CVE-2026-92040: Firefox WebAssembly Use-After-Free Flaw

CVE-2026-92040 Overview

CVE-2026-92040 is a use-after-free vulnerability in the JavaScript WebAssembly component shared by Mozilla Firefox and Thunderbird. The flaw affects how the JavaScript engine manages memory for WebAssembly objects, allowing references to freed memory to remain accessible. Mozilla addressed the issue in Firefox 156 and Thunderbird 156 through security advisories MFSA-2026-90 and MFSA-2026-94. An attacker who lures a user to a malicious web page containing crafted WebAssembly could trigger memory corruption within the browser process.

Critical Impact
Successful exploitation can lead to memory corruption in the renderer, creating conditions for arbitrary code execution within the browser sandbox.

Affected Products

Mozilla Firefox versions prior to 156
Mozilla Thunderbird versions prior to 156
JavaScript WebAssembly component in the SpiderMonkey engine

Discovery Timeline

2026-09-15 - CVE-2026-92040 published to the National Vulnerability Database
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92040

Vulnerability Analysis

The vulnerability is a use-after-free condition in the WebAssembly implementation within Mozilla's JavaScript engine. Use-after-free flaws occur when a program continues to reference memory after it has been released. In browser contexts, attackers can reclaim the freed allocation with controlled data and manipulate the dangling pointer to influence program execution.

WebAssembly (Wasm) executes low-level bytecode inside the JavaScript engine and interacts closely with garbage-collected JavaScript objects. Object lifetimes across the JavaScript-to-WebAssembly boundary create opportunities for reference-tracking errors. When such an error occurs, the engine may operate on stale pointers during compilation, instantiation, or execution of a Wasm module.

Mozilla addressed the defect in Firefox 156 and Thunderbird 156. Refer to the Mozilla Security Advisory MFSA-2026-90 and Mozilla Security Advisory MFSA-2026-94 for the vendor's technical details.

Root Cause

The root cause is improper lifetime management of an object within the WebAssembly component. Additional technical detail is tracked in Mozilla Bug Report #2024248, which is likely access-restricted while exploitation risk remains elevated.

Attack Vector

Exploitation requires a victim to load attacker-controlled web content that instantiates a malicious WebAssembly module. In Thunderbird, the risk is reduced because scripting is disabled in mail, but the shared engine remains vulnerable when the browser-like context is exposed. Successful triggering of the use-after-free can yield a controllable pointer, which is the first step toward remote code execution in the content process.

No verified proof-of-concept code is publicly available for CVE-2026-92040. See the referenced Mozilla advisories for further technical context.

Detection Methods for CVE-2026-92040

Indicators of Compromise

Unexpected crashes of firefox.exe, firefox, or thunderbird processes with signatures matching heap corruption or use-after-free
Renderer or content process spawning unusual child processes such as cmd.exe, powershell.exe, or shell interpreters
Outbound network connections initiated by the browser to newly registered or low-reputation domains after visiting an unfamiliar page

Detection Strategies

Inventory installed Firefox and Thunderbird versions across managed endpoints and flag any build below 156
Monitor browser crash telemetry for repeated faults originating in the JavaScript or WebAssembly modules
Correlate web proxy logs with endpoint browser telemetry to identify sessions that immediately precede browser process anomalies

Monitoring Recommendations

Ingest browser process telemetry into a centralized data lake for behavioral analysis across the fleet
Alert on browser child-process creation chains that deviate from baseline user activity
Track patch-compliance drift for Firefox and Thunderbird on a recurring schedule

How to Mitigate CVE-2026-92040

Immediate Actions Required

Upgrade Mozilla Firefox to version 156 or later on all endpoints
Upgrade Mozilla Thunderbird to version 156 or later
Restart browser and mail client processes after patch installation to ensure the vulnerable code is unloaded
Verify enterprise auto-update policies are enforced and not blocked by group policy or proxy configuration

Patch Information

Mozilla shipped fixes in Firefox 156 and Thunderbird 156. Consult the Mozilla Security Advisory MFSA-2026-90 for Firefox and the Mozilla Security Advisory MFSA-2026-94 for Thunderbird. Both advisories list the corresponding component-level changes.

Workarounds

Restrict WebAssembly execution through enterprise browser policy where operationally feasible until patches are applied
Enforce strict site isolation and process sandboxing controls in Firefox configuration
Deploy web filtering to block access to untrusted or newly registered domains for high-risk user groups

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