CVE-2026-109746

Executive Summary

CVE-2026-109746 is a mitigation bypass affecting the DOM: Security component of Mozilla Firefox, corrected in Firefox 158. The flaw allows an attacker to circumvent a browser security boundary (a "mitigation bypass") within the DOM security layer, which typically undermines protections such as same-origin enforcement, content-security-policy (CSP) restrictions, or other DOM-scoped security checks. Any user running a Firefox build prior to 158 is potentially affected; the primary impact is client-side compromise achieved through normal browsing of attacker-controlled or compromised web content.

Technical Analysis

The NVD description states: "Mitigation bypass in the DOM: Security component. This vulnerability was fixed in Firefox 158." The CNA title is empty and no CVSS vector, CWE, or detailed technical write-up is provided in the supplied data. Consequently, the following analysis is restricted strictly to what the description supports and to the general vulnerability class — mitigation bypass in a browser DOM security component.

Vulnerability class. A "mitigation bypass" in a browser security context is an issue in which a defense-in-depth control — rather than the core trust boundary itself — fails to hold. In Firefox, the DOM: Security component is responsible for enforcing a range of browser-side security policies that are evaluated against document and DOM state, including:

• Content Security Policy (CSP) enforcement and reporting,
• Mixed content blocking,
• Subresource and navigation restrictions,
• Origin/same-origin isolation checks applied to DOM APIs,
• Trusted Types and related DOM XSS mitigations.

A bypass in this layer means that a page which should have been prevented from loading or executing a resource, script, or navigation is permitted to proceed. Because the component evaluates the security policy, a bypass effectively disables the intended restriction for the affected code path.

Attack surface. The attack surface is the browser rendering and scripting engine as exercised by any web page the user visits. Exploitation is client-side and requires the victim to load attacker-influenced content (a malicious site, a compromised legitimate site, an injected advertisement, or attacker-supplied HTML rendered through a web application). The attacker does not need access to the victim host, credentials, or the network path — only the ability to deliver content that reaches the vulnerable DOM security path.

Root cause (at the vulnerability-class level). Mitigation bypasses typically arise from (a) incomplete coverage of a policy decision across all DOM entry points, (b) incorrect parsing or normalization of inputs used in the security decision, or (c) a check applied at the wrong lifecycle stage so that it can be raced or superseded. Without a vendor advisory or patch diff, it is not possible to attribute a specific root cause, function, symbol, or code path to CVE-2026-109746. No function names, API paths, or DOM interfaces are named in the supplied description and none are asserted here.

Fix status. The only remediation fact provided is that the issue was "fixed in Firefox 158." No patch source, commit reference, MFSA advisory number, or diff is present in the supplied dataset, and patch confidence is recorded as 0/4. Analysts should independently verify the exact fixed build and any ESR backports against Mozilla's own advisory channel before treating any specific version as confirmed-patched.

Relationship to defense-in-depth. Because this is a bypass of a mitigation rather than a standalone memory-safety or RCE primitive, its severity is contingent on what the bypassed control was protecting. In practice, a DOM security mitigation bypass is most often chained: the attacker uses the bypass to get past a restriction that a site or the browser relied upon, then relies on a second issue (e.g., a script-injection sink or an existing XSS) to achieve code execution in the page context. The supplied data provides no evidence of a chain, so this is stated as the class-level mechanism, not as a confirmed exploit path.

Exploitation Scenarios

No public PoC, exploit code, or Nuclei template is available for CVE-2026-109746. The attack chain below is inferred from the vulnerability class (mitigation bypass in a browser DOM security component) and is constrained to what the NVD description supports.

1. Reconnaissance / target identification. The attacker identifies victims by browser fingerprinting — serving the exploit content only to vulnerable Firefox builds while benign content is returned to other user agents. This is standard drive-by practice and requires no special access to the victim. Public scanning services and Shodan-style queries for Firefox-exposing services are of limited use here because the target is a client browser, not a server; the real targeting step is content delivery plus user-agent/feature gating.

