CVE-2026-92016: Firefox Use-After-Free Vulnerability

CVE-2026-92016 Overview

CVE-2026-92016 is a use-after-free vulnerability in the Disability Access APIs component of Mozilla Firefox, Firefox ESR, and Thunderbird. The flaw affects the accessibility subsystem responsible for exposing application state to assistive technologies. An attacker who can trigger the vulnerable code path may reference memory that has already been freed, which can lead to memory corruption and potentially arbitrary code execution within the browser or mail client process. Mozilla addressed the issue across multiple product branches, including Firefox 156, Firefox ESR 140.16, Firefox ESR 153.3, Thunderbird 156, Thunderbird 140.16, and Thunderbird 153.3.

Critical Impact
Successful exploitation of the use-after-free condition in the Disability Access APIs component can result in memory corruption inside a trusted browser or mail client process, providing a foundation for further exploitation.

Affected Products

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

Discovery Timeline

2026-09-15 - CVE-2026-92016 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92016

Vulnerability Analysis

CVE-2026-92016 is classified as a use-after-free [CWE-416] defect in the Disability Access APIs component shared by Firefox and Thunderbird. Accessibility APIs bridge the rendering engine with platform assistive technologies such as screen readers. These APIs maintain object references that mirror the lifetime of DOM and layout objects. A use-after-free occurs when code retains a pointer to an accessibility object after its underlying storage has been released. Dereferencing that stale pointer allows an attacker-controlled allocation to occupy the freed slot, corrupting internal state.

Mozilla's fix ships in Firefox 156, Firefox ESR 140.16, Firefox ESR 153.3, Thunderbird 156, Thunderbird 140.16, and Thunderbird 153.3. Full technical context is tracked in Mozilla Bug Report #2061327 and the associated advisories MFSA-2026-90, MFSA-2026-92, MFSA-2026-93, MFSA-2026-94, MFSA-2026-95, and MFSA-2026-96.

Root Cause

The root cause is improper lifetime management of accessibility objects in the Disability Access APIs component. When the accessibility layer holds a reference beyond the point where the underlying object is destroyed, subsequent method calls or property lookups operate on freed memory. Attackers can influence the heap layout so that a controlled allocation lands in the freed region.

Attack Vector

The vulnerability is reachable through content processed by Firefox or Thunderbird when the accessibility subsystem is active. A crafted web page or message that manipulates the DOM to force premature destruction of accessibility objects can trigger the free while another code path still holds a reference. Mozilla has not confirmed active exploitation, and no public proof-of-concept is currently linked in the advisory.

No verified proof-of-concept code is publicly available. Refer to the linked Mozilla advisories for authoritative technical details.

Detection Methods for CVE-2026-92016

Indicators of Compromise

Unexpected crashes of firefox.exe, firefox, or thunderbird processes referencing accessibility modules in crash dumps.
Creation of child processes or shell interpreters spawned by the browser or mail client without user interaction.
Outbound network connections initiated by the browser to previously unseen domains immediately following the rendering of untrusted content.

Detection Strategies

Inventory Firefox, Firefox ESR, and Thunderbird installations and flag any version older than the fixed builds (Firefox 156, ESR 140.16, ESR 153.3, Thunderbird 156, 140.16, 153.3).
Correlate browser crash telemetry with recently visited URLs to identify potential exploitation attempts targeting accessibility interfaces.
Monitor endpoint detection and response telemetry for anomalous process trees originating from Mozilla applications.

Monitoring Recommendations

Enable and centralize browser crash reporting to surface repeated faults in accessibility-related modules.
Alert on new persistence mechanisms, scheduled tasks, or registry autoruns created shortly after browser sessions.
Track file writes to user profile directories that deviate from normal browser behavior.

How to Mitigate CVE-2026-92016

Immediate Actions Required

Upgrade Firefox to version 156 or later on all managed endpoints.
Upgrade Firefox ESR deployments to 140.16 or 153.3 depending on the supported branch.
Upgrade Thunderbird to 156, 140.16, or 153.3 to remediate the mail client exposure.
Restart affected applications after patching to ensure the vulnerable code is unloaded from memory.

Patch Information

Mozilla resolved CVE-2026-92016 in Firefox 156, Firefox ESR 140.16, Firefox ESR 153.3, Thunderbird 156, Thunderbird 140.16, and Thunderbird 153.3. Consult MFSA-2026-90 through MFSA-2026-96 for the complete list of fixed issues in each release.

Workarounds

No vendor-sanctioned workaround is documented; applying the fixed versions is the recommended remediation.
For systems that cannot be immediately patched, restrict browsing to trusted sites and disable rendering of untrusted HTML content in Thunderbird.
Enforce least-privilege user accounts to limit the blast radius of any successful browser compromise.

bash

# Configuration example: verify installed Firefox and Thunderbird versions on Linux endpoints
firefox --version
thunderbird --version

# Debian/Ubuntu upgrade example
sudo apt update && sudo apt install --only-upgrade firefox firefox-esr thunderbird