrome version on Linux systems
google-chrome --version
# Force Chrome update check (Linux)
sudo apt-get update && sudo apt-get upgrade google-chrome-stable
# Fedora update command
sudo dnf update chromium
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
