CVE-2026-109750

Executive Summary

CVE-2026-109750 is a site isolation flaw in the DOM: Web Authentication component of Mozilla Firefox, addressed in Firefox 158. "Site isolation" failures in Firefox concern the browser's process-boundary enforcement, and the Web Authentication (WebAuthn) component is the affected subsystem. The impact is bounded by the fact that the NVD description provides no further technical detail — there is no public proof-of-concept, no exploit code, no Nuclei template, and no evidence of in-the-wild exploitation or CISA KEV listing.

Technical Analysis

The available source data consists of a single descriptive sentence: "Site isolation issue in the DOM: Web Authentication component. This vulnerability was fixed in Firefox 158." This is insufficient to reconstruct the precise root cause, the specific enforcement boundary that fails, or the exact preconditions for triggering the flaw.

What can be stated factually, based on the description alone:

Vulnerability class — site isolation failure. Site isolation is the browser security architecture in which content from different origins is placed into distinct operating-system processes, with cross-origin data access mediated by the browser's IPC and validation layer. When a site isolation issue exists, the isolation guarantee that normally prevents one origin's rendered content or internal state from being reachable by another origin is weakened or bypassable. This class of defect is typically relevant because it can undermine same-origin policy assumptions that other mitigations depend upon, such as the protections around credential and authentication surfaces.

Affected surface — DOM: Web Authentication. The description assigns the flaw to the DOM: Web Authentication component. "Web Authentication" in this context refers to Firefox's WebAuthn subsystem — the browser-side implementation supporting the Web Authentication API used for credential creation and authentication ceremonies (public-key credentials). The description places the issue in the "DOM" component grouping, indicating the defect is associated with how the DOM exposes or mediates Web Authentication behavior rather than, for example, the network stack or the certificate layer.

What is not established. The source data does not identify:

• The specific DOM interface, method, or event associated with the flaw
• The exact isolation boundary involved (e.g., process type, origin grouping, or IPC channel)
• The precise security property violated (confidentiality of credential material, cross-origin authentication state visibility, integrity of authentication results, or another property)
• Whether the issue is a race, a lifetime/ownership error, a message-validation gap, or a logic defect
• The attacker's required preconditions beyond rendering attacker-controlled content in the user's browser

Any statement below that asserts a specific function name, class name, API parameter, or code path for this CVE would be fabrication. There are no such identifiers in the provided NVD description or CNA title beyond "DOM," "Web Authentication," and the fixed version "Firefox 158." No source-verbatim identifiers for this CVE were supplied.

Because the CNA title field is empty and NVD provides only the sentence above, the technical analysis is necessarily at the vulnerability-class level. Analysts should treat the following as the maximum defensible characterization: a site isolation defect in Firefox's WebAuthn-related DOM code that could, in the general case for this class, degrade origin-isolation guarantees around authentication flows. The practical exploitability and exact security impact cannot be assessed from the available intelligence.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template exists for CVE-2026-109750. The attack chain below is inferred strictly from the vulnerability class (site isolation) and the affected component (DOM: Web Authentication). It deliberately avoids inventing specific function names, endpoints, or parameters, none of which are provided in the source data.

1. Reconnaissance / target identification. An attacker identifies targets running a Firefox build below version 158. This requires either version fingerprinting of a victim's browser from a web context, or a social-engineering/drive-by delivery where the attacker provides the browsing context. Because the flaw is in the browser rather than a server-side product, there is no network service to scan; the "target" is the user's browser session.

2. Triggering the flaw class. In a site isolation class of defect, the attacker causes the victim to load attacker-controlled content that is intended to be isolated from a second, sensitive origin (here, an origin that participates in Web Authentication ceremonies). The attacker's content is crafted to cross or bypass the isolation boundary that the Web Authentication component is expected to enforce in the DOM. The exact trigger — whether it involves navigating the victim between origins, embedding the sensitive origin, or forcing some DOM interaction with the Web Authentication surface — is not specified in the available data and must not be assumed.

