CVE-2026-109751: Firefox Networking Information Disclosure

CVE-2026-109751 Overview

CVE-2026-109751 is an information disclosure vulnerability in the Networking component of Mozilla Firefox and Thunderbird. The flaw allows a remote attacker to expose limited sensitive information when a user interacts with attacker-controlled content. Mozilla fixed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. The weakness maps to [CWE-200] Exposure of Sensitive Information to an Unauthorized Actor. No public exploit, proof-of-concept, or in-the-wild exploitation has been reported, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
A remote attacker can trigger disclosure of limited sensitive data from the Networking component through attacker-controlled content requiring user interaction.

Affected Products

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

Discovery Timeline

2026-10-13 - CVE-2026-109751 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109751

Vulnerability Analysis

The vulnerability resides in the Networking component shared by Firefox and Thunderbird. The component mishandles data during network request processing, allowing portions of information to leak to attacker-controlled contexts. The attack requires user interaction, such as visiting a malicious page or opening crafted email content rendered by Thunderbird. Successful exploitation affects confidentiality only; integrity and availability remain unaffected. The flaw is tracked under [CWE-200] Exposure of Sensitive Information to an Unauthorized Actor.

Root Cause

The root cause involves improper handling of data boundaries within the Networking subsystem. Mozilla has not published granular technical details beyond the advisory text. Refer to the Mozilla Bug Report #2068372 and the associated advisories (MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, MFSA-2026-110) for vendor-provided analysis.

Attack Vector

An attacker hosts malicious web content or crafted email that the victim renders in a vulnerable Firefox or Thunderbird client. When the user interacts with the content, the Networking component discloses information the attacker should not receive. No authentication is required, and no elevated privileges are needed. The attacker operates remotely over the network.

// No verified exploit code is available for CVE-2026-109751.
// See the Mozilla security advisories listed above for technical details.

Detection Methods for CVE-2026-109751

Indicators of Compromise

No public indicators of compromise have been published for CVE-2026-109751.
Monitor for Firefox or Thunderbird builds reporting versions earlier than Firefox 158, Firefox ESR 153.5, or Thunderbird 158/153.5 in managed inventories.

Detection Strategies

Inventory browser and mail client versions across the fleet and flag endpoints running unpatched Mozilla builds.
Correlate web proxy and DNS logs with user visits to untrusted domains that could deliver crafted Networking content.
Review Thunderbird telemetry for abnormal outbound connections tied to remote-content loading in email messages.

Monitoring Recommendations

Enable centralized version reporting from endpoint management tooling to identify systems missing the fix.
Track Mozilla advisory feeds and update detections when additional technical details or indicators become public.
Alert on anomalous network traffic patterns originating from browser or mail client processes to atypical destinations.

How to Mitigate CVE-2026-109751

Immediate Actions Required

Upgrade Firefox to version 158 or later on all managed endpoints.
Upgrade Firefox ESR deployments to version 153.5 or later.
Upgrade Thunderbird installations to version 158 or 153.5 or later, depending on the deployed branch.
Prioritize patching for users who routinely browse untrusted content or process external email.

Patch Information

Mozilla released fixes in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Review the vendor advisories (MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, MFSA-2026-110) and roll updates through standard change management.

Workarounds

Disable automatic loading of remote content in Thunderbird to reduce exposure from crafted email messages.
Restrict browsing to trusted sites via web filtering until endpoints are updated.
Enforce automatic update policies for Mozilla products through enterprise policy or configuration management.

bash

# Example: enforce automatic updates for Firefox via policies.json
# Place this file at the Firefox distribution directory on managed endpoints
{
  "policies": {
    "AppAutoUpdate": true,
    "DisableAppUpdate": false,
    "DisableTelemetry": false
  }
}