CVE-2021-21201: Google Chrome Use After Free Vulnerability

CVE-2021-21201 Overview

CVE-2021-21201 is a use-after-free vulnerability in the permissions component of Google Chrome prior to version 90.0.4430.72. This 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 represents a significant security risk as it enables attackers to break out of Chrome's sandbox isolation, potentially gaining broader system access.

Critical Impact

An attacker who has compromised Chrome's renderer process can exploit this use-after-free vulnerability to escape the browser sandbox, potentially achieving arbitrary code execution outside of the sandboxed environment and compromising the underlying system.

Affected Products

• Google Chrome versions prior to 90.0.4430.72
• Debian Linux 10.0
• Fedora 32, 33, and 34

Discovery Timeline

• April 14, 2021 - Google releases security patch in Chrome 90.0.4430.72
• April 26, 2021 - CVE-2021-21201 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-21201

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to reference memory after it has been freed. In the context of Google Chrome's permissions system, the vulnerability arises from improper memory management where freed memory objects are subsequently accessed.

The permissions component in Chromium handles various browser permission requests including geolocation, camera, microphone, and notifications. When the underlying memory management fails to properly track object lifecycles, it creates an exploitable condition. The use-after-free condition can be triggered through specific interactions with the permissions system via a malicious HTML page.

What makes this vulnerability particularly dangerous is its role as a sandbox escape vector. Chrome employs a multi-process architecture where web content runs in sandboxed renderer processes with limited system access. An attacker who can exploit CVE-2021-21201 after compromising the renderer can potentially escape this sandbox, bypassing Chrome's security isolation and gaining access to the broader system with the privileges of the Chrome process.

Root Cause

The root cause of CVE-2021-21201 lies in improper lifecycle management of memory objects within Chrome's permissions handling code. When certain permission-related operations are performed in a specific sequence, the code continues to reference memory that has already been deallocated. This dangling pointer situation creates an exploitable condition where an attacker can manipulate the freed memory to gain control of program execution.

The vulnerability specifically affects how Chrome manages permission request objects and their associated data structures. When these objects are freed prematurely or when references to them are not properly invalidated, subsequent access to this memory leads to undefined behavior that attackers can leverage.

Attack Vector

The attack requires a multi-stage exploitation approach. First, an attacker must compromise the Chrome renderer process through a separate vulnerability or attack technique. Once the renderer is compromised, the attacker can craft specific interactions with Chrome's permissions system to trigger the use-after-free condition.

The exploitation flow involves:

1. Attacker delivers a malicious HTML page to the victim
2. The malicious page triggers vulnerability in the permissions component
3. With renderer process already compromised, attacker manipulates freed memory
4. Successful exploitation allows sandbox escape and potential code execution outside the sandbox

The vulnerability requires user interaction (visiting a malicious page), but given that Chrome is one of the most widely used browsers, this attack surface is substantial. Technical details of the vulnerability can be found in the Chromium Bug Report #1025683.

Detection Methods for CVE-2021-21201

Indicators of Compromise

• Unusual crash patterns in Chrome's permissions-related processes
• Unexpected memory access violations in chrome.exe or related Chromium processes
• Signs of sandbox escape such as child processes spawned outside normal Chrome architecture
• Anomalous network connections originating from Chrome that bypass normal browser controls

Detection Strategies

• Monitor Chrome process behavior for signs of sandbox escape, including unusual child process creation or file system access outside expected directories
• Deploy endpoint detection solutions capable of identifying use-after-free exploitation patterns and memory corruption attacks
• Implement browser version tracking to identify systems running vulnerable Chrome versions (prior to 90.0.4430.72)
• Analyze browser crash dumps for exploitation signatures related to permissions component failures

Monitoring Recommendations

• Enable Chrome crash reporting and analyze reports for patterns consistent with memory corruption exploitation
• Monitor endpoints for Chrome versions and ensure automated updates are functioning properly
• Track network traffic for unusual patterns that could indicate post-exploitation activity following a sandbox escape
• Implement behavioral monitoring for Chrome processes accessing sensitive system resources outside normal patterns

How to Mitigate CVE-2021-21201

Immediate Actions Required

• Update Google Chrome to version 90.0.4430.72 or later immediately
• Enable automatic Chrome updates to ensure timely security patches
• Apply distribution-specific patches for Debian and Fedora systems running Chromium
• Review Chrome enterprise policies to ensure updates are not being blocked

Patch Information

Google addressed this vulnerability in Chrome version 90.0.4430.72 released on April 14, 2021. The fix resolves the use-after-free condition in the permissions component by implementing proper memory lifecycle management. For detailed information, refer to the Google Chrome Stable Channel Update.

Distribution-specific patches are available:

• Debian Security Advisory DSA-4906
• Gentoo GLSA 202104-08
• Fedora updates available through standard package management channels

Workarounds

• If immediate patching is not possible, consider temporarily using an alternative browser until Chrome can be updated
• Implement strict browsing policies to limit access to untrusted websites
• Deploy network-level controls to block known malicious domains and phishing sites
• Enable Site Isolation in Chrome to provide additional protection against renderer compromises

# Check Chrome version on Linux systems
google-chrome --version

# Update Chrome on Debian/Ubuntu
sudo apt update && sudo apt upgrade google-chrome-stable

# Update Chromium on Fedora
sudo dnf update chromium