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 versions prior to 87.0.4280.141. A remote attacker who had already compromised the renderer process could leverage this flaw to potentially escape the Chrome sandbox through a crafted HTML page. The issue is tracked as [CWE-416] and was addressed in the Stable Channel update published by Google on January 7, 2021. The vulnerability also affects downstream distributions, including Fedora 32, Fedora 33, and Debian 10.

Critical Impact
A successful exploit chain involving CVE-2021-21108 enables sandbox escape from the browser renderer, allowing attacker-controlled code to break out of Chrome's security boundary on the victim host.

Affected Products

Google Chrome prior to 87.0.4280.141
Fedora Project Fedora 32 and 33
Debian Linux 10

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

The vulnerability resides in Chrome's media subsystem and is classified as a use-after-free condition. Use-after-free flaws occur when a program continues to use a pointer after the memory it references has been freed. An attacker who controls the freed allocation can place a crafted object in its place, causing the application to operate on attacker-controlled data when the dangling pointer is dereferenced.

The flaw becomes meaningful as part of a multi-stage exploit chain. According to Google's advisory, an attacker must first compromise the renderer process, typically through a separate renderer-level bug. CVE-2021-21108 then provides the primitive needed to escape the renderer sandbox and reach a higher-privilege context. Sandbox escape vulnerabilities are particularly valuable to threat actors building browser exploit chains targeting journalists, government entities, and enterprise users.

Root Cause

The defect is a memory lifetime management error inside the media component. Object references in the media pipeline are not invalidated correctly when the underlying object is destroyed, leaving a dangling pointer that can be reused. The condition is tracked internally in Chromium as bug 1155426.

Attack Vector

Exploitation requires that the attacker host a crafted HTML page and induce the victim to visit it, satisfying the user interaction requirement. The attacker must also already have code execution within the renderer process, which is typically achieved by chaining with a separate renderer-side vulnerability. Once both conditions are met, attacker-controlled logic in the renderer triggers the freed media object reuse to corrupt browser-process memory and break out of the sandbox.

No verified public proof-of-concept code is available for this issue. See the Chrome Stable Channel Update and the Crash Report for Bug 1155426 for vendor-published technical references.

Detection Methods for CVE-2021-21108

Indicators of Compromise

Chrome browser processes (chrome.exe, chrome) spawning unexpected child processes outside the normal renderer or GPU process tree.
Unexpected crashes in the Chrome media stack logged before or alongside suspicious post-exploitation activity.
Outbound network connections from the Chrome browser-process to unfamiliar infrastructure following visits to untrusted HTML content.

Detection Strategies

Inventory installed Chrome versions across managed endpoints and flag any host running a build below 87.0.4280.141.
Correlate Chrome renderer crash telemetry with subsequent process creation events to identify potential sandbox escape behavior.
Monitor for Chrome processes writing executables, modifying autorun keys, or loading unsigned modules, which would be inconsistent with normal browser activity.

Monitoring Recommendations

Aggregate browser process telemetry, including child process lineage and module loads, into a central analytics pipeline.
Track patch deployment status for Chrome, Fedora, and Debian package updates referenced in the vendor advisories.
Alert on anomalous use of media APIs or repeated renderer crashes that may indicate exploit attempts against memory corruption flaws.

How to Mitigate CVE-2021-21108

Immediate Actions Required

Update Google Chrome to version 87.0.4280.141 or later on all desktop platforms.
Apply Fedora and Debian distribution updates referenced in the vendor advisories for affected Linux hosts.
Restart Chrome after patching so the new binary is loaded across all sessions, including pinned profiles.

Patch Information

Google released the fix in the Chrome Stable Channel update on January 7, 2021. See the Chrome Stable Channel Update for full release notes. Distribution-specific patches are available through the Fedora Package Announcement, Gentoo GLSA 202101-05, and Debian DSA-4832 Security Advisory.

Workarounds

Enforce Chrome auto-update policies through enterprise management so end users cannot defer browser updates.
Restrict browsing of untrusted HTML content using web filtering or isolation technology until patches are confirmed deployed.
Disable or limit media playback features for high-risk user groups where business workflows allow.

bash

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

# Update Chrome package on Debian-based systems
sudo apt-get update && sudo apt-get install --only-upgrade google-chrome-stable

# Update Chromium on Fedora
sudo dnf upgrade chromium