CVE-2026-92022: Firefox DOM HTML Parser Use-After-Free

CVE-2026-92022 Overview

CVE-2026-92022 is a use-after-free vulnerability in the DOM HTML Parser component shared across Mozilla Firefox and Thunderbird. The flaw affects how the browser engine tracks memory during HTML parsing operations. An attacker who convinces a user to load crafted web content can trigger memory that has already been freed, potentially leading to memory corruption. Mozilla addressed the issue across multiple product lines, including 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
Use-after-free conditions in browser DOM parsers can lead to memory corruption and, in worst cases, arbitrary code execution within the content process sandbox.

Affected Products

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

Discovery Timeline

2026-09-15 - CVE-2026-92022 published to the National Vulnerability Database
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92022

Vulnerability Analysis

The vulnerability resides in the DOM HTML Parser, the component responsible for tokenizing and constructing the Document Object Model from incoming markup. A use-after-free occurs when code retains a reference to an object after that object has been deallocated. Subsequent operations on the dangling reference operate on invalid or attacker-influenced memory. In a browser parser, this class of bug typically emerges from reentrancy during script execution, mutation events, or asynchronous parsing steps that invalidate objects still tracked by the parser state machine.

Mozilla's advisory groups this issue among a batch of memory safety fixes distributed across MFSA-2026-90 through MFSA-2026-96. The parallel patching of Firefox, Firefox ESR, and Thunderbird indicates the defective code lives in shared Gecko engine components used by all three products.

Root Cause

The root cause is improper lifetime management of an object referenced during HTML parsing. When the parser continues to operate on a freed allocation, the heap slot can be reclaimed and repopulated by attacker-controlled data before the stale pointer is dereferenced.

Attack Vector

Exploitation requires a victim to render attacker-controlled HTML in a vulnerable Firefox or Thunderbird build. In Firefox, this is achieved by visiting a malicious page. In Thunderbird, remote content rendered from an HTML email can reach the same parser paths. See Mozilla Security Advisory MFSA-2026-90 and the related advisories for the exact affected code paths.

No public proof-of-concept is available. The vulnerability is described in prose only; refer to Mozilla Bug Report #2068059 for technical detail if it becomes publicly accessible.

Detection Methods for CVE-2026-92022

Indicators of Compromise

Unexpected crashes of firefox.exe, firefox-bin, or thunderbird content processes with heap corruption signatures
Content process termination events referencing HTML parsing frames in crash telemetry
Outbound connections from browser child processes to previously unseen domains hosting obfuscated HTML payloads

Detection Strategies

Inventory installed Firefox and Thunderbird versions across managed endpoints and flag builds below the fixed releases
Monitor endpoint telemetry for browser child-process crashes correlated with visits to suspicious URLs or receipt of HTML-formatted email
Alert on browser processes spawning shells, script interpreters, or performing suspicious file writes, consistent with post-exploitation of a content-process bug

Monitoring Recommendations

Ingest browser and mail client crash reports into a centralized logging pipeline for correlation with web proxy and email gateway logs
Track patch compliance for Firefox 156 and the fixed ESR and Thunderbird branches through configuration management tooling
Baseline normal child-process behavior for firefox and thunderbird and alert on deviations such as unexpected process ancestry

How to Mitigate CVE-2026-92022

Immediate Actions Required

Update Firefox to version 156 or later on all endpoints
Update Firefox ESR deployments to 115.41, 140.16, or 153.3 depending on the branch in use
Update Thunderbird to 156, 140.16, or 153.3 to close the same parser flaw in the mail client
Prioritize systems used by high-risk users such as administrators, developers, and executives

Patch Information

Mozilla shipped fixes in Firefox 156, Firefox ESR 115.41, 140.16, and 153.3, and Thunderbird 156, 140.16, and 153.3. Details are published in Mozilla Security Advisory MFSA-2026-90, MFSA-2026-91, MFSA-2026-92, MFSA-2026-93, MFSA-2026-94, MFSA-2026-95, and MFSA-2026-96.

Workarounds

Disable rendering of remote HTML content in Thunderbird and prefer plain-text mail display until patches are deployed
Enforce browser isolation or restrict access to untrusted web content from sensitive endpoints
Deploy web content filtering to block known malicious domains that host exploit payloads

bash

# Configuration example
# Verify Firefox version on Linux endpoints
firefox --version

# Verify Thunderbird version
thunderbird --version

# Enforce plain text mail rendering in Thunderbird (user.js)
user_pref("mailnews.display.prefer_plaintext", true);
user_pref("mailnews.display.html_as", 1);