CVE-2021-29953: Mozilla Firefox for Android XSS Vulnerability

CVE-2021-29953 Overview

CVE-2021-29953 is a Universal Cross-Site Scripting (UXSS) vulnerability affecting Firefox for Android. A malicious webpage could force a Firefox for Android user into executing attacker-controlled JavaScript in the context of another domain, bypassing the browser's same-origin policy. This vulnerability is particularly dangerous as it enables attackers to execute scripts across any domain the user visits, potentially leading to credential theft, session hijacking, and unauthorized actions on behalf of the user.

Critical Impact

This Universal XSS vulnerability allows attackers to execute arbitrary JavaScript in the security context of any domain, enabling complete compromise of user sessions, theft of sensitive data, and unauthorized actions across all websites the victim visits.

Affected Products

• Mozilla Firefox for Android < 88.1.3
• Mozilla Firefox < 88.0.1 (limited platform scope)

Discovery Timeline

• 2021-06-24 - CVE-2021-29953 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-29953

Vulnerability Analysis

This vulnerability is classified as a Universal Cross-Site Scripting (UXSS) flaw (CWE-79), which represents one of the most severe categories of XSS vulnerabilities. Unlike traditional reflected or stored XSS that affects a single application, UXSS vulnerabilities exist within the browser itself, allowing malicious code execution in the context of any domain.

The vulnerability specifically affects Firefox for Android, with other operating systems remaining unaffected. When exploited, an attacker can craft a malicious webpage that, when visited by a Firefox for Android user, forces the execution of attacker-controlled JavaScript within the security context of a completely different domain.

Root Cause

The root cause of CVE-2021-29953 lies in improper enforcement of the browser's same-origin policy within Firefox for Android. The same-origin policy is a critical security mechanism that restricts how scripts from one origin can interact with resources from another origin. A flaw in the Android-specific implementation allowed this security boundary to be bypassed, enabling cross-domain script execution.

Additional technical details were temporarily withheld by Mozilla to allow users time to update before full exploitation methods became public knowledge.

Attack Vector

The attack leverages a network-based vector requiring user interaction. An attacker would need to lure a Firefox for Android user to a malicious webpage. Once the user visits this page, the attacker's JavaScript can execute in the context of other domains the user has sessions with. This could enable:

• Stealing authentication cookies and session tokens from banking sites
• Reading sensitive emails or messages from webmail services
• Performing unauthorized transactions on e-commerce platforms
• Extracting personal data from social media accounts

The exploitation requires no special privileges and works across different domain boundaries, making it particularly dangerous for mobile users who frequently access multiple sensitive services.

Detection Methods for CVE-2021-29953

Indicators of Compromise

• Unusual JavaScript execution patterns in browser console logs originating from unexpected domains
• Cross-origin requests being made without proper CORS headers from the user's browser
• Unexpected modifications to session cookies or authentication tokens
• Anomalous network traffic patterns indicating data exfiltration to third-party domains

Detection Strategies

• Monitor for suspicious redirections or iframe injections on web servers that may indicate UXSS exploitation attempts
• Implement Content Security Policy (CSP) headers to detect policy violations that may indicate cross-origin script execution
• Analyze web application logs for unusual authentication patterns or session anomalies
• Deploy browser-level telemetry to identify JavaScript execution in unexpected contexts

Monitoring Recommendations

• Enable enhanced logging on critical web applications to detect unauthorized cross-origin access
• Monitor user agent strings for vulnerable Firefox for Android versions (< 88.1.3) accessing sensitive resources
• Implement anomaly detection for session activity that suggests compromised authentication
• Review CSP violation reports for patterns consistent with UXSS exploitation

How to Mitigate CVE-2021-29953

Immediate Actions Required

• Update Firefox for Android to version 88.1.3 or later immediately
• Update Firefox desktop to version 88.0.1 or later as a precautionary measure
• Review recent session activity on sensitive accounts for unauthorized access
• Clear browser cookies and re-authenticate to critical services after updating

Patch Information

Mozilla has released security patches addressing this vulnerability. Users should update to Firefox for Android version 88.1.3 or later, and Firefox desktop version 88.0.1 or later. The fix addresses the same-origin policy bypass that enabled Universal XSS attacks.

For detailed patch information, refer to the Mozilla Security Advisory MFSA-2021-20 and the Mozilla Bug Report #1701684.

Workarounds

• Use an alternative mobile browser until Firefox for Android can be updated
• Avoid clicking links from untrusted sources while using vulnerable Firefox versions
• Log out of sensitive accounts when not actively using them to limit session exposure
• Enable additional authentication factors on critical accounts to mitigate session hijacking risks
• Consider using private browsing mode to limit persistent session exposure

# Verify Firefox for Android version
# Navigate to: Settings > About Firefox
# Ensure version is 88.1.3 or higher

# For organizations managing mobile devices, verify browser versions via MDM
# Example policy check for compliant browser version
adb shell dumpsys package org.mozilla.firefox | grep versionName
# Should return versionName=88.1.3 or higher