CVE-2026-6762: Mozilla Firefox Spoofing Vulnerability

CVE-2026-6762 Overview

CVE-2026-6762 is an Authentication Bypass vulnerability affecting Mozilla Firefox and Thunderbird applications. The vulnerability exists in the DOM: Core & HTML component, allowing attackers to perform spoofing attacks. This issue enables malicious actors to manipulate how content is presented to users, potentially deceiving them into trusting malicious content or disclosing sensitive information.

Critical Impact

Attackers can exploit this DOM spoofing vulnerability to deceive users by manipulating the visual representation of web content, potentially leading to credential theft, phishing attacks, or unauthorized actions performed on behalf of the user.

Affected Products

• Mozilla Firefox (versions before 150)
• Mozilla Firefox ESR (versions before 115.35 and 140.10)
• Mozilla Thunderbird (versions before 150 and 140.10)

Discovery Timeline

• April 21, 2026 - CVE-2026-6762 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6762

Vulnerability Analysis

This vulnerability is classified under CWE-290 (Authentication Bypass by Spoofing), indicating a weakness where an attacker can bypass authentication mechanisms through identity spoofing techniques. The DOM: Core & HTML component in affected Mozilla products fails to properly validate or handle certain DOM operations, creating an opportunity for content spoofing.

The vulnerability requires user interaction to exploit, meaning a victim must visit a malicious webpage or interact with crafted content. Once exploited, the attacker can achieve limited impact on confidentiality, integrity, and availability of the affected system. The network-based attack vector means this can be exploited remotely without requiring prior authentication.

Root Cause

The root cause stems from improper handling within the DOM: Core & HTML component of Mozilla's rendering engine. The component fails to adequately validate or sanitize certain DOM manipulations, allowing attackers to spoof content presentation. This type of vulnerability typically occurs when the browser's DOM parser does not properly enforce security boundaries between different content origins or when visual rendering does not accurately reflect the true source of content.

Attack Vector

The attack is conducted over the network with low complexity. An attacker must craft a malicious webpage or email (in the case of Thunderbird) that exploits the DOM handling flaw. When a user visits the malicious page or views the crafted email content, the vulnerability can be triggered.

The exploitation scenario typically involves:

1. Attacker creates a webpage containing specially crafted DOM elements
2. Victim navigates to the malicious page or receives malicious email content
3. The DOM spoofing vulnerability allows the attacker to manipulate how content appears
4. The victim may be deceived into entering credentials, clicking malicious links, or trusting spoofed content

For technical details on the specific exploitation mechanism, refer to the Mozilla Bug Report #2021080 and the official Mozilla Security Advisories.

Detection Methods for CVE-2026-6762

Indicators of Compromise

• Unexpected DOM manipulation patterns in browser logs or network traffic
• Users reporting visual inconsistencies or suspicious content appearance on known legitimate websites
• Unusual iframe behavior or content rendering anomalies in Firefox or Thunderbird
• Network requests to suspicious domains following user interaction with seemingly legitimate content

Detection Strategies

• Monitor endpoint behavior for Firefox and Thunderbird processes exhibiting unusual DOM rendering patterns
• Implement content security policies (CSP) to detect and block unauthorized content manipulation attempts
• Deploy browser extension or endpoint detection rules that identify known spoofing techniques
• Review web application firewall logs for requests containing suspicious DOM manipulation payloads

Monitoring Recommendations

• Enable enhanced logging for Firefox and Thunderbird installations to capture DOM-related events
• Monitor for version numbers of Mozilla products to identify unpatched instances across the environment
• Implement network-level monitoring to detect exploitation attempts targeting this vulnerability
• Track user reports of visual anomalies or suspicious content presentation in email or web browsers

How to Mitigate CVE-2026-6762

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 depending on your installation track
• Audit all endpoints to ensure vulnerable versions are identified and scheduled for patching

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple product versions. Organizations should reference the official Mozilla Security Advisories for detailed patch information:

• Mozilla Security Advisory MFSA-2026-30
• Mozilla Security Advisory MFSA-2026-31
• Mozilla Security Advisory MFSA-2026-32
• Mozilla Security Advisory MFSA-2026-33
• Mozilla Security Advisory MFSA-2026-34

Workarounds

• Implement strict Content Security Policy (CSP) headers on internal web applications to reduce DOM manipulation risks
• Consider disabling JavaScript execution for untrusted sources as a temporary measure
• Educate users about verifying website authenticity through address bar indicators rather than page content
• Use email filtering to block suspicious HTML content in Thunderbird environments until patching is complete

# Verify Firefox version on Linux/macOS
firefox --version

# Verify Thunderbird version
thunderbird --version

# Check for vulnerable versions and schedule updates
# Firefox versions before 150 are vulnerable
# Firefox ESR versions before 115.35 or 140.10 are vulnerable
# Thunderbird versions before 150 or 140.10 are vulnerable