CVE-2026-92049: Firefox Win32 Use-After-Free Vulnerability

CVE-2026-92049 Overview

CVE-2026-92049 is a use-after-free vulnerability in the Widget: Win32 component of Mozilla Firefox and Thunderbird on Windows. The flaw affects the widget subsystem that manages native Win32 UI elements within the browser and mail client. Mozilla addressed the issue in Firefox 156, Firefox ESR 153.3, Thunderbird 156, and Thunderbird 153.3.

Use-after-free conditions in browser widget code can lead to memory corruption. Depending on the object lifetime and heap layout at the time of exploitation, an attacker may achieve arbitrary code execution within the affected process.

Critical Impact
Successful exploitation of a browser widget use-after-free can result in memory corruption inside the Firefox or Thunderbird content or parent process on Windows, potentially enabling code execution in the context of the user.

Affected Products

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

Discovery Timeline

2026-09-15 - CVE-2026-92049 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92049

Vulnerability Analysis

The vulnerability resides in the Widget: Win32 component, which implements native window, event, and input handling for Firefox and Thunderbird on Microsoft Windows. A use-after-free occurs when code retains and dereferences a pointer to a widget object that has already been freed. Reuse of the freed allocation by attacker-influenced data can corrupt object state, virtual function tables, or callback pointers.

Mozilla published fixes across multiple concurrent advisories, including MFSA-2026-90, MFSA-2026-93, MFSA-2026-94, and MFSA-2026-96. Additional technical context is tracked in Mozilla Bug Report #2061295.

Root Cause

The root cause is unsafe object lifetime management in the Win32 widget layer. A widget or associated resource is freed while another code path still holds and later uses a reference to that memory. This class of defect maps to Use After Free ([CWE-416]).

Attack Vector

Exploitation typically requires the target to load attacker-controlled content in Firefox or open a crafted message in Thunderbird that triggers the vulnerable widget code path. The flaw is specific to the Win32 build, so only Windows installations are affected. The published NVD record does not enumerate CWE identifiers or a CVSS vector at this time.

No public proof-of-concept, exploit code, or in-the-wild exploitation has been reported. The vulnerability is not listed on the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-92049

Indicators of Compromise

Unexpected crashes of firefox.exe or thunderbird.exe on Windows, particularly with access violation exceptions inside widget or xul.dll frames.
Windows Error Reporting entries or crash dumps referencing the widget subsystem shortly after loading untrusted web content or email.
Child process spawns from Firefox or Thunderbird that do not match normal browser or mail workflows.

Detection Strategies

Inventory installed Firefox, Firefox ESR, and Thunderbird versions across Windows endpoints and flag any build older than Firefox 156, Firefox ESR 153.3, or Thunderbird 156 / 153.3.
Monitor endpoint telemetry for anomalous memory access violations and heap corruption events originating from Mozilla processes.
Correlate browser or mail client crashes with recent navigation to newly observed domains or receipt of unusual attachments.

Monitoring Recommendations

Ingest Windows application and WER logs into a centralized analytics platform and alert on repeated Firefox or Thunderbird faults.
Track process lineage from firefox.exe and thunderbird.exe for suspicious child processes such as cmd.exe, powershell.exe, or rundll32.exe.
Alert on outbound network connections from Mozilla processes to low-reputation destinations following a crash event.

How to Mitigate CVE-2026-92049

Immediate Actions Required

Upgrade Firefox to version 156 or later on all Windows endpoints.
Upgrade Firefox ESR to version 153.3 or later where extended support builds are deployed.
Upgrade Thunderbird to version 156 or version 153.3 or later, depending on release channel.
Enforce update compliance through management tooling and verify installed versions after deployment.

Patch Information

Mozilla fixed CVE-2026-92049 in Firefox 156, Firefox ESR 153.3, Thunderbird 156, and Thunderbird 153.3. Refer to MFSA-2026-90, MFSA-2026-93, MFSA-2026-94, and MFSA-2026-96 for the corresponding advisory metadata and fixed-version mappings.

Workarounds

No vendor-supplied workaround is documented; patching is the required remediation.
Restrict Thunderbird to plain-text message rendering and disable remote content to reduce the exposure of complex widget code paths to untrusted input.
Apply browser hardening such as strict content filtering, script controls, and least-privilege user accounts on Windows to limit blast radius if exploitation occurs.

bash

# Verify installed Firefox version on Windows (PowerShell)
Get-ItemProperty "HKLM:\SOFTWARE\Mozilla\Mozilla Firefox" | Select-Object CurrentVersion

# Verify installed Thunderbird version on Windows (PowerShell)
Get-ItemProperty "HKLM:\SOFTWARE\Mozilla\Mozilla Thunderbird" | Select-Object CurrentVersion