CVE-2026-92042: Firefox DOM Race Condition Vulnerability

CVE-2026-92042 Overview

CVE-2026-92042 is a race condition vulnerability in the DOM: Content Processes component shared by Mozilla Firefox and Thunderbird. The flaw affects how concurrent operations are handled within content processes that render web content and messages. Mozilla addressed the issue in Firefox 156, Firefox ESR 153.3, Thunderbird 156, and Thunderbird 153.3.

Critical Impact
A race condition in the Firefox and Thunderbird content process can lead to unpredictable state within the browser sandbox, with potential for memory corruption or security control bypass when triggered by crafted web content.

Affected Products

Mozilla Firefox versions prior to 156
Mozilla Firefox ESR versions prior to 153.3
Mozilla Thunderbird versions prior to 156 and 153.3

Discovery Timeline

2026-09-15 - CVE-2026-92042 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92042

Vulnerability Analysis

The vulnerability resides in the DOM: Content Processes component that Firefox and Thunderbird use to isolate and execute untrusted web content. A race condition occurs when two or more concurrent operations access shared state without proper synchronization. Attackers who can influence timing between operations may reach an unsafe state during a narrow execution window.

Mozilla shipped the fix across four coordinated security advisories: MFSA-2026-90, MFSA-2026-93, MFSA-2026-94, and MFSA-2026-96. The consistent guidance across product lines indicates a shared code path used by both the browser and the mail client for rendering HTML content.

Root Cause

The root cause is missing or insufficient synchronization between concurrent operations in the DOM content process. Race conditions in browser content processes typically stem from unsynchronized access to shared DOM state, IPC message ordering assumptions, or lifetime management of objects referenced across threads. Refer to Mozilla Bug 2049342 for the underlying implementation details.

Attack Vector

Exploitation requires an attacker to serve crafted web content or a crafted HTML email that triggers the vulnerable code path. In Firefox, a user visiting a malicious page is the primary vector. In Thunderbird, previewing or opening a crafted HTML message can invoke the same rendering logic. Successful triggering depends on winning a timing window between concurrent operations.

No public proof-of-concept, exploit code, or evidence of exploitation in the wild has been recorded. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Verified technical details are described in the Mozilla Security Advisory MFSA-2026-90 and its companion advisories.

Detection Methods for CVE-2026-92042

Indicators of Compromise

Unexpected crashes of Firefox or Thunderbird content processes, particularly repeated tab crash events on the same origin.
Crash telemetry entries referencing DOM content process threads or IPC message handling.
Outbound connections from browser or mail processes to unfamiliar domains immediately following a content-process crash.

Detection Strategies

Inventory endpoints running Firefox and Thunderbird and flag any version below Firefox 156, Firefox ESR 153.3, Thunderbird 156, or Thunderbird 153.3.
Correlate browser and mail client crash events with subsequent process creation or network activity to identify attempted exploitation.
Monitor for child processes spawned by firefox.exe or thunderbird.exe that do not match expected browser subprocess patterns.

Monitoring Recommendations

Ingest endpoint process telemetry, crash logs, and DNS queries into a central analytics platform for correlation.
Alert on repeated content-process crashes across multiple users visiting the same URL or receiving the same message.
Track patch compliance for Mozilla products as an ongoing metric in vulnerability management dashboards.

How to Mitigate CVE-2026-92042

Immediate Actions Required

Update Firefox to version 156 or later, and Firefox ESR to 153.3 or later, on all managed endpoints.
Update Thunderbird to version 156 or later, or to 153.3 on the ESR-aligned channel.
Prioritize systems where users regularly render untrusted web content or open external HTML email.

Patch Information

Mozilla released fixes in Firefox 156, Firefox ESR 153.3, Thunderbird 156, and Thunderbird 153.3. Detailed release notes and affected component listings are available in MFSA-2026-90, MFSA-2026-93, MFSA-2026-94, and MFSA-2026-96.

Workarounds

Configure Thunderbird to display messages as plain text to reduce exposure of the HTML rendering path.
Restrict browsing to trusted origins using enterprise policy or a secure web gateway until patches are deployed.
Disable JavaScript for untrusted origins where operationally acceptable to reduce the surface for timing-based exploitation.

bash

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

# Verify installed Thunderbird version
thunderbird --version

# Example: enforce minimum version via configuration management
# (Ansible task fragment)
# - name: Ensure Firefox is patched
#   package:
#     name: firefox
#     state: latest