CVE-2026-109722

Executive Summary

CVE-2026-109722 is a JIT miscompilation vulnerability in the JavaScript Engine (JIT component) of Mozilla Firefox. The flaw was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5. An attacker who can deliver crafted JavaScript to a victim's browser may trigger the miscompilation, potentially leading to memory corruption and arbitrary code execution within the browser's renderer process.

Technical Analysis

The NVD description states only that this is a "JIT miscompilation in the JavaScript Engine: JIT component." This is a vulnerability class rather than a mechanism-specific disclosure. The available data does not include the specific JIT optimization pass, function, or bytecode sequence involved, and no CWE is assigned in the source data.

JIT miscompilation vulnerabilities arise when a just-in-time compiler's optimization pass produces machine code that does not preserve the semantic guarantees of the original JavaScript. Typical root causes within this class include:

• Incorrect type inference: The JIT may infer a narrower type for a value than is actually possible at runtime, allowing the generated code to operate on the wrong memory representation.
• Bounds-check elimination errors: An optimization may prove a bounds check redundant and remove it based on assumptions that a later mutation invalidates.
• Speculation and deoptimization mismatches: If the JIT speculates on a property or map shape and the deoptimization path fails to correctly reconstruct interpreter state, the engine can continue executing under stale assumptions.

Because the JIT emits native code directly, a miscompilation can result in type confusion, out-of-bounds access, or use of an incorrectly sized value — all of which are memory-safety primitives that an attacker can chain into arbitrary read/write within the renderer process.

Attack surface: The JavaScript engine is reachable by any web content the browser loads. This makes the vulnerability network-reachable and, critically, remote-triggerable through a single page visit, requiring no prior authentication.

Insufficient technical detail available: The NVD entry does not specify the JIT component internals, the triggering JavaScript construct, or the precise memory-safety primitive. This report avoids fabricating any such details.

Exploitation Scenarios

No public proof-of-concept, Nuclei template, or exploit code is known for CVE-2026-109722. The attack chain below is inferred at the vulnerability-class level from the NVD description; specific function names, bytecode patterns, or heap layouts are deliberately not asserted because the source data does not provide them.

1. Reconnaissance / target identification. An attacker identifies targets running an affected Firefox build (pre-158, pre-115.43 ESR, or pre-153.5 ESR). This can be done passively via browser fingerprinting, user-agent parsing, or by hosting content that conditionally serves the payload only to matching client builds. Affected versions are widely used in both consumer and enterprise endpoints.

2. Delivery and trigger of the JIT miscompilation. The attacker hosts or injects malicious JavaScript into a page (compromised site, malvertising, or watering-hole). When the victim loads the page, the crafted script executes in the JavaScript engine. As a JIT miscompilation flaw, exploitation requires the attacker to construct JavaScript that drives the engine into the faulty optimization. The precise construct is not disclosed in the NVD description and is therefore not reproduced here. At the class level, the attacker supplies input that causes the JIT to emit incorrect native code, producing a type confusion or memory-safety violation analogous to CWE-843.

3. Payload / execution. Once a memory-safety primitive is obtained inside the renderer, the attacker escalates within the sandboxed renderer. Exploitation typically requires chaining the JIT primitive with a separate escape primitive to cross the browser sandbox. No public chain is known for this CVE; the specific chain is not derivable from the provided data.

4. Post-exploitation impact. Successful exploitation yields code execution in the context of the browser. In a compromised renderer, this enables data exfiltration from loaded pages, credential and session-token theft, and — if a sandbox escape is chained — broader host-level execution. The CVSS confidentiality/integrity/availability impact would be High if a full chain is achieved.

Note: Because no PoC is available, the above chain is inferred from vulnerability class. Confidence in the specific trigger mechanism is low; confidence that the class is remotely triggerable via crafted JavaScript is high based on the NVD description.

Impact Assessment

CVE-2026-109722 affects Mozilla Firefox and its Extended Support Release channels. Because the JIT component processes any JavaScript executed by the browser, every user browsing the web with an affected version is a potential target — the blast radius is effectively the entire installed base of unpatched Firefox clients.

Impact characteristics based on the vulnerability class:

• Confidentiality: High — renderer-level code execution can expose page content, cookies, tokens, and in-memory secrets.
• Integrity: High — arbitrary in-memory modification is possible if a write primitive is achieved.
• Availability: High — memory corruption can crash or destabilize the browser process.

CVSS score and vector are officially N/A in the provided data; the vector shown in METADATA is an inference from the vulnerability class and must not be treated as the official NVD vector. The EPSS score has not yet been assigned by FIRST.org and is intentionally not fabricated. CISA KEV status is NO at time of writing. However, JIT engine vulnerabilities in major browsers are high-value targets for exploit brokers, and weaponization should be assumed possible even without a public PoC.

