CVE-2026-6746: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-6746 Overview

CVE-2026-6746 is a use-after-free vulnerability discovered in the DOM: Core & HTML component of Mozilla Firefox and Thunderbird. This memory corruption flaw occurs when the browser engine attempts to access memory that has already been freed during DOM operations, potentially leading to application crashes or denial of service conditions.

Use-after-free vulnerabilities in browser DOM implementations are particularly concerning as they can be triggered remotely through maliciously crafted web content. An attacker could exploit this vulnerability by enticing a user to visit a specially crafted website or open a malicious email in Thunderbird.

Critical Impact

This use-after-free vulnerability in the DOM: Core & HTML component can be exploited remotely without authentication, potentially causing application crashes and denial of service for affected Mozilla Firefox and Thunderbird users.

Affected Products

• Mozilla Firefox versions prior to 150
• Mozilla Firefox ESR versions prior to 115.35 and 140.10
• Mozilla Thunderbird versions prior to 150 and 140.10

Discovery Timeline

• April 21, 2026 - CVE-2026-6746 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6746

Vulnerability Analysis

This vulnerability (CWE-416: Use After Free) resides in the DOM: Core & HTML component of Mozilla's browser engine. Use-after-free conditions occur when a program continues to use a pointer after the memory it references has been deallocated. In the context of DOM manipulation, this typically happens when JavaScript code or internal browser operations reference a DOM node or object that has already been destroyed.

The vulnerability can be exploited over the network without requiring user credentials, and the attack complexity is low. While the confidentiality and integrity impacts are limited, successful exploitation can cause significant availability disruption through application crashes.

Root Cause

The root cause of CVE-2026-6746 lies in improper memory lifecycle management within the DOM: Core & HTML component. When DOM elements are manipulated through JavaScript or internal browser operations, there appears to be a race condition or improper reference counting that allows freed memory to be accessed. This can occur during complex DOM tree manipulations where object destruction and access operations are not properly synchronized.

Attack Vector

The vulnerability is exploitable via network-based attacks. An attacker can craft malicious HTML/JavaScript content that triggers specific DOM manipulation patterns, causing the browser to access freed memory. The attack scenario typically involves:

1. User visits an attacker-controlled website or opens malicious email content
2. The page executes JavaScript that triggers specific DOM operations
3. A race condition or improper lifecycle management causes memory to be freed prematurely
4. Subsequent DOM operations attempt to access the freed memory
5. The application crashes, resulting in denial of service

The vulnerability does not require any special privileges and can be exploited without user interaction beyond visiting a malicious page. Technical details can be found in the Mozilla Bug Report #2014596.

Detection Methods for CVE-2026-6746

Indicators of Compromise

• Unexpected Firefox or Thunderbird crashes, particularly when browsing untrusted websites
• Crash reports indicating memory access violations in DOM-related components
• Browser error logs showing heap corruption or access violation errors
• Repeated crashes when rendering specific web content or email messages

Detection Strategies

• Monitor endpoint detection systems for abnormal browser crash patterns that may indicate exploitation attempts
• Deploy browser version auditing across the organization to identify unpatched installations
• Review system event logs for repeated application crashes in Firefox or Thunderbird processes
• Implement network monitoring to detect traffic to known malicious domains attempting to exploit browser vulnerabilities

Monitoring Recommendations

• Enable Mozilla crash reporter and review crash telemetry for patterns indicative of use-after-free exploitation
• Configure SIEM alerts for multiple browser crashes from the same endpoint within short timeframes
• Monitor for unusual JavaScript execution patterns that could indicate DOM manipulation attacks
• Track browser versions across endpoints using software inventory management tools

How to Mitigate CVE-2026-6746

Immediate Actions Required

• Update Mozilla Firefox to version 150 or later immediately
• Update Mozilla Firefox ESR to version 115.35 or 140.10 depending on your ESR track
• Update Mozilla Thunderbird to version 150 or 140.10 immediately
• Enable automatic updates to ensure timely security patches for all Mozilla products
• Restrict access to untrusted websites until patches are applied

Patch Information

Mozilla has addressed this vulnerability in multiple product versions. Apply the appropriate update based on your deployment:

Workarounds

• Consider using alternative browsers temporarily until patches can be applied in enterprise environments
• Implement browser isolation solutions to contain potential exploitation impacts
• Configure content security policies to restrict JavaScript execution on untrusted sites
• Enable Firefox's Fission (Site Isolation) feature if not already active to limit cross-origin exploitation

# Check current Firefox version via command line
firefox --version

# For enterprise deployments, update Firefox via package manager
# Debian/Ubuntu:
sudo apt update && sudo apt upgrade firefox

# RHEL/CentOS/Fedora:
sudo dnf update firefox

# For Thunderbird:
sudo apt update && sudo apt upgrade thunderbird