CVE-2026-109761

Executive Summary

CVE-2026-109761 is a use-after-free vulnerability in the JavaScript: WebAssembly component of Mozilla Firefox, fixed in Firefox 158. An attacker who can cause a victim to load crafted web content may trigger memory corruption in the WebAssembly engine, potentially leading to arbitrary code execution in the context of the browser process. All Firefox versions prior to 158 are presumed affected, and no public proof-of-concept, exploit code, or CISA KEV listing currently exists—making exploit development plausible but presently unconfirmed.

Technical Analysis

The vulnerability is a use-after-free (CWE-416) located in the JavaScript: WebAssembly component. Use-after-free arises when memory backing an object is released while a reference to that object remains live. A subsequent dereference of the dangling reference can allow the attacker to read or write data at an attacker-influenced location, because the freed allocation may be reoccupied by attacker-controlled content (a "heap spray" or reallocation technique).

In the WebAssembly context, the attack surface is the compilation and execution pipeline for Wasm modules embedded in or loaded by web pages. WebAssembly enables low-level, memory-adjacent operations inside a sandboxed linear memory model. A use-after-free in this component typically involves lifecycle mismanagement of an internal object—for example, a Wasm module, instance, function reference, or associated metadata—where the object is torn down (on module unload, instance destruction, or garbage collection) but a still-referenced handle or stale reference remains accessible.

Two important caveats govern this analysis. First, the NVD description is minimal: it identifies only the component (JavaScript: WebAssembly), the vulnerability class (use-after-free), and the fixed version (Firefox 158). It does not disclose specific function names, code paths, or the precise trigger. No function, class, file path, or API parameter names can therefore be stated without fabrication. Second, the exploitation mechanics of a browser-engine use-after-free are well understood at the class level: the attacker must win a race or manipulate object lifetimes so that a freed memory region is reused with controlled data while a dangling reference still points to it.

The consequences of a successful use-after-free in a browser engine are severe because the JavaScript engine and WebAssembly runtime operate within the renderer process. Historically, such flaws are chained with a sandbox-escape vulnerability to achieve full system compromise; absent a sandbox escape, the attacker achieves code execution confined to the renderer process, but with access to page content, credentials, session tokens, and potentially same-origin data.

Because no CVSS vector, CWE mapping, or technical advisory accompanied the CVE record, all severity estimates in this report are inferred from the vulnerability class rather than from vendor-published metrics.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template is available for CVE-2026-109761. The following attack chain is inferred from the vulnerability class (use-after-free in a browser WebAssembly engine) and should be treated as a plausible model, not a confirmed reproduction.

1. Reconnaissance / target identification. The attacker identifies victims running Firefox versions prior to 158. This is trivially achievable: user-agent strings, browser fingerprinting scripts, or a malicious page that simply attempts the exploit and observes behavior can determine whether the target is vulnerable. No special access to the victim's network is required—only that the victim loads attacker-controlled web content.

2. Triggering the flaw (vulnerability class). The attacker crafts a web page that loads malicious WebAssembly content. The exploit repeatedly invokes operations that cause the vulnerable component to allocate and subsequently free an object, while retaining a reference to it. The precise trigger sequence is not disclosed in the CVE record; at the class level, the goal is to create a dangling reference and then cause the freed memory region to be reallocated with attacker-controlled data. This is typically achieved through timing loops, garbage-collection pressure, or object-creation patterns in Wasm and JavaScript.

3. Payload/execution (vulnerability class). Once the freed region is reoccupied by controlled data, the dangling reference is dereferenced. The attacker uses this to corrupt an object's type or a function pointer, redirecting control flow to attacker-supplied code or to a ROP/JOP chain. In a WebAssembly context, the corrupted object may permit reads/writes outside the Wasm linear memory sandbox, breaking the memory-safety boundary of the engine.

4. Post-exploitation impact. Under the most likely model, the attacker gains arbitrary code execution in the renderer process. From there, the attacker can steal data accessible to the page (cookies, tokens, form data), perform actions as the user within the browser, and—if chained with a sandbox-escape bug—escape the browser sandbox to compromise the host. Without a sandbox escape, impact is limited to the renderer sandbox but remains significant for data theft and session hijacking.

The lack of a public exploit means attackers would need to develop a working trigger independently or obtain one privately. The vulnerability's presence in a widely deployed browser ensures a large pool of potentially vulnerable targets for whoever does.

Impact Assessment

Business impact is potentially high for any organization whose users browse the web with Firefox versions prior to 158. The affected asset is the endpoint browser, not a server-side service, so the blast radius is user workstations and the data accessible through them. A use-after-free in a browser engine that permits arbitrary code execution can lead to credential theft, session hijacking, data exfiltration from web applications, and—if combined with a sandbox escape—full endpoint compromise and lateral movement.

Because the vendor published no CVSS vector, no CWE mapping, and no advisory URL, the following subscore reasoning is inferential: an unauthenticated remote attacker (AV:N) with low attack complexity at the class level (AC:L), requiring user interaction in the form of visiting a malicious page (PR:N/UI:R), yields scope-unchanged impact to confidentiality, integrity, and availability of the renderer (C:H/I:H/A:H) in the typical browser-engine use-after-free model. Organizations should not treat the absence of an official score as evidence of low severity—use-after-free in a browser engine is routinely rated High to Critical.

Compounding factors: Firefox is widely deployed, the flaw was fixed in version 158 (meaning all earlier versions are exposed until patched), and no CISA KEV listing, EPSS score, or public exploit currently exists. The absence of active exploitation evidence lowers the immediate likelihood but does not reduce the potential impact if an exploit emerges.