Detection Guidance

Sigma Rule

title: Suspicious Firefox Renderer Crash or Exploit Attempt Activity
id: 8f3a1c2e-4b7d-4e9a-b6c1-2d5f7a9e0b34
status: experimental
description: Detects Firefox renderer process crashes/restarts and anomalous child-process behavior potentially consistent with a JIT memory-corruption exploit (CVE-2026-109722).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109722
author: Valters IT Threat Intelligence
date: 2026/01/01
tags:
  - attack.t1203
logsource:
  category: process_creation
  product: windows
detection:
  selection_image:
    Image|endswith:
      - '\firefox.exe'
  selection_faulting:
    CommandLine|contains:
      - '--contentproc'
  selection_repeat:
    # Correlate multiple rapid renderer launches (crash/restart loops)
    ParentImage|endswith: '\firefox.exe'
  condition: selection_image and selection_faulting and selection_repeat
falsepositives:
  - Normal multi-process browsing with many content processes
  - Legitimate tab crash/restore behavior
level: low

YARA Rule

rule EXPLOIT_Firefox_JIT_Miscompile_CVE_2026_109722
{
    meta:
        description = "Heuristic detection for JavaScript payloads targeting Firefox JIT miscompilation (CVE-2026-109722). Class-level heuristic only; no public PoC exists."
        author = "Valters IT Threat Intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109722"
        date = "2026-01-01"
        severity = "high"

    strings:
        $js1 = "ArrayBuffer" ascii wide
        $js2 = "DataView" ascii wide
        $js3 = "WebAssembly" ascii wide
        $js4 = "new Function" ascii wide
        $obf1 = "\\x68\\x65\\x78" // "hex" obfuscation marker common in staged JS
        $obf2 = "fromCharCode" ascii wide

    condition:
        filesize < 512KB and
        (
            (2 of ($js*)) and
            ($obf1 or $obf2)
        )
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"EXPLOIT Firefox JavaScript JIT miscompilation exploitation attempt (CVE-2026-109722) - suspicious JS with ArrayBuffer/WebAssembly staging"; flow:established,to_server; content:"User-Agent|3a 20|"; http_header; content:"Firefox"; http_header; content:"ArrayBuffer"; http_client_body; nocase; content:"WebAssembly"; http_client_body; nocase; classtype:attempted-user; sid:2026109722; rev:1; metadata:created_at 2026_01_01, cve CVE-2026-109722;)

Elastic Detection

process where event.type == "start" and
process.parent.name == "firefox.exe" and
process.name == "firefox.exe" and
process.args : "*contentproc*"

Splunk SPL Query

index=endpoint (sourcetype=WinEventLog:Security OR sourcetype=sysmon)
(EventCode=1 OR EventID=1)
Image="*\\firefox.exe"
CommandLine="*contentproc*"
| stats count by host, user, CommandLine
| where count > 5
| sort - count

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109722,">
  <rule id="109722" level="6">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">firefox.exe</field>
    <field name="win.eventdata.image">firefox.exe</field>
    <field name="win.eventdata.commandLine">contentproc</field>
    <description>CVE-2026-109722: Repeated Firefox content-process launches, possible JIT exploit crash/restart loop</description>
    <mitre>
      <id>T1203</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Note the NVD description references fixes in specific Firefox releases, but the patch-confidence metadata supplied here records no verified patch source.

1. Immediate mitigations

   • Upgrade to Firefox 158, Firefox ESR 115.43, or Firefox ESR 153.5 or later, as these are the versions named in the NVD description as containing the fix.
   • Where immediate upgrade is not possible, enforce script-blocking or restricted-JavaScript policies via enterprise policy, or restrict general web browsing on affected builds to trusted origins only.
   • Enable browser sandboxing (default in current Firefox) and ensure OS-level exploit mitigations (ASLR, DEP) are active to raise the cost of chaining a renderer primitive.

2. Patch / upgrade steps

   • Push the fixed Firefox / Firefox ESR builds through your managed software deployment channel (e.g., ESR enterprise policies, package managers, or vendor MSI).
   • Verify the installed version with the browser's about dialog or a managed inventory query after deployment.
   • Track Mozilla Security Advisories for the corresponding MFSA that formally documents this CVE.

3. Long-term hardening

   • Maintain an aggressive browser patch SLA; browser renderer vulnerabilities are routinely weaponized in watering-hole and malvertising campaigns.
   • Deploy endpoint detection for renderer crash/restart loops (see Detection Guidance) and monitor for exploit-like child-process behavior.
   • Consider network-layer controls that block known exploit-delivery infrastructure and enforce content security policies on managed web assets.

References

• NVD — CVE-2026-109722: https://nvd.nist.gov/vuln/detail/CVE-2026-109722
• Mozilla Security Advisories (MFSA) index: https://www.mozilla.org/security/advisories/
• Mozilla Firefox release notes (158 / ESR 115.43 / ESR 153.5): https://www.mozilla.org/firefox/releases/
• CWE-843 Type Confusion (vulnerability class reference, not confirmed assignment): https://cwe.mitre.org/data/definitions/843.html
• CISA KEV Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog (CVE-2026-109722 not listed at time of writing)

No public proof-of-concept, exploit code, or in-the-wild campaign attribution is available for CVE-2026-109722 based on the provided data.