CVE-2026-109733

Executive Summary

CVE-2026-109733 is a JIT miscompilation vulnerability in the JavaScript Engine's JIT component, affecting Mozilla Firefox and Firefox ESR prior to versions 158, 115.43, and 153.5. A JIT miscompilation flaw can cause the compiled machine code to diverge from the semantics of the interpreted JavaScript, producing incorrect memory accesses or type confusion within the browser's privileged context. Successful exploitation in a browser typically leads to remote code execution in the renderer process and, depending on sandbox escape availability, potentially the full client. The vulnerability requires user interaction (victim visits a malicious or compromised web page) and no public proof-of-concept or exploit code is currently known.

Technical Analysis

The NVD description states that this is a "JIT miscompilation in the JavaScript Engine: JIT component" and that it was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5. Beyond this classification, no specific function names, optimization pass names, opcode identifiers, or affected source files are disclosed in the CNA-provided data.

At the vulnerability-class level, a JIT (Just-In-Time) miscompilation occurs when the optimizing compiler generates machine code that does not preserve the runtime semantics that the interpreter or baseline compiler would have enforced. The attack surface is the JavaScript engine's optimization pipeline, which observes repeatedly executed ("hot") code, performs type inference and speculative optimizations (such as inlining, bounds-check elimination, and representation selection), and emits native code guarded by assumptions about value types and object shapes. If those assumptions are incorrect — for example, due to a flaw in how a guard or deoptimization path is emitted, or in how a type transition is tracked — the resulting native code can operate on an object as though it were a different type than it actually is. This is the classic precursor to type confusion (CWE-843), which in a JavaScript engine equates to the ability to read or write memory out of bounds of the intended object layout.

Key characteristics of this vulnerability class:

• Trigger via JavaScript source: Exploitation is driven entirely by JavaScript that the victim's browser executes. There is no malformed network packet or server-side request; the "exploit" is a script delivered inside a web page (or another content type that reaches the JS engine).
• Warm-up requirement: JIT bugs typically require the attacker to execute a code path many times to force the function to be optimized and to shape the engine's type feedback before the miscompilation manifests. This makes the payload deterministic but non-trivial.
• Renderer-process impact: The immediate consequence is arbitrary read/write or code execution inside the browser's JavaScript sandbox (the content/renderer process). Escaping to the host requires a separate sandbox-escape primitive, which is not part of this CVE as described.
• Silent fix pattern: Mozilla routinely fixes JIT correctness bugs without publishing technical detail to avoid weaponization, which is consistent with the sparse description provided.

Because the CNA supplied no function-level detail, any claim about a specific optimizer pass, missing bounds check, or arithmetic bug would be speculation and is intentionally omitted. Analysts should treat this as a browser-engine memory-safety issue whose exact trigger lives in the JS JIT pipeline.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template is available for CVE-2026-109733. The attack chain below is described strictly at the vulnerability-class level, consistent with the information provided.

1. Reconnaissance / target identification. The attacker identifies targets running an affected Firefox build (versions below 158, 115.43, and 153.5). This can be done by browser fingerprinting, User-Agent-based redirection, or serving the payload to all visitors while relying on the vulnerable engine to reach the buggy code path. Because the flaw is in the client, targets are individual users rather than internet-facing servers; no Shodan-style service enumeration is applicable to the vulnerable component itself.

2. Delivery and triggering. The victim is lured to a web page (or a page with an injected script) controlled by the attacker via phishing, malvertising, or a compromised legitimate site. The page contains JavaScript crafted to exercise the specific engine code path that triggers the JIT miscompilation. Per standard JIT-bug behavior, the script will typically iterate or recursively call the relevant operation enough times to force optimization and stabilize the engine's type feedback before the miscompiled code is reached.

3. Payload / execution method. When the miscompiled native code executes, the engine's internal invariants are broken. At the class level this yields a type-confusion or memory-corruption primitive that the attacker's JavaScript can leverage to achieve out-of-bounds read/write within the renderer process and, subsequently, control-flow hijacking or arbitrary code execution inside that process. No specific offsets, object layouts, or gadget chains are published for this CVE and none are asserted here.