3. Execution / interaction method at the class level. For site isolation failures, "execution" generally means that attacker-controlled content obtains access to data or state that site isolation was supposed to keep out of reach. In the Web Authentication context, the value of such access would relate to authentication-related state. No payload, JavaScript pattern, or DOM manipulation sequence can be responsibly described for this CVE because the description does not disclose the mechanism. Assessor note: "No public PoC available; attack chain is inferred from vulnerability class."

4. Post-exploitation impact. Consistent with the class, the potential impact is the exposure or manipulation of authentication-related state across an isolation boundary, which — if realizable — could weaken the assurance that WebAuthn-based sign-in reflects only same-origin, user-visible intent. The severity of any realized impact is not quantifiable from the source data. There is no evidence of weaponization, no observed exploitation, and no known adversary campaign tied to this CVE.

Impact Assessment

CVSS. NVD has not assigned a CVSS score or vector for CVE-2026-109750; the provided score is N/A. No subscores exist to reference. The estimated vector in METADATA is an analytical placeholder derived from the class (network reachable, high attack complexity, requires user interaction, scope change possible, low confidentially/integrity impact) and must not be treated as an authoritative score. FIRST.org has not published an EPSS value; no exploitation-likelihood figure should be inferred.

Affected systems. Mozilla Firefox builds below version 158. The affected-version data supplied is empty beyond the fixed version, so no precise lower bound is available. Organizations with managed Firefox deployments, and especially those relying on WebAuthn/passkey-based authentication in Firefox, are the relevant population. Enterprise blast radius is difficult to estimate without either a CVE identifier and version range or a support-status check against Mozilla's release channels.

Business impact and blast radius. Because Web Authentication underpins modern phishing-resistant authentication, a site isolation weakness in that surface is, by class, security-relevant to identity assurance programs. However, the absence of a CVSS score, a public PoC, KEV listing, and exploit code means the demonstrated risk is currently low. The practical blast radius is limited to individual Firefox sessions on vulnerable builds; there is no evidence of server-side or multi-tenant exposure. Treat this as a low-confidence, class-level risk pending Mozilla's fuller advisory.

Detection Guidance

Detection content below is generic to the vulnerability class (browser on vulnerable Firefox; site isolation / WebAuthn abuse patterns). Because no CVE-specific indicators, endpoints, or signatures are published, these rules are best-effort and should be tuned. They are not guaranteed to fire on exploitation of this specific issue.

Sigma Rule

title: Possible Firefox Site Isolation / WebAuthn Abuse Activity (CVE-2026-109750 Context)
id: 4f3c1b90-2a7e-4c1d-9b6a-8e2f0d1c5a77
status: experimental
description: Class-level detection placeholder for site isolation / Web Authentication anomalies in Firefox below version 158. No CVE-specific indicators are published; tune before production use.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109750
author: valtersit.com
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_image:
    - Image|endswith: '\\firefox.exe'
  selection_version:
    - CommandLine|contains:
        - '--version'
  condition: selection_image and selection_version
  # Note: browser isolation abuse is largely observable client-side via EDR telemetry
  # (cross-process handle access, unusual child content-process spawns), not process creation.
falsepositives:
  - Legitimate version checks by IT inventory tooling
level: low

YARA Rule

rule EXPL_CVE_2026_109750_Firefox_SiteIsolation_Class
{
    meta:
        description = "Class-level hunting rule for Firefox builds below 158 associated with a DOM: Web Authentication site isolation flaw. No CVE-specific binary indicator published; use for version triage only."
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109750"
        date = "2026-01-01"
        tlp = "WHITE"

    strings:
        $mz = { 4D 5A }
        $ffx = "Mozilla Firefox" ascii wide nocase
        $prodname = "Firefox" ascii wide
        $ua = "Firefox/1" ascii wide   // Firefox 1xx user-agent/product string prefix

    condition:
        $mz at 0 and filesize < 200MB and $ffx and $prodname
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109750 - Mozilla Firefox version below 158 (DOM: Web Authentication site isolation) client fingerprint"; flow:established,to_server; http.user_agent; content:"Firefox/"; nocase; pcre:"/Firefox\/(?:[0-9]{1,2}|1[0-5][0-9])(?:\.|$)/"; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109750; classtype:attempted-user; sid:1097501; rev:1;)

Note: this is a version/fingerprint heuristic, not an exploit signature. No network-observable exploit signature for this CVE is published.

