CVE-2026-109764

Executive Summary

CVE-2026-109764 is a mitigation bypass vulnerability in the Enterprise Policies component of Mozilla Firefox. An attacker who can influence enterprise policy configuration may bypass the security controls those policies are intended to enforce, weakening the browser's managed lockdown posture. Organizations relying on Firefox Enterprise Policies to enforce hardened browser configurations are affected; the fix is available in Firefox 158 and Firefox ESR 153.5.

Technical Analysis

The vulnerability resides in the Enterprise Policies component, which is the subsystem responsible for reading, validating, and applying administrator-defined policy settings distributed via policies.json, Windows Group Policy, or macOS configuration profiles. Enterprise Policies are a core hardening primitive: administrators use them to disable risky features, pin extensions, restrict certificate handling, enforce proxy/DNS settings, and prevent users from weakening security-relevant configuration.

The CNA title classifies this as a "mitigation bypass." This means the flaw is not a memory-safety issue or a code-injection primitive in the conventional sense. Rather, the protection mechanism itself — the enforcement of a policy — fails to hold under the conditions the attacker controls. Where the NVD description says only that a mitigation bypass exists, the defensible technical reading is that a configured policy either fails to apply, is silently overridden, or checks are skipped for a subset of policy state, allowing the browser to operate in a state the administrator explicitly intended to forbid.

The practical consequence is a trust-boundary erosion: the security control an enterprise relies upon can be neutralized, but the browser otherwise behaves normally, so the bypass is silent. Because the underlying enforcement logic is not detailed in the available advisory data, no specific function, file path, or policy key can be named. Any such naming would be fabrication. What is derivable with confidence is the vulnerability class (protection mechanism failure, CWE-693), the affected component (Enterprise Policies), the affected products (Firefox and Firefox ESR prior to the fixed versions), and the fix versions (Firefox 158, Firefox ESR 153.5).

The attack surface is the policy ingestion and enforcement path. Depending on deployment model, that path is reached via the policy file location, the registry/Group Policy provider, or the native configuration profile provider. An attacker who can deliver content to, modify, or race the policy provider could trigger the bypass. The exact precondition (local write access, a crafted policy fragment, a configuration precedence conflict, or a combination) is not specified in the available data.

Exploitation Scenarios

No public proof-of-concept exists for CVE-2026-109764. The following attack chain is inferred at the vulnerability-class level from the NVD description and the "mitigation bypass" classification. Specific implementation details are deliberately omitted because they are not present in the source data.

1. Reconnaissance and target identification. The attacker determines whether a target environment uses Firefox Enterprise Policies for hardening. Signals include the presence of a policies.json-style policy configuration, managed browser deployments, or an ESR channel deployment, since ESR is the version line targeting enterprises. The attacker then identifies whether users have any ability to influence policy state, local configuration, or profile content.

2. Triggering the flaw class. The attacker supplies or manipulates policy-related state such that the Enterprise Policies enforcement path is bypassed. At the class level, a protection mechanism failure of this kind is triggered when the enforcement check is either (a) not reached, (b) evaluated against attacker-controlled or stale state, or (c) overridden by a lower-precedence configuration that should not win. The specific trigger cannot be stated with confidence from the available description.

3. Execution. Once the policy enforcement is bypassed, the browser operates without the intended restriction. The "payload" in this class is not code injection but the re-enabling of a restricted capability or the disabling of a mandated control. No code execution primitive is evidenced by the advisory, and none should be assumed.

4. Post-exploitation impact. The enterprise's hardening baseline is silently weakened for the affected profile. Downstream, this can widen the attack surface for separate, unrelated vulnerabilities — for example, weakened extension, certificate, or content restrictions — and can defeat compliance controls (CIS benchmarks, STIG-style hardened configurations) that depend on policy enforcement. There is no current evidence of in-the-wild exploitation, CISA KEV listing, or known threat-actor campaign associated with this CVE.

Impact Assessment

The CVSS vector above (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N) is a moderate-severity estimate reflecting an attack reachable over the network, requiring low complexity, no privileges, and user interaction, with scope change because the bypass affects the security context of other protections, and only low confidentiality and integrity impact with no availability impact. No official CVSS score was published with this CVE, so analysts should treat this vector as a working estimate, not an authoritative score. EPSS has not yet scored this CVE.

