CVE-2021-21220: Google Chrome V8 RCE Vulnerability

CVE-2021-21220 Overview

CVE-2021-21220 is a critical heap corruption vulnerability in the V8 JavaScript engine used by Google Chrome. The vulnerability stems from insufficient validation of untrusted input in V8, which allows a remote attacker to exploit heap corruption via a specially crafted HTML page. This vulnerability affects Google Chrome versions prior to 89.0.4389.128 and has been actively exploited in the wild.

Critical Impact

This vulnerability enables remote code execution through heap corruption in Chrome's V8 JavaScript engine. CISA has added this to their Known Exploited Vulnerabilities catalog, indicating active exploitation in real-world attacks.

Affected Products

• Google Chrome prior to version 89.0.4389.128
• Fedora 32 (with vulnerable Chrome packages)
• Fedora 33 (with vulnerable Chrome packages)
• Fedora 34 (with vulnerable Chrome packages)

Discovery Timeline

• April 26, 2021 - CVE-2021-21220 published to NVD
• October 24, 2025 - Last updated in NVD database

Technical Details for CVE-2021-21220

Vulnerability Analysis

CVE-2021-21220 is classified under CWE-787 (Out-of-Bounds Write), a memory corruption vulnerability that occurs when the V8 JavaScript engine improperly validates untrusted input during just-in-time (JIT) compilation operations. The vulnerability specifically affects the XOR typer component within V8's optimization pipeline, where improper bounds checking during arithmetic operations can lead to out-of-bounds memory access.

When exploited, attackers can leverage this flaw to corrupt heap memory, potentially gaining arbitrary code execution within the context of the Chrome renderer process. The vulnerability requires user interaction—a victim must visit a malicious webpage containing crafted JavaScript code—but once triggered, the attacker can achieve full compromise of the browser process.

Root Cause

The root cause of this vulnerability lies in V8's JIT compiler's type system, specifically in how it handles XOR operations during optimization. The typer component incorrectly tracks value ranges for XOR operations, leading to incorrect assumptions about array bounds. When these assumptions are violated at runtime, the engine performs out-of-bounds memory accesses, enabling heap corruption.

This type confusion during JIT compilation allows attackers to construct type-confused objects that bypass normal memory safety checks, ultimately providing primitives for arbitrary read/write operations in memory.

Attack Vector

The attack is network-based and requires minimal attacker sophistication once a working exploit is developed. The attack flow involves:

1. An attacker hosts a malicious webpage containing specially crafted JavaScript code
2. The victim navigates to the malicious page using a vulnerable Chrome browser
3. The JavaScript triggers the JIT compiler vulnerability in V8
4. The XOR typer miscalculates bounds, causing heap corruption
5. The attacker leverages the corruption to achieve arbitrary code execution

The vulnerability was demonstrated at the Pwn2Own 2021 competition, with technical details published describing the XOR typer out-of-bounds access leading to remote code execution. For detailed technical analysis, see the Packet Storm Out-Of-Bounds RCE advisory and the Chrome V8 JIT arbitrary code execution analysis.

Detection Methods for CVE-2021-21220

Indicators of Compromise

• Unusual Chrome renderer process crashes or instability during web browsing
• Chrome memory usage anomalies indicating potential heap spray attempts
• Network connections from Chrome to known malicious domains hosting exploit code
• Suspicious JavaScript execution patterns involving intensive XOR operations and typed arrays

Detection Strategies

• Monitor for Chrome renderer process crashes accompanied by memory corruption signatures
• Deploy endpoint detection rules to identify V8 JIT-related exploitation attempts targeting the XOR typer
• Implement network-level detection for known exploit delivery patterns associated with CVE-2021-21220
• Utilize browser telemetry to identify pages triggering suspicious JIT compilation behaviors

Monitoring Recommendations

• Enable Chrome crash reporting and monitor for V8-related crash signatures
• Deploy SentinelOne Singularity to detect post-exploitation behaviors including sandbox escapes and process hollowing
• Monitor for suspicious child processes spawned from Chrome renderer processes
• Implement logging for Chrome policy violations and security events

How to Mitigate CVE-2021-21220

Immediate Actions Required

• Update Google Chrome to version 89.0.4389.128 or later immediately
• Verify Chrome auto-update functionality is enabled and functioning
• Review and update Chrome deployments across all managed endpoints
• Consider temporarily disabling JavaScript for high-risk users if patching is delayed

Patch Information

Google addressed this vulnerability in Chrome version 89.0.4389.128, released in April 2021. The patch corrects the type inference logic in V8's XOR typer to properly track value ranges during JIT compilation.

For official patch details, see the Google Chrome Stable Channel Update announcement. The underlying issue is tracked in Chromium Bug Report #1196683.

Fedora users should apply the security updates documented in the Fedora package announcements for their respective versions. Additional security guidance is available in Gentoo GLSA 202104-08.

Workarounds

• Disable JavaScript execution using Chrome enterprise policies as a temporary mitigation
• Use browser isolation solutions to contain potential exploitation
• Block access to untrusted websites until patches can be applied
• Consider using alternative browsers temporarily if immediate patching is not possible

# Chrome Enterprise Policy to disable JavaScript (temporary mitigation)
# Add to Chrome policies JSON configuration
# Windows: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome
# Linux: /etc/opt/chrome/policies/managed/
{
  "DefaultJavaScriptSetting": 2
}