CVE-2021-37973: Google Chrome Use After Free Vulnerability

CVE-2021-37973 Overview

CVE-2021-37973 is a Use After Free vulnerability in the Portals component of Google Chrome prior to version 94.0.4606.61. This critical memory corruption flaw allows a remote attacker who has already compromised the renderer process to potentially perform a sandbox escape 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.

Critical Impact

This vulnerability enables sandbox escape from a compromised Chrome renderer process, potentially allowing attackers to execute arbitrary code with elevated privileges outside the browser sandbox. Active exploitation has been observed in the wild.

Affected Products

• Google Chrome prior to version 94.0.4606.61
• Fedora 33 and Fedora 35
• Debian Linux 10.0 and 11.0

Discovery Timeline

• October 8, 2021 - CVE-2021-37973 published to NVD
• October 24, 2025 - Last updated in NVD database

Technical Details for CVE-2021-37973

Vulnerability Analysis

CVE-2021-37973 is a Use After Free (CWE-416) vulnerability affecting the Portals API implementation in Google Chrome. The Portals feature in Chrome allows web developers to embed content from other pages in a way that enables seamless navigation transitions. The vulnerability occurs when memory that has been freed is subsequently accessed, leading to undefined behavior that can be exploited by an attacker.

The flaw specifically manifests in scenarios where an attacker who has already gained code execution within the renderer process can leverage this memory corruption bug to escape the Chrome sandbox. The sandbox is a critical security boundary designed to limit the impact of renderer process compromises, making this vulnerability particularly dangerous as it undermines one of Chrome's primary security mechanisms.

This vulnerability requires user interaction (such as visiting a malicious webpage) but can be exploited remotely over the network without authentication. The scope-changing nature of this vulnerability means a successful exploit can impact resources beyond the vulnerable component, potentially affecting the entire system.

Root Cause

The root cause of CVE-2021-37973 lies in improper memory management within the Portals implementation. A Use After Free condition occurs when code continues to reference memory after it has been deallocated. In the context of the Portals API, specific sequences of operations can trigger a state where a memory object is freed while references to it still exist. When these dangling references are subsequently accessed, the attacker can potentially control the memory contents, leading to arbitrary code execution.

The Portals API manages complex object lifecycles during page transitions and embeddings. The vulnerability appears to stem from a race condition or improper state management that allows a freed object to be accessed during portal navigation or activation operations.

Attack Vector

The attack vector for CVE-2021-37973 is network-based and requires user interaction. An attacker must first compromise the renderer process (potentially through another vulnerability) and then exploit this Use After Free to escape the sandbox.

The exploitation chain typically involves:

1. Luring a victim to a malicious website containing crafted HTML content
2. Triggering the Use After Free condition in the Portals component
3. Leveraging heap manipulation techniques to control the freed memory contents
4. Achieving code execution outside the sandbox boundaries

While no verified proof-of-concept code is publicly available, the vulnerability is confirmed to have been exploited in the wild. Technical details can be found in the Chromium Bug Report #1251727 and the Chrome Release Update.

Detection Methods for CVE-2021-37973

Indicators of Compromise

• Unusual Chrome renderer process behavior or crashes followed by suspicious system activity
• Anomalous memory allocation patterns in Chrome processes
• Evidence of heap spray or memory corruption techniques in browser memory
• Unexpected child processes spawned from Chrome with elevated privileges

Detection Strategies

• Monitor Chrome version across endpoints and flag systems running versions prior to 94.0.4606.61
• Implement endpoint detection rules for suspicious process chains originating from Chrome
• Deploy memory forensics capabilities to detect Use After Free exploitation patterns
• Enable Chrome's built-in crash reporting and analyze crash dumps for exploitation indicators

Monitoring Recommendations

• Configure centralized logging for browser crashes and security events
• Establish baseline behavior for Chrome processes and alert on deviations
• Monitor for network connections to known malicious domains serving exploit content
• Track CISA KEV updates and cross-reference with environment vulnerability data

How to Mitigate CVE-2021-37973

Immediate Actions Required

• Update Google Chrome to version 94.0.4606.61 or later immediately
• Verify Chrome auto-update functionality is enabled and operational across all endpoints
• Review systems for indicators of compromise given confirmed wild exploitation
• Apply Fedora and Debian security updates for Chromium-based browsers

Patch Information

Google has released a security update addressing CVE-2021-37973 in Chrome version 94.0.4606.61. This fix was announced in the Chrome Stable Channel Update for Desktop on September 24, 2021. Linux distribution users should apply the relevant security updates:

• Debian: DSA-5046
• Fedora: Security updates available via package-announce mailing list

Workarounds

• If immediate patching is not possible, consider restricting access to untrusted websites
• Implement network-level blocking of known malicious domains
• Enable strict site isolation in Chrome via chrome://flags/#enable-site-per-process
• Consider using application allowlisting to prevent unauthorized code execution

# Verify Chrome version from command line
google-chrome --version

# Force Chrome update check (Linux)
sudo apt update && sudo apt upgrade google-chrome-stable

# Check Fedora Chromium version
rpm -qa | grep chromium