CVE-2026-100825: Firefox JavaScript JIT Use-After-Free Flaw

CVE-2026-100825 Overview

CVE-2026-100825 is a use-after-free vulnerability [CWE-416] in the Just-In-Time (JIT) compiler component of the JavaScript engine used across Mozilla Firefox and Thunderbird. An attacker who convinces a user to load a crafted web page can trigger memory corruption in the browser process. Successful exploitation can lead to arbitrary code execution within the affected application's context. Mozilla addressed the flaw in Firefox 157, Firefox ESR 153.4, Thunderbird 157, and Thunderbird 153.4.

Critical Impact
Remote attackers can achieve code execution in the browser or mail client process by luring a user to visit malicious content that exercises the vulnerable JIT code path.

Affected Products

Mozilla Firefox versions prior to 157
Mozilla Firefox ESR versions prior to 153.4
Mozilla Thunderbird versions prior to 157 and 153.4

Discovery Timeline

2026-09-29 - CVE-2026-100825 published to the National Vulnerability Database
2026-09-30 - Last updated in NVD database

Technical Details for CVE-2026-100825

Vulnerability Analysis

The flaw resides in the JavaScript engine's JIT compiler, which converts hot JavaScript code paths into optimized native machine code at runtime. A use-after-free condition occurs when the engine continues to reference a memory object after it has been released. An attacker who controls the JavaScript executed by the JIT can influence allocation and free operations to reclaim the freed region with attacker-controlled data. Manipulating the dangling reference then allows corruption of program state within the content or parent process.

Because the JIT emits native code, exploitation of memory corruption in this component is a well-established path to arbitrary code execution. User interaction is required — the victim must load attacker-controlled content — but no privileges or authentication are needed. In Thunderbird, script execution is disabled in email by default, but the shared engine remains reachable through other content surfaces.

Root Cause

The root cause is improper lifetime management of an object referenced by generated JIT code, classified as [CWE-416] Use After Free. Mozilla has not published low-level technical details in the public advisories referenced by this CVE.

Attack Vector

Exploitation is network-based. An attacker hosts a malicious page or delivers crafted content that a user opens in an unpatched Firefox or Thunderbird build. When the JIT compiles and executes the crafted JavaScript, the use-after-free is triggered, enabling memory corruption primitives that lead to code execution.

// No verified public proof-of-concept is available for CVE-2026-100825.
// Refer to Mozilla Foundation Security Advisories MFSA-2026-97, -100, -101, and -103
// for vendor guidance.

Detection Methods for CVE-2026-100825

Indicators of Compromise

Unexpected child process creation from firefox.exe or thunderbird.exe, particularly command shells, script interpreters, or LOLBins.
Browser or mail client process crashes with access violation signatures originating from JIT-generated code regions.
Outbound connections from the browser process to newly registered domains immediately after rendering untrusted content.

Detection Strategies

Inventory installed Firefox, Firefox ESR, and Thunderbird versions across managed endpoints and flag builds older than 157 / 153.4.
Alert on anomalous memory allocation patterns and RWX regions in browser processes indicative of JIT abuse.
Correlate web proxy logs with endpoint telemetry to identify users who visited suspicious sites before browser crashes.

Monitoring Recommendations

Ingest browser and mail client crash telemetry into centralized logging for pattern analysis.
Monitor for post-exploitation behavior such as credential access, persistence mechanisms, and lateral movement following a browser session.
Track Mozilla advisories MFSA-2026-97, MFSA-2026-100, MFSA-2026-101, and MFSA-2026-103 for updated indicators.

How to Mitigate CVE-2026-100825

Immediate Actions Required

Update Firefox to version 157 or later on all endpoints.
Update Firefox ESR deployments to 153.4 or later.
Update Thunderbird to 157 or 153.4 as appropriate for the deployed channel.
Restart the affected applications after patching to ensure updated binaries load.

Patch Information

Mozilla has released fixed builds addressing the use-after-free. Full details are available in the Mozilla Security Advisory MFSA-2026-97, MFSA-2026-100, MFSA-2026-101, and MFSA-2026-103. The upstream tracking bug is documented in Mozilla Bug Report #2057465.

Workarounds

Restrict browsing to trusted sites via web filtering until patches are deployed.
Ensure Thunderbird's default setting that disables JavaScript in email remains enforced by policy.
Deploy browser isolation for high-risk user groups where immediate patching is not feasible.

bash

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

# Verify installed Thunderbird version
thunderbird --version

# Windows: query installed version via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion