CVE-2026-109761: Firefox WebAssembly Use-After-Free Flaw

CVE-2026-109761 Overview

CVE-2026-109761 is a use-after-free vulnerability [CWE-416] in the JavaScript WebAssembly component shared by Mozilla Firefox and Thunderbird. The flaw allows a remote attacker to trigger access to freed memory when a victim loads crafted web content that exercises the affected WebAssembly code path. Mozilla fixed the issue in Firefox 158 and Thunderbird 158.

Exploitation requires user interaction, such as visiting a malicious page or opening message content that renders attacker-controlled WebAssembly. Successful exploitation can lead to limited disclosure of process memory or integrity impact within the renderer context.

Critical Impact
A remote attacker who convinces a user to load crafted content can trigger use-after-free conditions in the WebAssembly engine, potentially leaking memory or corrupting state in the Firefox or Thunderbird process.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Thunderbird versions prior to 158
JavaScript WebAssembly component shipped in SpiderMonkey

Discovery Timeline

2026-10-13 - CVE-2026-109761 published to the National Vulnerability Database (NVD)
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109761

Vulnerability Analysis

The vulnerability is a use-after-free condition in the JavaScript WebAssembly subsystem used by Firefox and Thunderbird. In this class of defect, code retains a reference to a memory object after that object has been released back to the allocator. Subsequent operations on the stale reference read or write memory that may now hold unrelated data or attacker-influenced content.

According to the Common Vulnerability Scoring System (CVSS) vector, the attack is delivered over the network and requires user interaction, with low complexity and no privileges. Impact is limited to low confidentiality and low integrity, with no availability impact recorded. See the Mozilla Security Advisory MFSA-2026-109 and Mozilla Security Advisory MFSA-2026-106 for vendor technical details.

Root Cause

The root cause is improper object lifetime management in the WebAssembly code path. A reference used by the engine outlives the backing allocation, producing a dangling pointer. When that pointer is dereferenced, the engine operates on freed memory, which can be reclaimed and repopulated by other allocations in the same process.

Attack Vector

An attacker hosts a page or crafts HTML message content that loads a specific WebAssembly module and triggers the vulnerable sequence. The victim must open the page in Firefox or render the content in Thunderbird. No authentication is required, and the attack is delivered entirely over the network.

No verified public exploit code or proof-of-concept is referenced in the advisory. Additional reproduction details are tracked in Mozilla Bug Report #2071764.

Detection Methods for CVE-2026-109761

Indicators of Compromise

Unexpected Firefox or Thunderbird process crashes with signatures referencing the JavaScript or WebAssembly subsystem
Browser or mail client telemetry showing repeated crashes shortly after loading pages that deliver WebAssembly modules
Outbound connections from end-user hosts to unfamiliar domains immediately preceding renderer crashes

Detection Strategies

Inventory installed Firefox and Thunderbird versions across the fleet and alert on any instance below version 158
Correlate web proxy logs for application/wasm responses from low-reputation domains with subsequent process instability on the same endpoint
Monitor for child-process anomalies spawned by firefox.exe or thunderbird.exe following rendering of untrusted content

Monitoring Recommendations

Ingest endpoint crash reports and browser version telemetry into a central analytics platform for continuous version compliance checks
Track WebAssembly download patterns at the network perimeter to identify suspicious delivery infrastructure
Review authenticated user sessions on hosts that experienced renderer crashes to confirm no follow-on credential or data access occurred

How to Mitigate CVE-2026-109761

Immediate Actions Required

Upgrade Firefox to version 158 or later on all managed endpoints
Upgrade Thunderbird to version 158 or later on all managed endpoints
Validate deployment through software inventory tooling and remove any older side-by-side installations

Patch Information

Mozilla addressed CVE-2026-109761 in Firefox 158 and Thunderbird 158. Administrators should consult Mozilla Security Advisory MFSA-2026-109 and Mozilla Security Advisory MFSA-2026-106 for the authoritative list of fixed builds and bundled security fixes. Enterprise administrators using the Extended Support Release should confirm that their ESR channel includes the backported patch before deferring the standard release upgrade.

Workarounds

Disable WebAssembly execution in Firefox by setting javascript.options.wasm to false through enterprise policy where business workflows allow
Restrict Thunderbird from rendering remote content in messages until the upgrade is deployed
Enforce web filtering policies that block known malicious domains delivering untrusted WebAssembly payloads

bash

# Example Firefox enterprise policy to disable WebAssembly as a temporary workaround
# Place in policies.json under the distribution directory
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}