2. Triggering the flaw class. The victim navigates to (or is caused to load) attacker-controlled or attacker-injected content. That content is constructed so that a resource, script, or navigation which the DOM: Security layer should block is instead permitted to proceed. At the class level, the attacker arranges for the security decision — CSP enforcement, mixed-content blocking, or an origin/isolation check applied to a DOM operation — to be evaluated in a way that the vulnerable implementation does not catch. The attacker controls the content and its timing; the victim controls only whether the page is loaded.

3. Payload / execution method. Because this is a mitigation bypass, the payload is whatever the bypassed control was supposed to prevent from executing. In the general case, the attacker uses the bypass to load or execute script that the policy should have blocked. No specific injection sink, DOM API, event, or parameter name is named in the supplied description, and none is asserted here. Where a bypass alone does not yield code execution, the attacker would need to pair it with a second, independent injection primitive on the same origin.

4. Post-exploitation impact. Once the bypassed mitigation no longer holds, the attacker operates with the privileges of the affected page/origin within the browser process — for example reading or exfiltrating data accessible to that origin, interacting with authenticated sessions, or manipulating content the user sees and trusts. The impact is confined to the browser's client-side security model unless chained to a further memory-corruption or sandbox-escape issue; no such chain is evidenced in the supplied data.

Impact Assessment

CVSS. No CVSS score or vector is provided by NVD for this CVE (Score: N/A; Vector: N/A). EPSS has not been scored by FIRST.org at the time of this report, and no value should be inferred. CISA KEV status: NO — there is no evidence of known exploitation in the wild within the supplied data. The vector shown in the METADATA block is a class-appropriate analytical baseline for a client-side mitigation bypass and must not be confused with an official NVD score.

Business impact. The affected asset class is end-user workstations running Firefox builds earlier than 158, including developer, engineering, and administrative endpoints where Firefox is the primary or secondary browser. A successful bypass weakens site-isolation and policy controls, increasing exposure to client-side attacks such as script injection into trusted origins and theft of data available to the browser. The blast radius is the individual browser session and the origins that session is authenticated to; there is no evidence of server-side or network-wide impact.

Affected systems. Any system running a Firefox version below 158. Downstream builds and distributions that embed Firefox (ESR variants, Linux distribution packages, and other Gecko-based consumers that track the Firefox 158 baseline) should be reviewed against their vendor advisory, because the supplied dataset contains no affected-version list and no OSV package data.

Severity context. Because a mitigation bypass alone is not typically a self-contained code-execution primitive and no exploit or PoC is public, the near-term operational risk is moderate and contingent on chaining. Risk rises materially for any organization where browser-side policy enforcement (CSP, mixed content, origin isolation) is part of the security posture for sensitive web applications.

Detection Guidance

Detection for a client-side DOM security mitigation bypass must focus on browser version inventory, browser process telemetry, and network-side signals of policy circumvention (blocked-then-allowed resource loads, CSP violation reports that cease while suspicious loads continue). The rules below are deliberately generic to the vulnerability class, since the supplied data names no function, endpoint, path, or parameter.

Sigma Rule

title: Suspicious Firefox Version Below 158 Process Execution Context
id: 9f1c4b2e-7d3a-4c8b-9a11-2f6e5d0b7c31
status: experimental
description: Detects execution of Firefox binaries consistent with versions prior to the Firefox 158 fix baseline for CVE-2026-109746 (DOM Security mitigation bypass). Version must be confirmed via product_version or external inventory.
author: Threat Intelligence
date: 2026/01/01
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        Image|endswith:
            - '\firefox.exe'
            - '\firefox'
    filter_monitor:
        CommandLine|contains:
            - '-headless'
    condition: selection_img and not filter_monitor
fields:
    - Image
    - CommandLine
    - CurrentDirectory
falsepositives:
    - Legitimate Firefox usage on hosts pending update to version 158 or later
    - Detection cannot by itself confirm the vulnerable build; correlate with inventory
level: low
tags:
    - attack.t1189
    - cve.2026.109746

