CVE-2026-92018: Firefox DOM Privilege Escalation Vulnerability

CVE-2026-92018 Overview

CVE-2026-92018 is a sandbox escape vulnerability affecting the DOM: Core & HTML component in Mozilla Firefox and Thunderbird. The flaw allows content running inside the browser's rendering sandbox to break out of its security boundary. Mozilla addressed the issue across multiple product lines, including current release and Extended Support Release (ESR) branches. Fixed versions include Firefox 156, Firefox ESR 115.41, Firefox ESR 140.16, Firefox ESR 153.3, Thunderbird 156, Thunderbird 140.16, and Thunderbird 153.3.

Critical Impact
A sandbox escape in the DOM engine can allow attacker-controlled web content to break process isolation, expanding the impact of any subsequent renderer compromise.

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-2026-92018 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92018

Vulnerability Analysis

The vulnerability resides in the DOM: Core & HTML component, which handles parsing, tree construction, and lifecycle management for Document Object Model (DOM) nodes. A defect in this component allows content executing inside the renderer sandbox to escape process isolation. Sandbox escapes in browsers are high-impact because they remove the primary boundary that limits what a compromised renderer can access on the host system.

Mozilla shipped fixes across the release and ESR branches simultaneously, indicating the code path is present in long-lived DOM logic shared across supported Gecko versions. Thunderbird is affected because it embeds the same rendering engine when processing HTML email content.

Root Cause

Mozilla has not published detailed root-cause information at the time of writing. The advisory attributes the defect to the DOM: Core & HTML subsystem. Refer to Mozilla Bug Report #2064287 and the Mozilla Security Advisory MFSA-2026-90 for further technical detail as it becomes available.

Attack Vector

Exploitation requires a victim to load attacker-controlled web content in a vulnerable Firefox build, or to render a crafted HTML email in a vulnerable Thunderbird build. Successful exploitation escapes the content process sandbox, giving code the privileges of the browser process on the host. This is typically chained with a separate renderer-level memory corruption bug to achieve full compromise.

No public proof-of-concept or exploit code is available. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

Detection Methods for CVE-2026-92018

Indicators of Compromise

No vendor-published indicators of compromise are available for CVE-2026-92018 at this time.
Presence of Firefox or Thunderbird builds below the fixed versions on managed endpoints indicates exposure.

Detection Strategies

Inventory installed Firefox and Thunderbird versions across the fleet and flag hosts running builds earlier than the fixed releases listed by Mozilla.
Correlate browser child-process crashes with subsequent unexpected parent-process behavior, which can indicate sandbox escape attempts.
Monitor for browser processes spawning unexpected child processes or writing to sensitive filesystem locations outside standard profile directories.

Monitoring Recommendations

Enable telemetry from endpoint agents to capture Firefox and Thunderbird process trees, module loads, and file writes.
Alert on outbound connections from renderer or browser processes to newly registered or low-reputation domains hosting active web content.
Track patch compliance metrics for Mozilla products through software asset management to reduce detection blind spots.

How to Mitigate CVE-2026-92018

Immediate Actions Required

Upgrade Firefox to version 156 or the appropriate ESR release: 115.41, 140.16, or 153.3.
Upgrade Thunderbird to version 156, 140.16, or 153.3 depending on the deployed branch.
Prioritize patching on endpoints used for general web browsing and on systems that process untrusted HTML email.

Patch Information

Mozilla addressed CVE-2026-92018 in Firefox 156, Firefox ESR 115.41, Firefox ESR 140.16, Firefox ESR 153.3, Thunderbird 156, Thunderbird 140.16, and Thunderbird 153.3. Detailed advisories are published as MFSA-2026-90 through MFSA-2026-96. Enterprise administrators should deploy updates through managed update channels or configuration tools such as Group Policy for Firefox ESR.

Workarounds

Disable rendering of remote HTML content in Thunderbird and configure it to display messages as plain text where operationally feasible.
Restrict browser use on high-value systems and route general browsing through hardened, patched workstations.
Apply network-level controls that block access to untrusted or uncategorized web content until endpoints are patched.

bash

# Verify installed Firefox version on Linux endpoints
firefox --version

# Verify installed Thunderbird version on Linux endpoints
thunderbird --version

# Windows: query installed version via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion