CVE-2026-2802: Mozilla Firefox Race Condition Vulnerability

CVE-2026-2802 Overview

A race condition vulnerability has been identified in the JavaScript Garbage Collection (GC) component of Mozilla Firefox and Thunderbird. This flaw exists in versions prior to 148 for both applications, allowing potential exploitation through concurrent access to memory resources during garbage collection operations.

Critical Impact

Attackers may exploit this race condition to achieve limited confidentiality and integrity impact through specially crafted JavaScript that triggers concurrent GC operations.

Affected Products

• Mozilla Firefox versions prior to 148
• Mozilla Thunderbird versions prior to 148

Discovery Timeline

• 2026-02-24 - CVE-2026-2802 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-2802

Vulnerability Analysis

This vulnerability is classified as CWE-362 (Race Condition), occurring within the JavaScript engine's garbage collection subsystem. The flaw arises when multiple threads or execution contexts attempt to access or modify memory simultaneously during GC cycles without proper synchronization.

In the context of browser JavaScript engines, garbage collection is responsible for reclaiming memory that is no longer in use. When a race condition exists in this component, it creates a window of opportunity where memory states can become inconsistent, potentially leading to information leakage or corruption of JavaScript object data.

The vulnerability requires user interaction to exploit, as a victim must visit a malicious webpage or open malicious email content (in the case of Thunderbird) that contains specially crafted JavaScript designed to trigger the race condition during GC cycles.

Root Cause

The root cause stems from insufficient synchronization mechanisms within the JavaScript GC component. When garbage collection processes execute concurrently with JavaScript operations that manipulate object references, a Time-of-Check Time-of-Use (TOCTOU) window can occur. This timing gap allows for inconsistent memory states when the GC marks objects for collection while other threads simultaneously access or modify those objects.

Attack Vector

The attack vector is network-based, requiring the attacker to lure a victim to a malicious website or deliver malicious content through Thunderbird. The exploitation scenario involves:

1. Victim navigates to attacker-controlled webpage or opens malicious email content
2. Malicious JavaScript executes and creates specific object allocation patterns
3. The script triggers garbage collection under controlled timing conditions
4. Race condition is exploited during concurrent GC and object manipulation
5. Attacker achieves limited confidentiality or integrity compromise

The vulnerability mechanism involves manipulating JavaScript object allocation and reference patterns to create timing conditions where the GC component's internal state becomes inconsistent. Detailed technical information is available in Mozilla Bug Report #2011069.

Detection Methods for CVE-2026-2802

Indicators of Compromise

• Unusual JavaScript execution patterns involving rapid object creation and deletion cycles
• Browser crashes or unexpected behavior during intensive JavaScript operations
• Memory anomalies in Firefox or Thunderbird processes during web browsing or email viewing

Detection Strategies

• Monitor for exploitation attempts by analyzing JavaScript patterns that create high volumes of object allocations followed by forced GC triggers
• Deploy endpoint detection solutions capable of identifying race condition exploitation behaviors in browser processes
• Implement web filtering to block access to known malicious domains attempting to exploit this vulnerability

Monitoring Recommendations

• Enable enhanced logging for Firefox and Thunderbird to capture crash reports and JavaScript errors
• Monitor network traffic for connections to suspicious domains delivering JavaScript-heavy content
• Review browser crash dumps for patterns consistent with GC-related race condition exploitation

How to Mitigate CVE-2026-2802

Immediate Actions Required

• Update Mozilla Firefox to version 148 or later immediately
• Update Mozilla Thunderbird to version 148 or later immediately
• Enable automatic updates to ensure timely deployment of future security patches
• Consider restricting JavaScript execution on untrusted websites using browser extensions or security policies

Patch Information

Mozilla has released security patches addressing this vulnerability in Firefox 148 and Thunderbird 148. Detailed patch information is available in the official security advisories:

• Mozilla Security Advisory MFSA-2026-13 (Firefox)
• Mozilla Security Advisory MFSA-2026-16 (Thunderbird)

Organizations should prioritize deployment of these updates across all managed Firefox and Thunderbird installations.

Workarounds

• Temporarily disable JavaScript in Firefox and Thunderbird if updates cannot be immediately applied (note: this significantly impacts functionality)
• Use NoScript or similar browser extensions to selectively block JavaScript on untrusted sites
• Employ network-level web filtering to restrict access to potentially malicious content
• Configure Thunderbird to display emails in plain text mode to reduce JavaScript exposure

# Check Firefox version from command line
firefox --version

# Check Thunderbird version from command line
thunderbird --version

# Example: Verify installed version meets minimum requirement (>=148)