YARA Rule

rule CVE_2026_109746_Firefox_DOM_Security_Marker
{
    meta:
        description = "Analytical placeholder rule for CVE-2026-109746 (Firefox DOM Security mitigation bypass, fixed in Firefox 158). No code-level indicators are available; this rule intentionally matches only generic Firefox version/build strings and must not be treated as a reliable detector."
        author = "Threat Intelligence"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109746"
        severity = "informational"
    strings:
        $fx = "Mozilla Firefox" ascii wide nocase
        $ua = "Firefox/" ascii wide
    condition:
        any of them
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109746 - Firefox DOM Security mitigation bypass - outbound Firefox client asset request (analytical)"; flow:to_server,established; http.user_agent; content:"Firefox/"; nocase; reference:cve,2026-109746; classtype:attempted-user; sid:2026109746; rev:1; metadata:created_at 2026_01_01, updated_at 2026_01_01;)

Note: This is an analytical, low-fidelity rule. CVE-2026-109746 is a client-side browser issue; no network signature can reliably identify exploitation from the supplied data.

Elastic Detection

process where event.type == "start" and
  process.name in ("firefox", "firefox.exe") and
  not process.command_line : "*-headless*"

Correlate with endpoint software inventory to confirm the browser build is below the Firefox 158 baseline. Broad activity rule only.

Splunk SPL Query

index=endpoint (sourcetype=WinEventLog:Security OR sourcetype=sysmon) EventCode IN (1,4688)
| search (Image="*\\firefox.exe" OR Image="/usr/lib/firefox/firefox")
| stats count values(CommandLine) as cmd by host, Image
| where count > 0
| sort - count

Use in combination with an inventory lookup of Firefox versions; flag hosts reporting a version lower than 158.

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109746,">
  <rule id="2026109" level="3">
    <decoded_as>json</decoded_as>
    <field name="win.eventdata.image">firefox.exe$</field>
    <description>CVE-2026-109746: Firefox process observed - verify build against Firefox 158 fix baseline (DOM Security mitigation bypass).</description>
    <options>no_full_log</options>
    <group>process_monitor,cve_2026_109746,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The NVD description states the issue "was fixed in Firefox 158," but no patch source, commit, or advisory is present in the supplied dataset; the fix status must be independently verified against Mozilla's release channel.

1. Immediate mitigations

• Update every Firefox installation to Firefox 158 or later as the primary action. Where the update cannot be applied immediately, restrict browsing of untrusted content on affected builds.
• Reduce exposure to the vulnerability class: enforce restrictive Content Security Policy where you control web applications, and avoid rendering untrusted HTML in privileged browser contexts.
• Where the organization depends on browser-side mitigations (CSP, mixed content blocking, origin isolation) for protecting sensitive applications, treat those controls as unreliable on unpatched builds and add compensating controls (network segmentation for admin interfaces, per-session isolation, phishing-resistant MFA for sensitive apps).

2. Patch / upgrade steps

• Track Mozilla security advisories (MFSA) for the Firefox 158 release and any associated extended-support backports; the supplied data does not include an advisory number.
• Deploy the fixed build through existing patch-management pipelines; verify version reporting across managed endpoints.
• Re-test downstream/embedded Gecko-based products against the 158 baseline; the supplied dataset contains no affected-version list or OSV package data.

3. Long-term hardening

• Maintain accurate browser version inventory and enforce auto-update or update-on-check policies.
• Monitor browser console/telemetry for CSP violation reports and blocked resource behavior; a decline in violations with continued suspicious loads can indicate a policy bypass in use.
• Harden web applications against the class of issue by adopting defense-in-depth: strict CSP, Trusted Types where available, output encoding, and isolation of untrusted content.
• Track CVE/CWE class trends in browser DOM security components to anticipate chaining risks.

References

• NVD — CVE-2026-109746: https://nvd.nist.gov/vuln/detail/CVE-2026-109746
• Mozilla Foundation Security Advisories (MFSA) — Firefox 158 release advisories (verify the specific advisory covering this CVE): https://www.mozilla.org/security/advisories/
• Mozilla Firefox release notes / version 158: https://www.mozilla.org/firefox/releases/
• CWE-693 — Protection Mechanism Failure: https://cwe.mitre.org/data/definitions/693.html
• CISA Known Exploited Vulnerabilities Catalog (current KEV status: not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat actor campaigns / exploit-in-the-wild evidence: None available in the supplied data. CISA KEV status is NO, no public PoC or exploit code is known, and no campaign attribution is supported by the provided dataset.