CVE-2026-109778: Firefox WebExtensions Privilege Escalation

CVE-2026-109778 Overview

CVE-2026-109778 is a privilege escalation vulnerability in the WebExtensions component of Mozilla Firefox and Thunderbird. The flaw allows a crafted extension or web content to perform operations beyond its intended privilege boundary. Mozilla addressed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. The weakness is classified as improper privilege management [CWE-269].

Critical Impact
Successful exploitation enables an attacker to elevate privileges within the browser extension sandbox, potentially gaining access to sensitive user data, cross-origin resources, or protected browser APIs when a user interacts with malicious content.

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-2026-109778 published to the National Vulnerability Database (NVD)
2026-10-14 - Last updated in NVD database

Technical Details for CVE-2026-109778

Vulnerability Analysis

The vulnerability resides in the WebExtensions subsystem, which implements the API surface used by browser extensions. The component is responsible for enforcing privilege boundaries between extension code, web content, and the browser's privileged runtime. An improper privilege management flaw [CWE-269] in this subsystem lets an attacker perform actions that should require higher privileges.

Exploitation requires user interaction, consistent with a scenario in which a victim installs a malicious extension or visits attacker-controlled content that interacts with an installed extension. Successful exploitation can compromise confidentiality, integrity, and availability of data processed by the browser or mail client.

Because Thunderbird shares the Gecko and WebExtensions code base with Firefox, the same defect affects mail client deployments. See Mozilla Security Advisory MFSA-2026-106 and related advisories for Mozilla's technical description.

Root Cause

Mozilla attributes the issue to improper privilege management within the WebExtensions component. The subsystem failed to correctly constrain an operation to the privilege set granted to extension contexts, enabling escalation across the extension-to-browser trust boundary. Specific implementation details are tracked in Mozilla Bug Report #2070547.

Attack Vector

The vulnerability is reachable over the network and requires user interaction. A typical attack path involves convincing a user to install a crafted extension or to load web content that triggers the vulnerable WebExtensions code path. No pre-authentication to a service is required; exploitation occurs client-side within the browser or mail client process.

No public proof-of-concept exploit code is available in the enriched data, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the Mozilla advisories for vendor-authored technical detail.

Detection Methods for CVE-2026-109778

Indicators of Compromise

Installation of unsigned, side-loaded, or recently published extensions on endpoints running vulnerable Firefox or Thunderbird builds.
Unexpected child processes spawned by firefox.exe or thunderbird.exe, or anomalous file writes into user profile directories.
Outbound network connections from browser processes to low-reputation domains following extension installation.

Detection Strategies

Inventory installed browser extensions across the fleet and compare against an approved allowlist.
Monitor endpoint telemetry for Firefox or Thunderbird processes performing privileged file, registry, or credential-store access.
Correlate browser version data from asset management with the fixed versions listed in the Mozilla advisories.

Monitoring Recommendations

Enable browser extension telemetry collection where supported by enterprise policy management.
Alert on new extension IDs observed in extensions.json within user profiles.
Track Firefox and Thunderbird version strings in EDR inventory and flag hosts running builds below the patched versions.

How to Mitigate CVE-2026-109778

Immediate Actions Required

Update Firefox to version 158, Firefox ESR to 153.5, Thunderbird to 158, or Thunderbird 153.5 as applicable.
Audit installed extensions and remove any that are untrusted, unmaintained, or installed outside the organization's approved process.
Enforce extension installation policies through enterprise policy templates to restrict users to vetted add-ons.

Patch Information

Mozilla released fixed builds concurrent with advisories MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110. Apply the vendor-supplied updates through standard patch management or Mozilla's auto-update mechanism.

Workarounds

Disable or remove non-essential WebExtensions until the patched browser version is deployed.
Use the Firefox ExtensionSettings enterprise policy to restrict installation to an approved allowlist.
Isolate browsing of untrusted content using separate browser profiles or application containment.

bash

# Example Firefox enterprise policy restricting extension installation
# File: /etc/firefox/policies/policies.json
{
  "policies": {
    "ExtensionSettings": {
      "*": {
        "installation_mode": "blocked"
      }
    },
    "DisableAppUpdate": false
  }
}