CVE-2026-6765: Mozilla Firefox Information Disclosure Flaw

CVE-2026-6765 Overview

CVE-2026-6765 is an information disclosure vulnerability affecting the Form Autofill component in Mozilla Firefox and Thunderbird. This vulnerability allows potential exposure of sensitive user data through improper handling of autofill information. The flaw has been classified under CWE-359 (Exposure of Private Personal Information to an Unauthorized Actor), indicating that sensitive user information stored in the browser's autofill feature could be accessed by unauthorized parties.

Critical Impact

Attackers could potentially access sensitive personal information stored in the Form Autofill component, including names, addresses, payment information, and other autofill data without user consent or knowledge.

Affected Products

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

Discovery Timeline

• April 21, 2026 - CVE-2026-6765 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6765

Vulnerability Analysis

This vulnerability exists within the Form Autofill component of Mozilla Firefox and Thunderbird. The Form Autofill feature is designed to store and automatically populate form fields with user data such as addresses, credit card information, and personal details for convenience. The information disclosure flaw allows this sensitive data to be exposed to unauthorized actors through a network-based attack vector that requires no user interaction or special privileges.

The vulnerability is categorized under CWE-359 (Exposure of Private Personal Information to an Unauthorized Actor), which describes scenarios where applications inadvertently expose sensitive user data. In this case, the Form Autofill component fails to properly protect stored personal information, potentially allowing it to leak to malicious websites or actors.

Root Cause

The root cause stems from improper data protection mechanisms within the Form Autofill component. The vulnerability allows confidential autofill data to be accessed without proper authorization checks, enabling information disclosure. Technical details are available in Mozilla Bug Report #2022419.

Attack Vector

The attack vector is network-based, meaning exploitation can occur remotely without requiring local access to the target system. The attack complexity is low and requires no privileges or user interaction. A malicious website or attacker could potentially craft requests or leverage browser behavior to extract sensitive autofill information from the victim's browser. The vulnerability only affects confidentiality—there is no impact on integrity or availability of the system.

Detection Methods for CVE-2026-6765

Indicators of Compromise

• Unusual network requests originating from the browser's autofill component to external domains
• Unexpected form submission activity or autofill data being populated on untrusted websites
• Browser logs showing abnormal access patterns to autofill storage mechanisms

Detection Strategies

• Monitor browser processes for unusual data access patterns related to autofill storage
• Implement network monitoring to detect potential data exfiltration from browser processes
• Deploy endpoint detection solutions to identify exploitation attempts targeting browser components
• Review browser extension activity that may attempt to access autofill data

Monitoring Recommendations

• Enable enhanced browser telemetry to track autofill component behavior
• Configure SIEM rules to alert on suspicious browser-related network traffic patterns
• Monitor for bulk data extraction attempts from user profile directories
• Implement Data Loss Prevention (DLP) rules to detect potential autofill data exfiltration

How to Mitigate CVE-2026-6765

Immediate Actions Required

• Update Mozilla Firefox to version 150 or later immediately
• Update Mozilla Firefox ESR to version 140.10 or later
• Update Mozilla Thunderbird to version 150 or later
• Update Mozilla Thunderbird ESR to version 140.10 or later
• Review and clear sensitive autofill data stored in affected browser versions

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple product lines. The fixes are documented in the following security advisories:

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

Organizations should prioritize updating to Firefox 150, Firefox ESR 140.10, Thunderbird 150, or Thunderbird 140.10 to remediate this vulnerability.

Workarounds

• Disable the Form Autofill feature in Firefox/Thunderbird settings until patches can be applied
• Clear existing autofill data from browser storage as a precaution
• Consider using a dedicated password manager instead of browser-based autofill
• Restrict browsing to trusted websites only until the browser is updated

# Firefox/Thunderbird: Disable Form Autofill via about:config
# Navigate to about:config and set:
extensions.formautofill.addresses.enabled = false
extensions.formautofill.creditCards.enabled = false

# Or via user.js in profile directory:
user_pref("extensions.formautofill.addresses.enabled", false);
user_pref("extensions.formautofill.creditCards.enabled", false);