Elastic Detection

// EQL — class-level: browser content processes spawning unexpected children
// and rapid cross-origin navigation bursts to the same host in Firefox clients.
// No CVE-specific fields are published; treat as hunting aid.
process where process.parent.name in ("firefox.exe", "firefox-bin") and
  process.name in ("cmd.exe", "powershell.exe", "sh", "bash", "curl", "wget")

Splunk SPL Query

index=proxy OR index=web OR index=firefox_telemetry
| search user_agent="*Firefox/*"
| rex field=user_agent "Firefox/(?<ff_ver>[0-9]+)"
| where ff_ver < 158
| stats count dc(src_ip) AS unique_hosts values(ff_ver) AS versions BY dest
| sort - count

Purpose: surface clients on vulnerable Firefox builds to prioritize patching; not an exploit detector.

Wazuh OSSEC Rule

<group name="cve,browser,mozilla,">
  <rule id="109750" level="4">
    <if_sid>5920</if_sid>
    <field name="win.eventdata.image">firefox\.exe</field>
    <description>CVE-2026-109750 context: Firefox process activity on a potentially unpatched (&lt; 158) build.</description>
    <group>cve_2026_109750,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED (no patch evidence from any tracked source). The only remediation fact in the source data is that the issue "was fixed in Firefox 158." No vendor advisory URL, commit hash, KB article, or binary diff is available in the provided material, so no specific patch reference can be cited.

1. Immediate mitigations (compensating controls while patch confidence is low).

   • Record vulnerable Firefox builds (< 158) as an asset risk and prioritize upgrade in the next maintenance window for users relying on WebAuthn/passkeys.
   • Where Firefox version cannot be controlled, restrict reliance on WebAuthn within Firefox for high-assurance workflows and prefer a browser at a minimum supported version.
   • Standardize on a single, patched browser baseline via enterprise policy; block or restrict unsupported older browser installations.
   • Continue monitoring Mozilla's security advisories for the definitive CVE-to-version mapping, because the affected version lower bound is not provided here.

2. Patch / upgrade steps.

   • Update Firefox to version 158 or later from the official Mozilla distribution channel.
   • Confirm the installed build version post-update; verify on both managed endpoints and user-installed copies.
   • If the ESR channel is in use, confirm the corresponding ESR release that includes the Firefox 158 fix and upgrade to that build.

3. Long-term hardening.

   • Maintain a browser patch SLA and telemetry-based version inventory, since browser flaws are reachable client-side, not server-side.
   • Treat site isolation and WebAuthn-related components as high-value surfaces and monitor them for future advisories.
   • Assume the Web Authentication and WebAuthn code paths will continue to be targeted and apply defense-in-depth beyond same-origin assumptions.

References

1. NVD — CVE-2026-109750: https://nvd.nist.gov/vuln/detail/CVE-2026-109750
2. Mozilla Security Advisories (Firefox): https://www.mozilla.org/security/advisories/
3. Mozilla Firefox Security Advisories (mfsa index): https://www.mozilla.org/security/advisories/mfsa/
4. MITRE ATT&CK T1189 — Drive-by Compromise: https://attack.mitre.org/techniques/T1189/
5. CISA Known Exploited Vulnerabilities Catalog (listed here as a reference source; CVE-2026-109750 is NOT currently in KEV): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaign or exploit-in-the-wild evidence is known for CVE-2026-109750 based on the provided data.