CVE-2026-109749: Firefox WebAssembly Use-After-Free Flaw

CVE-2026-109749 Overview

CVE-2026-109749 is a use-after-free vulnerability [CWE-416] in the JavaScript WebAssembly component shared by Mozilla Firefox and Thunderbird. Mozilla patched the issue in Firefox 158 and Thunderbird 158. An attacker can trigger the flaw by convincing a user to load a crafted web page or email content that exercises the vulnerable WebAssembly code path. Successful exploitation can corrupt memory in the browser or mail client process, potentially leading to limited information disclosure or integrity impact. The flaw requires user interaction and can be reached across the network through standard web content.

Critical Impact
Memory corruption in the WebAssembly engine of Firefox and Thunderbird, reachable from crafted web or email content, with limited confidentiality and integrity impact.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Thunderbird versions prior to 158
Any downstream distribution bundling the affected Gecko/SpiderMonkey engine

Discovery Timeline

2026-10-13 - CVE-2026-109749 published to the National Vulnerability Database
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109749

Vulnerability Analysis

The defect sits inside the WebAssembly subsystem of SpiderMonkey, the JavaScript engine used by Firefox and Thunderbird. A use-after-free occurs when the engine continues to reference a memory object after it has been released. When attacker-controlled WebAssembly code triggers the dangling reference, the engine operates on memory that may already be reallocated for a different purpose. This mismatch can expose adjacent memory contents or allow controlled writes into reused allocations.

Mozilla fixed the issue in Firefox 158 and Thunderbird 158, as documented in Mozilla Security Advisory MFSA 2026-106 and Mozilla Security Advisory MFSA 2026-109. The upstream engineering notes are tracked in Mozilla Bug Report #2067174.

Root Cause

The root cause is improper lifetime management of an object referenced by the WebAssembly runtime. The engine frees the backing allocation while another code path still holds a pointer to it. Subsequent access to that pointer dereferences freed memory, violating the single-owner invariant expected by the garbage collector.

Attack Vector

An attacker hosts a malicious page that serves a crafted WebAssembly module. When a user visits the page in a vulnerable Firefox build, the module drives the engine into the vulnerable code path. In Thunderbird, equivalent content delivered through remote email resources can reach the same code when scripting is enabled. User interaction is required to load the content.

No public exploit code is available for CVE-2026-109749, and no verified proof-of-concept has been released. Technical reproduction details are restricted to the referenced Mozilla advisories and the upstream bug tracker.

Detection Methods for CVE-2026-109749

Indicators of Compromise

Unexpected crashes of firefox.exe, firefox, or thunderbird processes with signatures referencing SpiderMonkey or WebAssembly frames
Browser crash reports mentioning js::wasm modules or heap-use-after-free sanitizer messages
Outbound connections from browser or mail client processes to newly observed domains serving .wasm payloads

Detection Strategies

Inventory installed Firefox and Thunderbird versions across managed endpoints and flag any build earlier than 158
Correlate browser crash telemetry with web proxy logs to identify sites that triggered the fault
Hunt for anomalous child processes spawned by firefox or thunderbird, which may indicate post-exploitation activity following memory corruption

Monitoring Recommendations

Forward endpoint process, crash, and network telemetry to a central analytics tier for retrospective hunting
Alert on repeated SpiderMonkey crashes affecting the same user or originating from the same URL
Monitor DNS and HTTP logs for retrieval of application/wasm content from untrusted origins

How to Mitigate CVE-2026-109749

Immediate Actions Required

Upgrade Firefox to version 158 or later on all managed endpoints
Upgrade Thunderbird to version 158 or later, including portable and user-installed copies
Restart affected applications after patching to ensure the vulnerable process memory is unloaded
Prioritize deployment to users who browse untrusted content or process external email with remote content enabled

Patch Information

Mozilla has shipped fixes in Firefox 158 and Thunderbird 158. Refer to Mozilla Security Advisory MFSA 2026-106 and Mozilla Security Advisory MFSA 2026-109 for the authoritative patch metadata. Linux distributions should pull the corresponding backported packages from their vendor repositories.

Workarounds

Disable JavaScript on untrusted sites using enterprise policy or extensions such as content blockers
Set Thunderbird to block remote content in messages, reducing exposure from email-delivered payloads
Restrict WebAssembly execution via the javascript.options.wasm preference where operationally acceptable until patching is complete

bash

# Enforce minimum Firefox version via enterprise policy (policies.json)
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}