CVE-2026-92015: Firefox WebExtensions Privilege Escalation

CVE-2026-92015 Overview

CVE-2026-92015 is a privilege escalation vulnerability in the WebExtensions component of Mozilla Firefox and Thunderbird. The flaw allows an attacker to elevate privileges within the browser extension subsystem when a user interacts with malicious content. Mozilla addressed the issue across multiple release trains, including Firefox 156, Firefox ESR 115.41, Firefox ESR 140.16, Firefox ESR 153.3, Thunderbird 156, Thunderbird 140.16, and Thunderbird 153.3. The vulnerability is categorized under [CWE-269] Improper Privilege Management.

Critical Impact
Successful exploitation lets an attacker gain elevated permissions inside the browser's extension boundary, potentially exposing user data, browsing sessions, and extension-controlled resources.

Affected Products

Mozilla Firefox versions prior to 156
Mozilla Firefox ESR versions prior to 115.41, 140.16, and 153.3
Mozilla Thunderbird versions prior to 156, 140.16, and 153.3

Discovery Timeline

2026-09-15 - CVE-2026-92015 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92015

Vulnerability Analysis

The vulnerability resides in the WebExtensions component, the subsystem that manages browser extension APIs, permissions, and privilege boundaries. WebExtensions run with elevated capabilities compared to standard web content, including access to tabs, storage, network requests, and cross-origin resources. A privilege escalation flaw in this component enables an attacker-controlled context to acquire permissions it should not hold.

Exploitation requires user interaction, consistent with a browser-delivered attack. A victim visiting a malicious page or handling a crafted message in Thunderbird can trigger the flaw, allowing the attacker to bypass the extension permission model. The impact spans confidentiality, integrity, and availability of data reachable through the browser process.

Root Cause

The issue maps to [CWE-269] Improper Privilege Management within the WebExtensions permission enforcement path. Mozilla's advisories (MFSA-2026-90 through MFSA-2026-96) describe the fix as tightening privilege boundaries in the extension component. Full technical detail is restricted while patches propagate, and the Mozilla Bug Report #2060235 remains the authoritative reference.

Attack Vector

The attack is delivered over the network through the browser or mail client. An attacker hosts crafted content that, when rendered or acted upon by the victim, invokes the vulnerable WebExtensions code path. No prior authentication is required. Because Thunderbird shares the Gecko rendering stack, message preview and remote content loading extend the attack surface beyond web browsing.

No verified public proof-of-concept code is available. See the Mozilla Bug Report #2060235 and the Mozilla Security Advisory MFSA-2026-90 for vendor-published technical details.

Detection Methods for CVE-2026-92015

Indicators of Compromise

Unexpected WebExtensions with elevated permissions appearing in user profiles without administrator approval.
Firefox or Thunderbird process versions below the patched builds still running in the environment.
Outbound connections from browser processes to unfamiliar domains shortly after visiting untrusted content.

Detection Strategies

Inventory installed browser and mail client versions across endpoints and flag any instance older than the patched releases.
Monitor extension install events and permission grant changes in the Firefox profile directory (extensions.json, addonStartup.json.lz4).
Correlate suspicious child process creation from firefox.exe or thunderbird.exe with recent browsing or mail activity.

Monitoring Recommendations

Enable endpoint telemetry for file writes to browser profile directories and extension storage paths.
Alert on Firefox or Thunderbird processes spawning shells, scripting interpreters, or LOLBins.
Track browser update compliance through configuration management to ensure ESR channels remain current.

How to Mitigate CVE-2026-92015

Immediate Actions Required

Upgrade Firefox to version 156 or later on all managed endpoints.
Upgrade Firefox ESR to 115.41, 140.16, or 153.3 based on the deployed ESR channel.
Upgrade Thunderbird to 156, 140.16, or 153.3 as appropriate for the installed channel.
Audit installed WebExtensions and remove any unrecognized or unnecessary add-ons.

Patch Information

Mozilla published fixes across the MFSA-2026-90 through MFSA-2026-96 advisory set. Administrators should apply the update matching their release channel. Enterprise deployments using Firefox ESR should validate compatibility with existing policies before rolling out.

Workarounds

Restrict WebExtension installation through the ExtensionSettings enterprise policy, allowlisting only vetted add-ons.
Disable remote content loading in Thunderbird to reduce message-borne exposure until patching completes.
Enforce browser update automation so ESR and rapid-release channels receive fixes without user intervention.

bash

# Example Firefox enterprise policy restricting WebExtension installation
# policies.json placed in the Firefox distribution directory
{
  "policies": {
    "ExtensionSettings": {
      "*": {
        "installation_mode": "blocked",
        "blocked_install_message": "Contact IT to request an approved extension."
      }
    },
    "DisableAppUpdate": false
  }
}