CVE-2026-109720

Executive Summary

CVE-2026-109720 is a critical JIT (Just-In-Time) miscompilation vulnerability in the JavaScript: WebAssembly component of Mozilla Firefox. This flaw allows an attacker to trigger memory corruption or type confusion through specially crafted WebAssembly bytecode, potentially leading to arbitrary code execution within the browser context. Organizations and individuals running Firefox versions prior to 158 or Firefox ESR prior to 153.5 are affected, with significant risk to enterprise environments where Firefox is a primary endpoint browser.

Technical Analysis

The vulnerability resides in the WebAssembly JIT compiler of Mozilla's SpiderMonkey JavaScript engine. JIT miscompilation occurs when the compiler incorrectly translates WebAssembly bytecode into native machine code, typically due to an incorrect assumption about type states, bounds checks, or register allocation. This can result in type confusion — where the engine treats a value as one type while the underlying memory holds another — or out-of-bounds memory access.

The attack surface is the WebAssembly execution pipeline: any web page that delivers a WebAssembly module can trigger the flaw if the browser processes malicious bytecode. The vulnerability is not a logic bug in the interpreter but a fault in the optimizing compiler, meaning it may only manifest under specific optimization tiers (e.g., IonMonkey or the Wasm baseline compiler) and specific control-flow patterns.

The NVD description explicitly states: "JIT miscompilation in the JavaScript: WebAssembly component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5." No further detail on the exact miscompilation pattern is provided. The vulnerability class is consistent with CWE-843 (Type Confusion) and its exploitation leads to memory corruption.

Because the issue is in JIT compilation, the attacker must cause the browser to compile and execute a WebAssembly module that hits the miscompilation path. This typically requires repeated execution or specific loop structures to trigger optimization. The resulting miscompilation can cause the JIT to emit code that reads or writes memory outside the intended buffer, or that treats a floating-point value as a pointer (or vice versa), enabling control-flow hijacking.

Exploitation Scenarios

No public proof-of-concept (PoC) or exploit code is available for CVE-2026-109720 at this time. The following attack chain is inferred from the vulnerability class (JIT miscompilation in WebAssembly) and the affected component.

1. Reconnaissance / Target Identification: An attacker identifies a victim using a vulnerable version of Firefox (pre-158 or pre-153.5 ESR). This can be done via browser fingerprinting or by targeting a watering-hole site known to be visited by the victim. The attacker does not need network access to the browser; the exploit is delivered through a web page.

2. Triggering the Flaw: The attacker hosts a malicious web page containing a WebAssembly module designed to trigger the JIT miscompilation. The module is crafted to cause the JIT compiler to make an incorrect optimization decision — for example, by exhausting type feedback or creating a specific loop structure that leads to a miscompiled function. When the victim visits the page, the browser compiles and executes the module, and the miscompilation manifests as memory corruption (e.g., an out-of-bounds read/write or type confusion).

3. Payload Execution: The attacker leverages the memory corruption to gain read/write primitives within the browser's renderer process. From there, they can overwrite function pointers or JIT code, redirecting execution to shellcode. Because the exploit runs in the renderer sandbox, the attacker must then chain a sandbox escape to achieve full code execution on the host. Alternatively, the attacker may use the corrupted state to leak sensitive information (e.g., cookies, session tokens) if the corruption allows arbitrary reads.

4. Post-Exploitation Impact: Successful exploitation can lead to arbitrary code execution in the context of the Firefox process. In a compromised enterprise endpoint, this could allow the attacker to install malware, exfiltrate data, pivot to internal networks, or disable security controls. If the renderer sandbox is bypassed, the attacker gains user-level access to the operating system.

No specific function names, API endpoints, or bytecode patterns are provided in the NVD description; therefore, the exact technical trigger cannot be detailed further without fabrication.

Impact Assessment

The CVSS v3.1 vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H) indicates a high-severity vulnerability. The attack is network-accessible (AV:N) with low complexity (AC:L) and requires no privileges (PR:N), but does require user interaction (UI:R) — the victim must visit a malicious page. The scope is unchanged (S:U), but confidentiality, integrity, and availability impacts are all High (C:H/I:H/A:H).

Business impact includes:

• Endpoint compromise: Firefox is widely deployed in enterprises; a successful exploit can lead to full system compromise if chained with a sandbox escape.
• Data breach: Attackers may steal credentials, session tokens, or sensitive documents accessible from the browser.
• Lateral movement: Compromised endpoints can serve as a foothold for further network intrusion.
• Reputation damage: Zero-day exploitation in a widely used browser can erode trust and trigger regulatory scrutiny.

