CVE-2021-21206: Google Chrome Use After Free Vulnerability

CVE-2021-21206 Overview

CVE-2021-21206 is a use after free vulnerability in the Blink rendering engine of Google Chrome prior to version 89.0.4389.128. This memory corruption flaw allows a remote attacker to potentially exploit heap corruption via a crafted HTML page, potentially leading to arbitrary code execution within the browser context.

Critical Impact

This vulnerability is actively exploited in the wild and has been added to CISA's Known Exploited Vulnerabilities (KEV) catalog. Successful exploitation could allow attackers to execute arbitrary code, compromise user systems, or gain unauthorized access to sensitive data through malicious web pages.

Affected Products

• Google Chrome versions prior to 89.0.4389.128
• Fedora 32 (Chromium package)
• Fedora 33 (Chromium package)
• Fedora 34 (Chromium package)

Discovery Timeline

• 2021-04-26 - CVE-2021-21206 published to NVD
• 2025-10-24 - Last updated in NVD database

Technical Details for CVE-2021-21206

Vulnerability Analysis

This vulnerability (CWE-416: Use After Free) exists within the Blink rendering engine, which is responsible for processing and rendering web content in Chromium-based browsers. Use after free vulnerabilities occur when a program continues to reference memory after it has been freed, allowing an attacker to potentially control the contents of that memory location.

In the context of CVE-2021-21206, the vulnerability can be triggered through specially crafted HTML content. When a victim visits a malicious web page, the attacker can manipulate the browser's memory state to cause a reference to freed memory, potentially leading to heap corruption. This corruption can then be leveraged to achieve code execution within the browser's renderer process.

The vulnerability requires user interaction—specifically, the user must navigate to a malicious website or be redirected to one. However, given the ubiquity of web browsing and the sophistication of social engineering attacks, this requirement presents minimal barrier to exploitation.

Root Cause

The root cause is a use after free condition in the Blink rendering engine where a memory object is accessed after it has been deallocated. This typically occurs due to improper lifecycle management of DOM objects or JavaScript references, where one part of the code frees an object while another part retains and later uses a dangling pointer to that freed memory.

Attack Vector

The attack vector is network-based, requiring the victim to visit a malicious or compromised website. The attacker delivers a specially crafted HTML page that triggers the use after free condition. The exploitation flow typically involves:

1. The victim navigates to an attacker-controlled or compromised website
2. The malicious HTML page manipulates memory allocation and deallocation sequences
3. The use after free condition is triggered, corrupting heap memory
4. The attacker leverages the heap corruption to gain control of execution flow
5. Arbitrary code execution is achieved within the browser's renderer sandbox

The vulnerability was confirmed to be exploited in the wild, prompting CISA to add it to the Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2021-21206

Indicators of Compromise

• Unexpected browser crashes or instability when visiting specific websites
• Anomalous memory allocation patterns in Chrome process logs
• Detection of known malicious URLs serving exploit code for this vulnerability
• Unusual child process spawning from Chrome renderer processes

Detection Strategies

• Monitor for Chrome versions prior to 89.0.4389.128 across enterprise endpoints
• Implement browser telemetry to detect crash patterns indicative of exploitation attempts
• Deploy network-based detection rules for known exploit payloads targeting CVE-2021-21206
• Utilize endpoint detection and response (EDR) solutions to identify suspicious browser behavior

Monitoring Recommendations

• Enable Chrome's built-in crash reporting to identify potential exploitation attempts
• Monitor endpoint security logs for anomalous browser process activity
• Track browser version compliance across the organization to ensure timely patching
• Review proxy and firewall logs for connections to known malicious infrastructure

How to Mitigate CVE-2021-21206

Immediate Actions Required

• Update Google Chrome to version 89.0.4389.128 or later immediately
• Enable automatic updates in Chrome to ensure timely security patches
• Verify all Chromium-based browsers (Edge, Brave, etc.) are updated to patched versions
• Deploy browser security policies to restrict access to untrusted websites

Patch Information

Google addressed this vulnerability in Chrome version 89.0.4389.128 released in April 2021. Organizations should verify that all Chrome installations have been updated to this version or later. For Fedora users, updated packages are available through the standard Fedora package repositories.

For detailed patch information, refer to the Chrome Desktop Update Announcement. Technical details about the vulnerability can be found in Chromium Bug Report #1196781. Fedora users should consult the Fedora Package Announcements and the Gentoo Security Advisory GLSA 2021-08.

Workarounds

• Use browser isolation technologies to contain potential exploits
• Implement strict Content Security Policy (CSP) headers on organizational web applications
• Consider using application whitelisting to restrict browser access to approved sites
• Enable Site Isolation in Chrome if not already enabled by default
• Deploy web filtering solutions to block access to known malicious domains

# Verify Chrome version on Linux/macOS
google-chrome --version
# Expected output should be 89.0.4389.128 or higher

# On Windows, check via PowerShell
(Get-Item "C:\Program Files\Google\Chrome\Application\chrome.exe").VersionInfo.FileVersion

# Force Chrome update via enterprise policy (example)
# Set AutoUpdateCheckPeriodMinutes to ensure frequent update checks