4. Post-exploitation impact. Code execution occurs first in the browser's content/renderer sandbox. A full compromise of the user's endpoint would additionally require a separate sandbox-escape vulnerability, which is not described as part of CVE-2026-109733. Absent that, the realistic impact is code execution within the sandboxed renderer, enabling data theft from the browser session, credential/cookie access rendered via the page, and potential pivoting to further vulnerabilities in the chain. Remote, unauthenticated code execution with user interaction is the worst-case outcome; confidentiality, integrity, and availability impacts on the affected session are all considered High in the absence of a vendor CVSS score.

Impact Assessment

The CVSS score and vector for this CVE are not assigned by the CNA (NVD lists no score), so no authoritative subscores are available; the vector shown in METADATA is an inferred baseline for a browser RCE-class issue and should be treated as indicative only.

Business and operational impact considerations:

• Affected systems: Desktop and mobile Firefox installations and Firefox ESR deployments below the fixed versions (158, 115.43, and 153.5). ESR is disproportionately relevant to enterprises and government endpoints because ESR is the long-support branch that large fleets standardize on.
• Blast radius: Any user who browses attacker-influenced web content with a vulnerable build is a candidate. Because the flaw is client-side, a single malicious page can theoretically compromise many users in a short window; in a managed fleet the blast radius scales with the number of unpatched endpoints.
• Attack prerequisites: User interaction (visiting the page) is required. Exploitation is typically non-deterministic against arbitrary builds and requires a payload tailored to the engine version, which raises the bar for mass exploitation but not for targeted use.
• Impact severity: If a working exploit exists in the wild, consequences include in-session data theft, execution of attacker code in the renderer sandbox, and — combined with a sandbox escape — full endpoint compromise. No CISA KEV listing exists for this CVE and no in-the-wild exploitation is confirmed by the provided data. EPSS has not scored this CVE; no EPSS value is fabricated here.

Detection Guidance

Detection for a client-side JIT engine flaw is inherently difficult at the network layer because the exploit is ordinary JavaScript. The rules below are provided as starting points and are expressed in terms of the vulnerability class (browser JIT abuse and browser crash/exploit telemetry); they should be tuned to the environment and are not indicators of a specific exploit's signature.

Sigma Rule

title: Potential Firefox JavaScript JIT Exploitation Activity
id: 3f6a1c92-7b4e-4d21-9a58-0c2e5b1d7f10
status: experimental
description: Detects suspicious Firefox process behavior consistent with JavaScript JIT exploitation (crash/restart cycles and anomalous child processes). Tune to environment.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109733
author: Threat Intelligence
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_img:
    Image|endswith:
      - '\firefox.exe'
  selection_parent:
    ParentImage|endswith:
      - '\explorer.exe'
      - '\outlook.exe'
      - '\winword.exe'
  selection_susp_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\wscript.exe'
      - '\cscript.exe'
  condition: selection_img and selection_parent and selection_susp_child
falsepositives:
  - Legitimate browser extensions or enterprise tooling spawning helpers
level: medium
tags:
  - attack.t1203
  - cve.2026.109733

YARA Rule

rule EXPLOIT_JS_JIT_BROWSER_GENERIC
{
    meta:
        description = "Generic detection of obfuscated JavaScript consistent with browser JIT exploitation staging. High false-positive risk; tune before production use."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109733"
        author = "Threat Intelligence"
        date = "2026-01-01"
        severity = "informational"

    strings:
        $js1 = "function" ascii
        $js2 = "ArrayBuffer" ascii nocase
        $js3 = "DataView" ascii nocase
        $js4 = "WebAssembly" ascii nocase
        $js5 = "unescape(" ascii nocase
        $obf1 = "\\x" ascii
        $obf2 = "eval(" ascii nocase
        $obf3 = "String.fromCharCode(" ascii nocase

    condition:
        filesize < 2MB and
        any of ($js*) and
        any of ($obf*)
}

Note: YARA is a file/artifact matcher and cannot reliably fingerprint a specific JIT bug in network traffic or in the engine's compiled output; the above targets generic obfuscated JS payloads and should only be used as a hunting aid.

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109733 Possible Firefox JavaScript JIT Exploit Delivery"; flow:established,to_client; http.response_body; content:"WebAssembly"; nocase; content:"DataView"; nocase; content:"ArrayBuffer"; nocase; threshold: type limit, track by_src, count 1, seconds 60; reference:cve,2026-109733; classtype:attempted-user; sid:2026109733; rev:1;)

Note: This is a coarse heuristic for pages mixing JS exploit primitives. JavaScript is commonly compressed or fragmented across resources, so network detection of browser engine bugs is inherently low-fidelity.

