CVE-2026-100817: Firefox & Thunderbird WebAssembly Vulnerability

CVE-2026-100817 Overview

CVE-2026-100817 affects the JavaScript WebAssembly component used in Mozilla Firefox and Thunderbird. Mozilla addressed the issue in Firefox 157 and Thunderbird 157. The vulnerability is categorized by Mozilla as an "Other issue" in the WebAssembly implementation, without a specific CWE assignment or public technical breakdown at the time of publication. Users running earlier versions of either product remain exposed until they update.

Critical Impact
The flaw resides in the WebAssembly engine, a component that executes untrusted code from web content, meaning exploitation vectors include any attacker-controlled page loaded in the browser or rendered content in Thunderbird.

Affected Products

Mozilla Firefox versions prior to 157
Mozilla Thunderbird versions prior to 157
Downstream distributions bundling affected Firefox or Thunderbird builds

Discovery Timeline

2026-09-29 - CVE-2026-100817 published to NVD
2026-09-30 - Last updated in NVD database

Technical Details for CVE-2026-100817

Vulnerability Analysis

The vulnerability exists in the WebAssembly (Wasm) subsystem of Mozilla's JavaScript engine. WebAssembly is a low-level bytecode format that runs inside the browser's JavaScript engine with near-native performance. Because Wasm modules are compiled and executed within the browser's sandbox, any defect in the compilation, validation, or runtime path can be reachable from any web page a user visits.

Mozilla's advisory classifies the finding as an "Other issue" without publishing a detailed root-cause narrative. Refer to Mozilla Security Advisory MFSA-2026-97 and Mozilla Security Advisory MFSA-2026-101 for the vendor's summary. The associated tracking record is available at Mozilla Bug Report #2068670.

Root Cause

Mozilla has not published a public root-cause description beyond identifying the WebAssembly component as the affected subsystem. No CWE identifier has been assigned. The corresponding Bugzilla entry remains the authoritative source for future disclosure of technical specifics.

Attack Vector

An attacker delivers a crafted WebAssembly module through a controlled or compromised web page. When a vulnerable version of Firefox loads the page, or Thunderbird renders remote content that reaches the Wasm pipeline, the malformed module triggers the defective code path. The attack requires no authentication and only the standard action of loading attacker-controlled content.

No verified proof-of-concept has been published for this CVE. The vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-100817

Indicators of Compromise

No public indicators of compromise have been released for CVE-2026-100817 at the time of writing.
Presence of Firefox or Thunderbird builds with a version string below 157 constitutes exposure rather than compromise.

Detection Strategies

Inventory endpoints for installed Firefox and Thunderbird versions and flag any build below 157.
Monitor process telemetry for unexpected child processes, crashes, or memory anomalies originating from firefox.exe, thunderbird.exe, or their Linux and macOS equivalents.
Correlate browser crash telemetry with visits to newly observed or low-reputation domains hosting WebAssembly content.

Monitoring Recommendations

Ingest endpoint process and crash telemetry into a central analytics platform to identify browser instability that may indicate exploitation attempts.
Track outbound network requests from browser processes to unusual destinations following Wasm module downloads (content types application/wasm).
Alert on privilege escalation or new persistence artifacts created by child processes spawned from Firefox or Thunderbird.

How to Mitigate CVE-2026-100817

Immediate Actions Required

Upgrade Firefox to version 157 or later across all managed endpoints.
Upgrade Thunderbird to version 157 or later on all workstations that process email with remote content enabled.
Validate that downstream Linux distributions have pulled the patched Mozilla packages before deploying.

Patch Information

Mozilla addressed the issue in Firefox 157 and Thunderbird 157. Consult Mozilla Security Advisory MFSA-2026-97 and Mozilla Security Advisory MFSA-2026-101 for the full advisory text and package references.

Workarounds

Disable WebAssembly execution as a temporary control by setting javascript.options.wasm to false in about:config on Firefox endpoints where updating is delayed.
In Thunderbird, keep remote content blocked in messages to reduce exposure to attacker-controlled Wasm content.
Restrict browsing to trusted domains via web filtering until patched versions are deployed.

bash

# Configuration example: disable WebAssembly in Firefox via policies.json
# Place at: <Firefox install>/distribution/policies.json (Windows) or /etc/firefox/policies/policies.json (Linux)
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}