CVE-2021-21108: Google Chrome Use After Free Vulnerability

CVE-2021-21108 Overview

CVE-2021-21108 is a use-after-free vulnerability in the media component of Google Chrome prior to version 87.0.4280.141. This critical memory corruption flaw allows a remote attacker who has already compromised the renderer process to potentially escape the browser sandbox through a specially crafted HTML page. The vulnerability represents a significant security risk as it can be chained with other exploits to achieve full system compromise.

Critical Impact

An attacker who has compromised the renderer process can leverage this use-after-free vulnerability to escape Chrome's sandbox, potentially gaining elevated privileges and access to the underlying operating system.

Affected Products

• Google Chrome versions prior to 87.0.4280.141
• Fedora Project Fedora (versions 32 and 33)
• Debian Linux 10.0

Discovery Timeline

• 2021-01-08 - CVE-2021-21108 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-21108

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) exists within Chrome's media handling component. Use-after-free conditions occur when a program continues to reference memory after it has been freed, leading to undefined behavior that attackers can exploit. In this case, the vulnerability allows an attacker who has already gained code execution within the renderer process to craft malicious HTML content that triggers the memory corruption condition.

The sandbox escape capability makes this vulnerability particularly dangerous. Chrome's sandbox architecture is designed to contain the impact of renderer process compromises, but this vulnerability provides a pathway to bypass those protections entirely. When successfully exploited, an attacker can break out of the sandboxed environment and execute code with the privileges of the Chrome process outside the sandbox.

Root Cause

The root cause of CVE-2021-21108 is improper memory management within Chrome's media processing code. When handling certain media-related operations, the code fails to properly track object lifetimes, resulting in a dangling pointer scenario. When this freed memory is subsequently accessed, the attacker-controlled data can influence program execution, enabling the sandbox escape.

Attack Vector

The attack requires network access and user interaction (visiting a malicious webpage). However, it also requires that the attacker has already compromised the renderer process through a separate vulnerability. This makes CVE-2021-21108 most dangerous when used as part of an exploit chain:

1. The attacker first exploits a separate vulnerability to gain code execution within Chrome's sandboxed renderer process
2. The attacker then crafts malicious HTML content that triggers the use-after-free condition in the media component
3. By manipulating the freed memory, the attacker can redirect program execution to escape the sandbox
4. Once outside the sandbox, the attacker has direct access to the user's system with Chrome's process privileges

The attack can be delivered through malicious websites, compromised advertisements, or phishing campaigns that direct users to attacker-controlled pages.

Detection Methods for CVE-2021-21108

Indicators of Compromise

• Unusual Chrome process crashes or error logs related to media component operations
• Detection of heap corruption or unexpected memory access patterns in Chrome processes
• Anomalous child process spawning from Chrome with elevated privileges
• Network connections to suspicious domains from Chrome processes that have escaped sandbox restrictions

Detection Strategies

• Monitor for unexpected Chrome process behavior including abnormal memory allocation patterns
• Implement endpoint detection rules for sandbox escape attempts targeting browser processes
• Deploy behavioral analysis to identify Chrome processes accessing resources outside normal sandbox boundaries
• Use memory forensics to detect signs of use-after-free exploitation in browser memory spaces

Monitoring Recommendations

• Enable Chrome's built-in crash reporting and review for media-related crashes
• Monitor system logs for privilege escalation attempts originating from browser processes
• Implement network monitoring for post-exploitation communication patterns
• Deploy SentinelOne Singularity platform for real-time detection of sandbox escape behaviors and memory corruption exploits

How to Mitigate CVE-2021-21108

Immediate Actions Required

• Update Google Chrome to version 87.0.4280.141 or later immediately
• Enable automatic updates to ensure timely delivery of security patches
• Review and apply relevant security advisories for Fedora and Debian systems running Chrome
• Consider restricting access to untrusted websites until patches are applied

Patch Information

Google has addressed this vulnerability in Chrome version 87.0.4280.141. The patch was announced in the Google Chrome Stable Channel Update. Additional security advisories have been released by affected Linux distributions:

• Debian Security Advisory DSA-4832
• Gentoo GLSA 202101-05
• Fedora Package announcements for Fedora 32 and 33

The detailed bug report is tracked as Chromium Bug #1155426.

Workarounds

• Disable JavaScript on untrusted sites to reduce the attack surface for renderer compromise
• Use browser isolation technologies to contain potential exploits
• Implement network segmentation to limit the impact of successful sandbox escapes
• Consider using enterprise browser policies to restrict media functionality if not required

# Verify Chrome version on Linux
google-chrome --version

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

# Update Chrome on Fedora
sudo dnf update chromium