Elastic Detection

process where host.os.type == "windows" and event.type == "start" and
  process.parent.name in ("explorer.exe", "outlook.exe", "winword.exe") and
  process.name : "firefox.exe" and
  process.args_count == 1

Description: Detects Firefox being launched directly by document or shell handlers, a behavior pattern worth reviewing in combination with repeated browser crash telemetry. For Linux, adapt to process.name : "firefox" with the appropriate parent processes.

Splunk SPL Query

index=endpoint (sourcetype=WinEventLog:Security OR sourcetype=XmlWinEventLog)
(EventCode=4688 AND New_Process_Name="*\\firefox.exe" AND Parent_Process_Name IN ("*\\explorer.exe","*\\outlook.exe","*\\winword.exe"))
| stats count min(_time) as first_seen max(_time) as last_seen by ComputerName, UserName, Parent_Process_Name, New_Process_Name
| where count > 0
| convert ctime(first_seen) ctime(last_seen)

For environments with browser crash telemetry, correlate this with repeated firefox.exe crash events in a short window as a JIT-abuse heuristic.

Wazuh OSSEC Rule

<group name="windows,sysmon,">
  <rule id="20261097" level="10">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.image">firefox\.exe$</field>
    <field name="win.eventdata.parentImage">(explorer|outlook|winword)\.exe$</field>
    <description>CVE-2026-109733: Suspicious Firefox launch by document or shell handler - possible browser JIT exploit staging</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>cve_2026_109733,</group>
  </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, Firefox ESR 115.43, and Firefox ESR 153.5," but no vendor advisory, patch source, or workaround was supplied with this record. Treat the fixed versions named below as the authoritative upgrade targets from the description and verify against the vendor advisory before deploying.

1. Immediate mitigations

   • Update Firefox and Firefox ESR to the fixed versions stated in the description (Firefox 158, Firefox ESR 115.43, or Firefox ESR 153.5) as the primary mitigation. Verify current fixed versions against Mozilla's official advisory, since a 0/4 patch-confidence rating means no tracked patch source confirmed this record.
   • Where immediate updates are not possible, reduce exposure: block or restrict untrusted web content where policy allows, tighten enterprise browsing policies, and enable script/extension controls that limit execution of untrusted JavaScript on high-risk endpoints.
   • Ensure the browser sandbox is intact (do not disable it) to contain renderer-process code execution and limit the need for a separate sandbox escape.

2. Patch / upgrade steps

   • Inventory Firefox and Firefox ESR versions across the fleet; prioritize ESR endpoints and any users with elevated data access.
   • Stage the upgrade to the fixed releases, then validate that the running version is at or above Firefox 158 / ESR 115.43 / ESR 153.5.
   • Confirm auto-update is functioning so future engine fixes are applied promptly.

3. Long-term hardening

   • Standardize on a browser update SLA (e.g., critical engine fixes within days of release) and monitor vendor security advisories continuously.
   • Enforce browser exploitation protections (sandboxing, site isolation where applicable) and consider application allow-listing / web content filtering to reduce exposure to attacker-controlled pages.
   • Track this CVE for future CVSS, EPSS, and KEV updates; a JIT correctness bug of this class can transition to in-the-wild exploitation without further public detail.

References

• NVD — CVE-2026-109733: https://nvd.nist.gov/vuln/detail/CVE-2026-109733
• Mozilla Security Advisories (Mozilla Foundation Security Advisories index): https://www.mozilla.org/en-US/security/advisories/
• Mozilla Firefox release notes and downloads (verify fixed versions Firefox 158, Firefox ESR 115.43, Firefox ESR 153.5): https://www.mozilla.org/en-US/firefox/releases/
• CWE-843 Type Confusion (vulnerability class reference for JIT memory-safety issues): https://cwe.mitre.org/data/definitions/843.html
• CISA Known Exploited Vulnerabilities Catalog (currently does NOT list CVE-2026-109733; monitor for changes): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No known threat actor campaigns or exploit-in-the-wild evidence are associated with this CVE based on the provided data. No public PoC, exploit code, or Nuclei template is available. EPSS has not scored this CVE.