Detection Guidance

Sigma Rule

title: Suspicious Firefox Renderer Crash or Memory Corruption - CVE-2026-109761
id: 8f2a1c74-3b6d-4e91-9a2f-c71d5e0b3a48
status: experimental
description: Detects Firefox renderer process crashes consistent with use-after-free exploitation in the WebAssembly component (CVE-2026-109761).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109761
author: valtersit.com
date: 2026/01/01
tags:
  - attack.t1203
logsource:
  category: application
  product: windows
detection:
  selection_image:
    Image|endswith:
      - '\firefox.exe'
  selection_events:
    EventID:
      - 1000
      - 1001
    Provider_Name: 'Application Error'
  selection_fault:
    Faulting_Module|contains:
      - 'xul.dll'
      - 'mozglue.dll'
      - 'firefox.exe'
  condition: selection_image and selection_events and selection_fault
falsepositives:
  - Legitimate Firefox crashes unrelated to exploitation
level: medium

YARA Rule

rule CVE_2026_109761_Firefox_Wasm_UAF
{
    meta:
        description = "Detects patterns associated with WebAssembly use-after-free exploitation attempts against Firefox prior to 158"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109761"
        date = "2026-01-01"
        severity = "high"

    strings:
        $wasm_magic = { 00 61 73 6D 01 00 00 00 }
        $uaf_marker1 = "wasm" ascii nocase
        $uaf_marker2 = "WebAssembly" ascii nocase
        $heap_spray = { 0C 0C 0C 0C 0C 0C 0C 0C }
        $jscript1 = "WebAssembly.instantiate" ascii
        $jscript2 = "WebAssembly.compile" ascii

    condition:
        $wasm_magic at 0 and
        (2 of ($uaf_marker*) or 1 of ($jscript*) or $heap_spray)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109761 Possible Firefox WebAssembly Use-After-Free Exploit Delivery"; flow:established,to_client; content:"Content-Type|3a| application/wasm"; http_header; content:"WebAssembly"; http_client_body; classtype:attempted-user; reference:cve,2026-109761; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109761; sid:2026109761; rev:1;)

Elastic Detection

process where process.name == "firefox.exe" and
  event.action in ("process_crash", "memory_violation") and
  process.command_line : "*wasm*" and
  process.exit_code != 0

Splunk SPL Query

index=main source=WinEventLog:Application (EventCode=1000 OR EventCode=1001)
  (Faulting_Module="*xul.dll*" OR Faulting_Module="*mozglue.dll*" OR Faulting_Module="*firefox.exe*")
  (Data="*ACCESS_VIOLATION*" OR Data="*0xc0000005*")
| stats count by host, Faulting_Module, Faulting_Application
| where count > 3
| sort - count

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109761,">
  <rule id="2026109" level="7">
    <decoded_as>json</decoded_as>
    <field name="EventID">^1000$</field>
    <field name="Faulting_Application">firefox\.exe</field>
    <description>CVE-2026-109761: Firefox renderer crash possibly related to WebAssembly use-after-free exploitation.</description>
    <mitre>
      <id>T1203</id>
    </mitre>
  </rule>
  <rule id="2026110" level="9">
    <if_sid>2026109</if_sid>
    <field name="Faulting_Module">xul\.dll</field>
    <description>CVE-2026-109761: Repeated Firefox crash in xul.dll - possible WebAssembly use-after-free exploit attempt.</description>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Note: the NVD description states the issue was fixed in Firefox 158, but no vendor advisory URL, commit, or patch file was provided in the intelligence data, so patch verification is not possible from available sources.

1. Immediate mitigations.

   • Upgrade Firefox to version 158 or later. This is the primary remediation. Verify the installed version through the browser's About dialog and confirm the build matches the official Mozilla release.
   • If immediate upgrade is not feasible, restrict browsing on vulnerable instances to trusted, allowlisted sites, and consider disabling or restricting JavaScript and WebAssembly execution where operationally acceptable (for example, via enterprise policy or a content-blocking extension). Note that disabling JavaScript broadly is disruptive.
   • Enforce browser isolation or a remote-rendering approach for high-risk browsing so that a renderer compromise does not touch the endpoint directly.
   • Deploy the detection rules above to identify crash patterns consistent with exploitation attempts.

2. Patch/upgrade steps.

   • Apply the Firefox 158 update through the vendor's standard update channel, or download the official build from Mozilla.
   • Confirm the version post-upgrade and re-run asset inventory to correct records still showing pre-158 versions.
   • Where Firefox Extended Support or managed deployments exist, check the vendor advisory for a corresponding fixed ESR release.

3. Long-term hardening.

   • Maintain an accelerated browser patch cadence; browser engine flaws are high-value targets and are frequently exploited in the wild shortly after technical details emerge.
   • Enable automatic updates and enforce them via enterprise policy to eliminate version drift.
   • Adopt exploit mitigations such as site isolation, sandbox hardening, and OS-level mitigations (ASLR, DEP, CFG) that raise the cost of turning a renderer use-after-free into full code execution.
   • Monitor vulnerability feeds and CISA KEV for future updates to this CVE, including exploit-in-the-wild notification.

References

1. NVD — CVE-2026-109761: https://nvd.nist.gov/vuln/detail/CVE-2026-109761
2. Mozilla Security Advisories (Firefox 158 release): https://www.mozilla.org/security/advisories/
3. CWE-416: Use After Free — https://cwe.mitre.org/data/definitions/416.html
4. MITRE ATT&CK T1203 — Exploitation for Client Execution: https://attack.mitre.org/techniques/T1203/
5. CISA Known Exploited Vulnerabilities Catalog (status check): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat-actor campaign, exploit-in-the-wild activity, or public proof-of-concept is documented for CVE-2026-109761 at the time of writing.