The blast radius includes any organization or individual using Firefox versions below 158 or ESR below 153.5. Given that Firefox ESR is common in enterprises for its extended support cycle, the affected population is substantial. No EPSS score or CISA KEV listing is available, but the lack of public exploit does not reduce the severity for targeted attacks.

Detection Guidance

Sigma Rule

title: Potential Firefox WebAssembly JIT Miscompilation Exploitation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects process creation of Firefox with unusual command-line arguments or crash patterns indicative of WebAssembly JIT exploitation.
author: Threat Intelligence Team
date: 2026-01-01
logsource:
  category: process_creation
  product: windows
detection:
  selection:
    Image|endswith:
      - '\firefox.exe'
      - '\firefox'
  filter_legit:
    CommandLine|contains:
      - '--headless'
      - '--screenshot'
  condition: selection and not filter_legit
fields:
  - CommandLine
  - ParentImage
  - User
falsepositives:
  - Legitimate Firefox launches with unusual flags
level: high

YARA Rule

rule CVE_2026_109720_Firefox_Wasm_JIT_Exploit
{
    meta:
        description = "Detects WebAssembly modules designed to trigger JIT miscompilation in Firefox < 158 / ESR < 153.5"
        author = "Threat Intelligence Team"
        date = "2026-01-01"
        reference = "CVE-2026-109720"
        hash = "unknown"
    strings:
        $wasm_magic = { 00 61 73 6D }
        $jit_trigger1 = "wasm_jit" ascii wide nocase
        $jit_trigger2 = "compile" ascii wide nocase
        $jit_trigger3 = "miscompile" ascii wide nocase
    condition:
        $wasm_magic at 0 and (any of ($jit_trigger*))
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109720 Firefox WebAssembly JIT Exploit Delivery Attempt"; flow:to_server,established; content:"application/wasm"; http_header; content:"|00 61 73 6D|"; depth:4; within:100; classtype:attempted-user; sid:10000001; rev:1;)

Elastic Detection

process where process.name == "firefox.exe" and process.args : ("--headless", "--screenshot") == false and process.parent.name != "explorer.exe"

Splunk SPL Query

index=main sourcetype=WinEventLog:Security EventCode=4688 process_name="firefox.exe" NOT CommandLine="*--headless*" NOT CommandLine="*--screenshot*" | stats count by ComputerName, User, CommandLine

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109720,">
  <rule id="100001" level="12">
    <decoded_as>json</decoded_as>
    <field name="process.name">firefox\.exe</field>
    <field name="process.args">--headless|--screenshot</field>
    <description>CVE-2026-109720: Potential Firefox WebAssembly JIT exploitation attempt detected</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The NVD description states the vulnerability was fixed in Firefox 158 and Firefox ESR 153.5, but no vendor advisory or patch reference is available in the provided data.

1. Immediate Mitigations:

   • Upgrade to Firefox 158 or Firefox ESR 153.5 immediately if these versions are available.
   • If upgrade is not possible, disable WebAssembly execution via about:config by setting javascript.options.wasm to false (note: this may break legitimate web applications).
   • Deploy browser isolation or sandboxing technologies to contain potential renderer compromises.
   • Enforce strict Content Security Policy (CSP) to limit the execution of untrusted scripts.

2. Patch/Upgrade Steps:

   • For Firefox: Navigate to Help > About Firefox and allow automatic updates, or download the latest version from mozilla.org.
   • For Firefox ESR: Update to version 153.5 via your enterprise software distribution system.
   • Verify the update by checking about:support for the version number.
   • Re-enable WebAssembly if it was disabled as a temporary mitigation.

3. Long-Term Hardening:

   • Implement a vulnerability management program that prioritizes browser updates.
   • Use endpoint detection and response (EDR) to monitor for suspicious browser child processes.
   • Restrict browser extensions to a whitelist.
   • Consider using browsers with hardened sandboxes (e.g., Firefox with security.sandbox.content.level set to maximum).
   • Conduct regular penetration testing that includes WebAssembly fuzzing.

References

• NVD — CVE-2026-109720: https://nvd.nist.gov/vuln/detail/CVE-2026-109720
• Mozilla Security Advisories: https://www.mozilla.org/en-US/security/advisories/
• Firefox Release Notes: https://www.mozilla.org/en-US/firefox/releases/
• CWE-843: Type Confusion: https://cwe.mitre.org/data/definitions/843.html
• FIRST EPSS: Not yet scored (as of report date).