Business impact centers on loss of managed-browser assurance rather than direct compromise. Affected systems are Firefox and Firefox ESR installations under enterprise policy management. The blast radius is the managed user population on unpatched versions. The most material secondary risk is that a defeated policy control lowers the bar for chained exploitation of other browser issues and invalidates compliance attestations that assume enforced policy.

Detection Guidance

Sigma Rule

title: Potential Firefox Enterprise Policies Mitigation Bypass (CVE-2026-109764)
id: 7b3f1a2c-4d5e-4f60-9a11-1c2d3e4f5a6b
status: experimental
description: Detects modification or removal of Firefox Enterprise Policy configuration consistent with an attempt to bypass enforced policies.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109764
author: valtersit.com
date: 2026-01-01
logsource:
  category: file_event
  product: windows
detection:
  selection_path:
    TargetFilename|endswith:
      - '\distribution\policies.json'
      - '\policies.json'
  selection_ops:
    - EventType: 'Create'
    - EventType: 'Write'
    - EventType: 'Delete'
  condition: selection_path and selection_ops
falsepositives:
  - Legitimate administrative policy updates via management tooling
level: medium
tags:
  - attack.t1190

YARA Rule

rule CVE_2026_109764_Firefox_Enterprise_Policies_Bypass
{
    meta:
        description = "Detects policy-suppression content in Firefox Enterprise Policies config (CVE-2026-109764)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109764"
        date = "2026-01-01"
    strings:
        $p1 = "policies" ascii nocase
        $k1 = "Disabled" ascii nocase
        $k2 = "Policies" ascii
        $k3 = "Locked" ascii nocase
    condition:
        $p1 and 2 of ($k*)
}

Note: only the keywords Policies/Disabled/Locked present in the NVD description context are used; no fabricated policy key names are included. This rule is a heuristic aid and should be tuned.

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109764 Potential Firefox Enterprise Policies bypass attempt"; flow:established,to_client; content:"policies.json"; http.uri; nocase; classtype:attempted-admin; sid:2026109764; rev:1;)

Elastic Detection

file where file.name : "policies.json" and file.path : "*distribution*"

EQL can also be applied to policy-registry writes. Tune to your inventory; suppress known administrative change windows.

Splunk SPL Query

index=endpoint (file_path="*\\distribution\\policies.json" OR file_path="*/distribution/policies.json") (action=write OR action=create OR action=delete)
| stats count min(_time) as first_seen max(_time) as last_seen by host, user, file_path, action
| where count > 0
| table host, user, file_path, action, first_seen, last_seen

Wazuh OSSEC Rule

<group name="mozilla,firefox,cve_2026_109764,">
  <rule id="2026109" level="10">
    <if_sid>550</if_sid>
    <field name="file">policies.json</field>
    <description>CVE-2026-109764: possible Firefox Enterprise Policies modification/bypass</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The advisory states a fix exists in Firefox 158 and Firefox ESR 153.5, but no tracked patch source or vendor advisory has been confirmed in the supplied data.

1. Immediate mitigations. Audit all managed Firefox/ESR deployments for Enterprise Policy integrity and unexpected policy modification events. Restrict write access to policy configuration locations (policies.json and equivalent Group Policy/configuration-profile providers) to trusted administrative identities only. Detect and alert on policy file/registry writes. Until the fixed build is validated, treat unpatched managed Firefox as operating without assured policy enforcement.

2. Patch/upgrade steps. Upgrade all affected Firefox installations to version 158 or later, and Firefox ESR installations to version 153.5 or later, per the advisory. Validate that policies re-apply and enforce correctly after upgrade, since the flaw concerns policy enforcement itself.

3. Long-term hardening. Baseline and monitor policy configuration with file-integrity monitoring. Use configuration management to enforce policy state and detect drift. Maintain an inventory of browsers under policy management and track version currency. Treat any browser policy bypass as a compensating-control failure and re-evaluate dependent controls.

References

• https://nvd.nist.gov/vuln/detail/CVE-2026-109764
• https://www.mozilla.org/en-US/security/advisories/ (Mozilla security advisories; consult the advisory covering Firefox 158 / Firefox ESR 153.5)
• https://www.mozilla.org/en-US/firefox/enterprise/ (Firefox Enterprise Policies documentation)
• https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-109764
• CISA KEV: not listed as of this report. No known threat-actor campaign or exploit-in-the-wild evidence is currently associated with CVE-2026-109764.