CVE-2026-109769: Firefox Cache Information Disclosure Flaw

CVE-2026-109769 Overview

CVE-2026-109769 is an information disclosure vulnerability in the Networking: Cache component of Mozilla Firefox and Thunderbird. The flaw allows a remote attacker to expose limited confidential data when a user interacts with crafted web content. Mozilla fixed the issue in Firefox 158 and Thunderbird 158. The weakness is categorized as [CWE-200] Exposure of Sensitive Information to an Unauthorized Actor. Exploitation requires user interaction and does not affect integrity or availability.

Critical Impact
An attacker-controlled web resource can trigger the Networking: Cache component to disclose confidential information to an unauthorized party, undermining the confidentiality boundary between origins.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Thunderbird versions prior to 158
Mozilla Networking: Cache component

Discovery Timeline

2026-10-13 - CVE-2026-109769 published to the National Vulnerability Database (NVD)
2026-10-13 - Last updated in the NVD database

Technical Details for CVE-2026-109769

Vulnerability Analysis

The vulnerability resides in the Networking: Cache component shared by Firefox and Thunderbird. The component handles HTTP cache storage and retrieval for network responses. Under specific conditions, the cache layer discloses confidential information to a context that should not have access to it. Mozilla classified the issue as an information disclosure flaw and addressed it in Firefox 158 and Thunderbird 158.

Successful exploitation requires the victim to interact with attacker-controlled web content, such as visiting a crafted page or opening an email that renders remote content in Thunderbird. The attacker gains access to limited sensitive data but cannot modify client state or disrupt the application. See Mozilla Security Advisory MFSA-2026-106 and Mozilla Security Advisory MFSA-2026-109 for Mozilla's official write-up.

Root Cause

The root cause is improper isolation or sanitization within the Networking: Cache component, consistent with [CWE-200]. Cache behavior exposes data that should remain restricted to its originating context. Full technical details are tracked in Mozilla Bug Report #2050313.

Attack Vector

The attack is delivered over the network and requires user interaction. An attacker hosts crafted web content or email content that triggers the vulnerable cache code path. When the victim loads the content in an unpatched Firefox or Thunderbird client, the browser or mail client discloses confidential data to the attacker's context.

No public proof-of-concept, exploit module, or CISA KEV listing is associated with this CVE at the time of publication.

Detection Methods for CVE-2026-109769

Indicators of Compromise

No public indicators of compromise have been published for CVE-2026-109769.
Review browser and mail client telemetry for Firefox and Thunderbird versions earlier than 158 across managed endpoints.

Detection Strategies

Inventory installed Firefox and Thunderbird builds using endpoint software inventory tools and flag versions below 158.
Correlate web proxy logs with user visits to untrusted origins by hosts still running vulnerable Mozilla builds.
Monitor Thunderbird clients for enabled remote content loading, which expands exposure to crafted email content.

Monitoring Recommendations

Alert when endpoints report Firefox or Thunderbird versions earlier than 158 after the organization's patch deadline.
Track outbound HTTP cache-related anomalies, such as unexpected cross-origin requests, in network telemetry.
Watch Mozilla's advisory feed for related follow-up CVEs in the Networking: Cache component.

How to Mitigate CVE-2026-109769

Immediate Actions Required

Upgrade Firefox to version 158 or later on all managed endpoints.
Upgrade Thunderbird to version 158 or later on all workstations that process email.
Verify auto-update is enabled for both Firefox and Thunderbird across the fleet.
Prioritize patching for high-exposure users who browse untrusted content or process external email.

Patch Information

Mozilla released fixes in Firefox 158 and Thunderbird 158. Refer to Mozilla Security Advisory MFSA-2026-106 and Mozilla Security Advisory MFSA-2026-109 for the authoritative patch references and version mapping.

Workarounds

Disable remote content loading in Thunderbird to reduce exposure from crafted email until the client is updated.
Restrict access to untrusted web content through web proxy filtering while patch deployment is in progress.
Enforce enterprise policy to block downgrade of Firefox and Thunderbird to pre-158 builds.

bash

# Verify installed Mozilla versions on Linux endpoints
firefox --version
thunderbird --version

# Enterprise policy example (Firefox policies.json) to enforce updates
# Place at /etc/firefox/policies/policies.json
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true
  }
}