CVE-2026-109748: Firefox WebAssembly Use-After-Free Flaw

CVE-2026-109748 Overview

CVE-2026-109748 is a use-after-free vulnerability [CWE-416] in the JavaScript WebAssembly component shared by Mozilla Firefox and Thunderbird. The flaw affects memory management inside the WebAssembly engine and requires user interaction, such as visiting a crafted web page or opening attacker-controlled content. Mozilla fixed the issue in Firefox 158 and Thunderbird 158. Successful exploitation can lead to limited impact on confidentiality and integrity. No public exploit code is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
Processing malicious WebAssembly content in an unpatched Firefox or Thunderbird build can trigger a use-after-free condition in the JavaScript engine, enabling memory corruption that attackers may chain into further compromise.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Thunderbird versions prior to 158
JavaScript WebAssembly component shared across both products

Discovery Timeline

2026-10-13 - CVE-2026-109748 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109748

Vulnerability Analysis

The vulnerability is a use-after-free condition inside the JavaScript WebAssembly subsystem used by Firefox and Thunderbird. A use-after-free occurs when code continues to reference memory after it has been released, allowing an attacker to influence the reused allocation. In a browser context, the WebAssembly engine compiles and executes untrusted bytecode delivered over the network, making it an attractive attack surface. Exploitation requires the victim to load attacker-controlled content, consistent with the user interaction requirement in the CVSS vector. Mozilla's advisories MFSA-2026-106 and MFSA-2026-109 document the fix landing in Firefox 158 and Thunderbird 158.

Root Cause

The root cause is improper lifetime management of an object in the WebAssembly code path. When the engine releases the underlying allocation but retains a dangling reference, subsequent operations dereference freed memory. Full technical details are tracked in the restricted Mozilla Bug Report #2067169.

Attack Vector

The attack vector is network-based with low complexity and no required privileges. An attacker hosts a web page or email content containing a crafted WebAssembly module. When the victim loads the page in Firefox or renders remote content in Thunderbird, the engine parses and executes the module, triggering the use-after-free. Because WebAssembly runs inside the content process sandbox, the immediate impact is limited to the renderer, but memory corruption can be chained with other flaws to escalate.

No verified public proof-of-concept code is available. Refer to the Mozilla Security Advisory MFSA-2026-106 and Mozilla Security Advisory MFSA-2026-109 for vendor-authoritative detail.

Detection Methods for CVE-2026-109748

Indicators of Compromise

Firefox or Thunderbird content processes crashing with memory access violations shortly after loading WebAssembly content
Outbound requests to domains serving unexpected .wasm modules from untrusted origins
Browser telemetry showing repeated renderer crashes tied to the JavaScript engine

Detection Strategies

Inventory Firefox and Thunderbird installations and flag any instance below version 158
Monitor endpoint crash reports and Windows Error Reporting or macOS CrashReporter entries referencing xul.dll, libxul.so, or JavaScript engine frames
Inspect web proxy logs for downloads of application/wasm content from low-reputation domains

Monitoring Recommendations

Enable centralized collection of browser crash telemetry and correlate spikes across users
Alert on process termination events for firefox.exe and thunderbird.exe that coincide with web browsing activity
Track patch compliance for Mozilla products through endpoint management or vulnerability scanning tooling

How to Mitigate CVE-2026-109748

Immediate Actions Required

Upgrade Firefox to version 158 or later on all managed endpoints
Upgrade Thunderbird to version 158 or later, including ESR channels where applicable
Confirm automatic update settings are enabled so future Mozilla patches deploy without delay

Patch Information

Mozilla released fixes in Firefox 158 and Thunderbird 158. Deployment guidance and advisory text are available in Mozilla Security Advisory MFSA-2026-106 and Mozilla Security Advisory MFSA-2026-109.

Workarounds

Disable JavaScript on untrusted sites using about:config by setting javascript.enabled to false where feasible
Block loading of remote content in Thunderbird by keeping the default setting that prevents remote resources in messages
Restrict WebAssembly execution through enterprise policy, for example by setting javascript.options.wasm to false for high-risk user groups until patching completes

bash

# Example Firefox enterprise policy to disable WebAssembly as a temporary workaround
# /etc/firefox/policies/policies.json
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}