CVE-2026-109767: Firefox Networking Information Disclosure

CVE-2026-109767 Overview

CVE-2026-109767 is an information disclosure vulnerability in the Networking component of Mozilla Firefox and Thunderbird. Mozilla fixed the issue in Firefox 158, Firefox Extended Support Release (ESR) 153.5, Thunderbird 158, and Thunderbird 153.5. The flaw is classified under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor. Exploitation requires user interaction over the network and can disclose limited confidential data without impacting integrity or availability.

Critical Impact
An attacker-controlled web resource can trigger limited information disclosure from the browser's Networking component when a user visits a crafted page.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Firefox ESR versions prior to 153.5
Mozilla Thunderbird versions prior to 158 and 153.5

Discovery Timeline

2026-10-13 - CVE-2026-109767 published to the National Vulnerability Database (NVD)
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109767

Vulnerability Analysis

The vulnerability resides in the Networking component shared by Firefox and Thunderbird. Mozilla categorizes the issue as information disclosure, tracked under CWE-200. An attacker exploits the flaw by convincing a user to interact with crafted web content or a message that triggers the vulnerable networking code path. The resulting leakage is limited to confidentiality; integrity and availability of the affected process remain intact. Full technical specifics are coordinated through Mozilla's advisories listed in the references.

Root Cause

Mozilla's public advisory describes the defect as information disclosure in networking code but does not disclose the specific function or data structure. See the Mozilla Bug Report #2074963 and Mozilla Security Advisory MFSA-2026-106 for coordinated details once Mozilla lifts access restrictions on the bug.

Attack Vector

Exploitation occurs over the network and requires user interaction, such as visiting a malicious website in Firefox or rendering attacker-controlled content in Thunderbird. No authentication is required, and the attack does not need elevated privileges on the target system. The scope remains unchanged, meaning the leaked data stays within the browser's security boundary. No public proof-of-concept exploit code is available at this time, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

No verified exploitation code is publicly available. Refer to the Mozilla Security Advisory MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110 for additional technical context.

Detection Methods for CVE-2026-109767

Indicators of Compromise

No specific indicators of compromise have been published for CVE-2026-109767.
Mozilla has not released file hashes, network signatures, or telemetry patterns associated with exploitation attempts.

Detection Strategies

Inventory endpoint browser and mail client versions to identify Firefox or Thunderbird installations below the fixed releases.
Correlate browser process telemetry with navigation to untrusted or newly observed domains that coincide with user-reported anomalies.
Monitor Thunderbird for rendering of remote content in HTML messages, which is a prerequisite for the user-interaction attack vector.

Monitoring Recommendations

Enable centralized logging of browser version strings via endpoint management or EDR telemetry.
Track execution of outdated Firefox and Thunderbird binaries after the patch deadline using software asset management tools.
Review web proxy logs for high-risk user interactions with uncategorized domains during the exposure window.

How to Mitigate CVE-2026-109767

Immediate Actions Required

Upgrade Firefox to version 158 or later on all managed endpoints.
Upgrade Firefox ESR to 153.5 for environments on the extended support channel.
Upgrade Thunderbird to version 158 or 153.5, depending on the deployed branch.
Validate that automatic updates are enabled and functioning for Mozilla products across the fleet.

Patch Information

Mozilla released fixes in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Patch details are documented in Mozilla Security Advisory MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110. Administrators should deploy the vendor-provided installers through standard patch management workflows.

Workarounds

Disable automatic loading of remote content in Thunderbird to reduce the user-interaction attack surface.
Restrict browsing to trusted domains using web proxy allowlists where feasible.
Encourage users to avoid clicking links from untrusted senders until patching is complete.

bash

# Verify installed Firefox version on Linux endpoints
firefox --version

# Verify installed Thunderbird version
thunderbird --version