CVE-2021-21109: Google Chrome Use After Free Vulnerability

CVE-2021-21109 Overview

CVE-2021-21109 is a use-after-free vulnerability in the payments component of Google Chrome prior to version 87.0.4280.141. This memory corruption flaw allows a remote attacker who has already compromised the renderer process to potentially perform a sandbox escape via a specially crafted HTML page. The vulnerability represents a serious security risk as successful exploitation could allow attackers to break out of Chrome's security sandbox and execute arbitrary code with elevated privileges on the victim's system.

Critical Impact

This use-after-free vulnerability enables sandbox escape from a compromised renderer process, potentially allowing attackers to execute code outside of Chrome's security boundaries and gain control of the affected system.

Affected Products

• Google Chrome versions prior to 87.0.4280.141
• Fedora 32 and Fedora 33 (via bundled Chromium packages)
• Debian Linux 10.0 (Buster)

Discovery Timeline

• 2021-01-08 - CVE-2021-21109 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-21109

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a dangerous memory corruption issue that occurs when a program continues to use a memory location after it has been freed. In the context of Chrome's payments component, the flaw exists in how the browser handles payment-related objects during certain operations.

The vulnerability requires user interaction—specifically, a victim must navigate to a malicious webpage. However, the exploitation scenario assumes the attacker has already compromised the renderer process through another vulnerability or attack vector. From this position, the use-after-free condition in the payments component provides a pathway to escape Chrome's sandbox, which is designed to contain malicious code within isolated processes.

Sandbox escapes are particularly dangerous because they defeat one of Chrome's primary security mechanisms. The sandbox restricts what a compromised renderer can do, limiting access to the operating system and user data. Breaking out of this containment allows attackers to perform actions such as installing malware, stealing credentials, or pivoting to other systems on the network.

Root Cause

The root cause stems from improper memory management in Chrome's payments implementation. When certain payment-related objects are deallocated, references to those memory locations are not properly invalidated. Subsequent code execution may attempt to access or manipulate these dangling pointers, leading to use-after-free conditions.

In use-after-free scenarios, the freed memory may be reallocated for different purposes. An attacker who can control what data occupies the freed memory region can potentially manipulate program execution flow, corrupt critical data structures, or achieve arbitrary code execution. The payments component's interaction with the renderer process creates an exploitable pathway for sandbox escape.

Attack Vector

The attack requires a multi-stage approach. First, an attacker must gain code execution within Chrome's renderer process—typically through a separate vulnerability such as a JavaScript engine bug. With renderer access established, the attacker can then craft specific interactions with the payments API to trigger the use-after-free condition.

By carefully timing memory allocations and crafting malicious HTML/JavaScript content, the attacker can exploit the dangling pointer to corrupt memory in ways that allow escaping the renderer sandbox. The crafted HTML page serves as the delivery mechanism for triggering the vulnerable code path and manipulating memory state to achieve exploitation.

This vulnerability demonstrates the importance of defense-in-depth security architectures. Even with Chrome's robust sandbox, memory safety issues in privileged components can undermine containment mechanisms.

Detection Methods for CVE-2021-21109

Indicators of Compromise

• Unusual crashes or instability in Google Chrome, particularly when visiting unfamiliar websites
• Unexpected child processes spawned from Chrome with elevated privileges
• Chrome renderer process attempting to access resources outside normal sandbox boundaries
• Suspicious network connections or file system activity originating from Chrome processes
• Memory access violations or exception logs related to Chrome's payments component

Detection Strategies

• Deploy endpoint detection and response (EDR) solutions to monitor for sandbox escape attempts and anomalous Chrome process behavior
• Implement browser-level monitoring to detect exploitation attempts targeting Chrome's internal APIs
• Monitor for unusual inter-process communication patterns between Chrome's renderer and browser processes
• Use memory protection technologies that can detect use-after-free exploitation techniques

Monitoring Recommendations

• Enable Chrome's crash reporting to identify potential exploitation attempts
• Monitor system logs for unexpected privilege escalation events associated with browser processes
• Implement network monitoring to detect command-and-control communications following successful exploitation
• Review endpoint telemetry for signs of post-exploitation activity such as credential theft or lateral movement

How to Mitigate CVE-2021-21109

Immediate Actions Required

• Update Google Chrome to version 87.0.4280.141 or later immediately
• Enable automatic updates for Chrome to ensure timely patching of future vulnerabilities
• Restart Chrome after updates to ensure the patched version is running
• Review and update Chromium-based browsers on Linux distributions including Fedora and Debian

Patch Information

Google addressed this vulnerability in the stable channel update released in January 2021. The fix is included in Chrome version 87.0.4280.141 and all subsequent releases. Detailed information about the security update is available in the Google Chrome Desktop Update announcement. The underlying bug details can be found in Chromium Bug Report #1152334.

For Linux distributions:

• Debian users should apply the patches detailed in Debian Security Advisory DSA-4832
• Fedora users should update via their package manager as addressed in the Fedora Package Announcements
• Gentoo users should reference GLSA 202101-05

Workarounds

• Limit exposure by avoiding untrusted websites until patching is complete
• Consider using browser isolation technologies for high-risk browsing activities
• Implement network-level controls to block access to known malicious domains
• Deploy web content filtering to reduce exposure to potentially malicious pages

# Verify Chrome version on Linux
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