CVE-2026-109739: Firefox WebExtensions Privilege Escalation

CVE-2026-109739 Overview

CVE-2026-109739 is a privilege escalation vulnerability in the WebExtensions component of Mozilla Firefox and Thunderbird. The flaw allows an attacker with limited privileges to elevate access within the browser extension framework, potentially gaining capabilities reserved for higher-trust contexts. Mozilla fixed the issue in Firefox 158 and Thunderbird 158. The weakness is categorized under [CWE-269] Improper Privilege Management, affecting the extension privilege boundary enforced by the browser.

Critical Impact
Successful exploitation grants an attacker elevated privileges within the WebExtensions runtime, enabling access to sensitive browser APIs and user data that should remain restricted.

Affected Products

Mozilla Firefox prior to version 158
Mozilla Thunderbird prior to version 158
WebExtensions component across both products

Discovery Timeline

2026-10-13 - CVE-2026-109739 published to NVD
2026-10-14 - Last updated in NVD database

Technical Details for CVE-2026-109739

Vulnerability Analysis

The vulnerability resides in the WebExtensions component, which governs how browser extensions interact with privileged browser APIs. Improper privilege management [CWE-269] allows an actor operating in a lower-privileged context to perform actions reserved for higher-privileged code paths. Mozilla's advisories (MFSA-2026-106 and MFSA-2026-109) list this as a privilege escalation fix shipped in Firefox 158 and Thunderbird 158.

The attack vector is network-based with low attack complexity. The attacker requires low privileges and no user interaction, and the impact spans confidentiality, integrity, and availability of the affected process.

Root Cause

The root cause is improper enforcement of privilege boundaries in the WebExtensions subsystem. A WebExtension or crafted interaction with the component can perform operations that should require higher trust, breaking the isolation model between extension code and browser-privileged code. Technical specifics are tracked in Mozilla Bug Report #2049069, which remains access-restricted at the time of publication.

Attack Vector

An attacker leverages the WebExtensions interface, typically through a malicious or compromised extension or content that interacts with extension messaging. The flaw allows that lower-privileged context to escalate to privileged extension capabilities, leading to arbitrary browser-side actions on behalf of the user.

See the Mozilla Security Advisory MFSA-2026-109 for the authoritative description of impacted code paths.

Detection Methods for CVE-2026-109739

Indicators of Compromise

Installation of unsigned or sideloaded WebExtensions on managed endpoints running Firefox or Thunderbird prior to version 158.
Unexpected extension manifest changes, new permissions, or extensions requesting elevated host permissions after installation.
Outbound connections from firefox.exe or thunderbird.exe to untrusted hosts correlated with extension activity.

Detection Strategies

Inventory Firefox and Thunderbird versions across endpoints and flag any build earlier than 158.
Audit the extensions directory and extensions.json for unauthorized or recently modified WebExtensions.
Correlate browser child process behavior with extension install events to surface privilege misuse.

Monitoring Recommendations

Monitor endpoint telemetry for Firefox and Thunderbird spawning unexpected child processes or writing to sensitive paths.
Track extension installation events through enterprise policy telemetry and group policy logs.
Alert on browser processes accessing credential stores, cookies, or profile data outside normal baselines.

How to Mitigate CVE-2026-109739

Immediate Actions Required

Upgrade Firefox to version 158 or later on all endpoints.
Upgrade Thunderbird to version 158 or later on all endpoints.
Review installed WebExtensions and remove any that are unsigned, unused, or not approved by policy.
Enforce enterprise policies that restrict extension installation to a vetted allowlist.

Patch Information

Mozilla released fixes in Firefox 158 and Thunderbird 158. Patch details are documented in MFSA-2026-106 and MFSA-2026-109. Administrators should deploy the vendor-supplied builds through their standard update channels or managed deployment tooling.

Workarounds

Use the ExtensionSettings enterprise policy to block installation of non-approved extensions until patches are deployed.
Disable or remove any WebExtensions that are not required for business operations.
Restrict user permissions to prevent installation of new extensions on endpoints that cannot be patched immediately.

bash

# Example Firefox enterprise policy to restrict extensions (policies.json)
{
  "policies": {
    "ExtensionSettings": {
      "*": {
        "installation_mode": "blocked"
      }
    },
    "DisableAppUpdate": false
  }
}