CVE-2021-21193: Google Chrome Use After Free Vulnerability

CVE-2021-21193 Overview

CVE-2021-21193 is a Use After Free vulnerability in the Blink rendering engine of Google Chrome prior to version 89.0.4389.90. This memory corruption flaw allows a remote attacker to potentially exploit heap corruption via a crafted HTML page, enabling arbitrary code execution within the context of the browser process.

Critical Impact

This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Successful exploitation could allow attackers to execute arbitrary code, potentially leading to complete system compromise through drive-by download attacks.

Affected Products

• Google Chrome (versions prior to 89.0.4389.90)
• Fedora 32 (with bundled Chromium packages)
• Debian Linux 10.0 (with bundled Chromium packages)

Discovery Timeline

• 2021-03-16 - CVE-2021-21193 published to NVD
• 2025-10-24 - Last updated in NVD database

Technical Details for CVE-2021-21193

Vulnerability Analysis

The vulnerability exists in the Blink rendering engine, which is responsible for parsing and rendering web content in Chromium-based browsers. Blink handles complex DOM operations, JavaScript execution, and layout calculations. The Use After Free condition (CWE-416) occurs when the application references memory after it has been freed, leading to heap corruption.

In this case, a specifically crafted HTML page can trigger a scenario where Blink continues to reference a memory object after it has been deallocated. When the freed memory is subsequently reallocated for a different purpose, the dangling pointer references corrupted data, allowing an attacker to manipulate program execution flow.

Root Cause

The root cause is a Use After Free memory safety vulnerability in Blink's memory management. When processing certain DOM structures or rendering operations, the engine fails to properly invalidate references to freed memory objects. This creates a race condition between object destruction and subsequent access, resulting in a dangling pointer that can be exploited.

The vulnerability is classified under CWE-416 (Use After Free), which occurs when a program continues to use a pointer after the memory it points to has been freed. In browser contexts, these vulnerabilities are particularly dangerous because attackers can craft malicious web pages to trigger the condition remotely.

Attack Vector

The attack vector is network-based and requires user interaction—specifically, a victim must navigate to a malicious webpage containing the crafted HTML payload. The attack does not require authentication or prior access to the target system.

A typical attack scenario involves:

1. An attacker crafts a malicious HTML page designed to trigger the Use After Free condition
2. The victim is lured to visit the page through phishing, malicious advertisements, or compromised legitimate websites
3. The Blink engine processes the malicious content, triggering the memory corruption
4. The attacker achieves arbitrary code execution within the browser's rendering process
5. Depending on sandbox escape techniques, the attacker may gain broader system access

The vulnerability can be exploited through drive-by download attacks, requiring no additional user interaction beyond visiting the malicious page. Given its listing in CISA's Known Exploited Vulnerabilities catalog, this attack pattern has been observed in real-world campaigns.

Detection Methods for CVE-2021-21193

Indicators of Compromise

• Unexpected browser crashes or instability when visiting specific websites
• Abnormal memory consumption patterns in Chrome or Chromium processes
• Evidence of suspicious JavaScript execution or DOM manipulation in browser logs
• Network connections to known malicious domains serving exploit content
• Anomalous child process spawning from browser processes

Detection Strategies

• Monitor browser process behavior for signs of memory corruption exploitation
• Implement network-based detection for known exploit delivery patterns
• Deploy endpoint detection rules to identify suspicious browser child process creation
• Analyze browser crash dumps for evidence of Use After Free exploitation attempts
• Correlate threat intelligence feeds for IOCs related to CVE-2021-21193 campaigns

Monitoring Recommendations

• Enable Chrome's built-in crash reporting and review crash patterns for exploitation signatures
• Monitor for browser processes executing unexpected system calls or spawning shells
• Track browser version deployments across the organization to identify vulnerable installations
• Implement web filtering to block access to known exploit kit infrastructure
• Review DNS query logs for indicators of malicious domain access

How to Mitigate CVE-2021-21193

Immediate Actions Required

• Update Google Chrome to version 89.0.4389.90 or later immediately
• Enable automatic updates for all Chromium-based browsers across the organization
• Apply corresponding security updates for Fedora and Debian systems running Chromium
• Consider temporary browser isolation for high-risk users until patching is complete
• Review CISA KEV remediation guidance for compliance requirements

Patch Information

Google released the security patch addressing CVE-2021-21193 in Chrome version 89.0.4389.90. The update was announced via the Google Chrome Update Announcement. Additional details are available in the Chrome Bug Report.

For Linux distributions:

• Fedora users should apply updates from the Fedora Package Announcement
• Debian users should refer to Debian Security Advisory DSA-4886
• Gentoo users can review the Gentoo GLSA Advisory

This vulnerability is tracked in the CISA Known Exploited Vulnerabilities Catalog, which mandates remediation for federal agencies.

Workarounds

• Implement browser isolation solutions to contain potential exploitation
• Use network-level filtering to block known malicious domains and exploit delivery infrastructure
• Consider disabling JavaScript on untrusted sites (significantly impacts browsing experience)
• Deploy content security policies where possible to limit script execution
• Utilize browser extensions that provide additional script control capabilities

# Verify Chrome version on Linux systems
google-chrome --version
# Expected output: Google Chrome 89.0.4389.90 or higher

# For Debian-based systems, update Chromium
sudo apt update && sudo apt upgrade chromium

# For Fedora systems, update Chromium
sudo dnf update chromium