CVE-2026-109766

Executive Summary

CVE-2026-109766 is a mitigation bypass vulnerability in the Enterprise Policies component of Mozilla Firefox, fixed in Firefox 158 and Firefox ESR 153.5. The Enterprise Policies component governs administrative configuration enforcement across managed Firefox deployments. A bypass of this component's controls could allow policy-imposed security restrictions to be circumvented on affected endpoints, weakening the security posture of enterprise-managed browsers. No CVSS score, CWE mapping, public proof-of-concept, or confirmed in-the-wild exploitation is currently available, limiting the ability to quantify severity precisely.

Technical Analysis

The NVD description states: "Mitigation bypass in the Enterprise Policies component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5." No CNA title, CWE identifier, CVSS vector, or affected version list was provided in the source intelligence. Consequently, the analysis below is constrained to the vulnerability class and component named, without fabricating implementation specifics.

The Enterprise Policies component is the mechanism by which Firefox enforces administrator-defined configuration on managed endpoints. It operates through policy configuration (for example, distribution policy files or equivalent managed policy sources) and controls settings such as extension installation allowances, proxy configuration, certificate handling, and other security-relevant browser behaviors. A "mitigation bypass" in this component means that a control intended to be enforced by policy could be evaded — that is, the policy restriction does not hold under certain conditions.

By vulnerability class, a mitigation bypass in a policy-enforcement component typically arises from one of the following mechanisms:

• Incomplete enforcement: a policy is applied at one layer but a parallel code path or configuration channel does not honor it.
• Ordering or precedence flaws: user-controlled or lower-privilege configuration overrides an administrator-set policy.
• Parsing/normalization gaps: policy values that are malformed, renamed, or otherwise nonstandard are silently ignored rather than enforced.

Because the source data does not specify which of these applies, the exact trigger condition is not determinable from the available intelligence. The attack surface is the enterprise policy enforcement boundary of the browser. An attacker who can influence browser state or configuration on a managed endpoint — whether through a local foothold, a malicious extension, or manipulation of a policy-consumption channel — may be positioned to exploit such a bypass.

The severity of a policy-enforcement bypass is contextual: its impact depends on which specific policies are affected. If the bypass permits circumvention of controls such as extension allow-listing, certificate pinning, or proxy restrictions, the downstream security impact can be substantial. If it affects a lower-risk policy, the impact is correspondingly limited. Determining which policies are affected requires verification against the actual patched code, which is not available in the provided data.

No CVSS vector or score was supplied, no CWE is mapped, and the EPSS score has not been published by FIRST.org. These absences should be treated as intelligence gaps rather than as indicators of low severity.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template exists for CVE-2026-109766, and no affected version list or patch diff is available. The attack chain is therefore described at the vulnerability-class level only, inferred from the fact that this is a mitigation bypass in the Enterprise Policies component.

1. Reconnaissance / target identification. An adversary targeting managed Firefox deployments first determines whether enterprise policies are in force. In managed environments, policy enforcement is a distinguishing characteristic — the presence of administrative configuration indicates a managed endpoint where a policy bypass would yield a meaningful security gain. The attacker profiles endpoints running Firefox versions below 158 (or Firefox ESR versions below 153.5), which the NVD entry identifies as the pre-fix state.

2. Triggering the flaw class. Because this is a mitigation bypass, the objective is to cause a policy-enforced restriction to not take effect. The specific input or condition that triggers the bypass is not described in the source intelligence. At the class level, the adversary would attempt to manipulate the policy-consumption path or the configuration state the policy governs such that enforcement fails or is overridden. The attacker must already have some ability to influence browser configuration or the environment in which policies are consumed; this is not necessarily a remote, unauthenticated network attack.

3. Execution / effect. Rather than delivering a memory-corruption payload, this class of vulnerability produces its effect by defeating a defensive control. The "payload" is the configuration or state change that the policy was intended to prevent — for example, an action that the enforced policy would otherwise block. No specific endpoint, parameter, or file path is identified in the available data, and none should be assumed.

4. Post-exploitation impact. Successful bypass weakens the integrity of the enterprise security baseline on the affected host. Its concrete consequences depend entirely on which policies are evaded. Because the affected policy set is unspecified, the downstream impact cannot be enumerated from the data provided; any specific outcome described would be speculation.

Note: No public PoC is available; the above attack chain is inferred from the vulnerability class and the named component only.

Impact Assessment

No CVSS base score, temporal score, or vector was provided, and the EPSS score has not been published. As a result, the impact assessment relies on qualitative reasoning anchored to the vulnerability class.

Affected systems are Firefox installations prior to version 158 and Firefox ESR installations prior to 153.5, specifically those operating under enterprise policy management. The practical blast radius is the managed fleet of an organization that relies on Firefox enterprise policies as a security control. Unmanaged consumer installations are theoretically affected at the code level but derive limited security value from the policy component, since the enforcement boundary is meaningful primarily where administrators impose restrictions.

From a business-impact perspective, the primary consequence is degradation of a security control, not direct code execution. The most credible risk pattern is that an attacker with a foothold on a managed endpoint uses the bypass to defeat a policy restriction that was intended to constrain them. Because the specific policies affected are unknown, the secondary impacts — such as whether the bypass facilitates persistence, data exfiltration, or certificate/key management abuse — cannot be stated with confidence.

CISA KEV status is negative: there is no evidence that this vulnerability is being exploited in the wild, and no known threat-actor campaign has been tied to it in the provided data. The absence of a public PoC further reduces the near-term likelihood of opportunistic exploitation. Organizations should nonetheless treat this as a control-integrity issue requiring prompt patching of managed Firefox fleets.

Detection Guidance

Sigma Rule

title: Firefox Enterprise Policies Mitigation Bypass - Policy Configuration Tampering
id: 8f2c1a44-9d3e-4b7a-b6c2-1e5f0d7a3c91
status: experimental
description: Detects modification of Firefox enterprise policy configuration files that may indicate exploitation of a mitigation bypass in the Enterprise Policies component (CVE-2026-109766).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109766
author: valtersit.com
date: 2026/01/01
tags:
  - attack.defense_evasion
  - attack.t1562
  - cve.2026.109766
logsource:
  category: file_event
  product: windows
detection:
  selection_paths:
    TargetFilename|endswith:
      - '\distribution\policies.json'
      - '\Mozilla\Firefox\policies.json'
  selection_actions:
    - EventType: 'create'
    - EventType: 'modify'
    - EventType: 'rename'
  condition: selection_paths and selection_actions
falsepositives:
  - Legitimate administrative policy deployment via management tooling
level: medium

YARA Rule

rule CVE_2026_109766_Firefox_Policy_Bypass
{
    meta:
        description = "Detects Firefox enterprise policy configuration content associated with mitigation bypass CVE-2026-109766"
        author = "valtersit.com"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109766"
        severity = "medium"

    strings:
        $policies = "policies" ascii nocase
        $firefox  = "Firefox" ascii nocase
        $enterprise = "Enterprise" ascii nocase

    condition:
        filesize < 1MB and
        (uint32be(0) == 0x7B0A2020 or uint32be(0) == 0x7B0D0A20) and
        all of ($policies, $firefox, $enterprise)
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109766 Mozilla Firefox Enterprise Policies Mitigation Bypass - Policy Update Fetch"; flow:established,to_server; http.method; content:"GET"; http.header; content:"User-Agent"; content:"Firefox"; nocase; http.uri; content:"policies"; nocase; classtype:attempted-user; sid:2026109766; rev:1; metadata:created_at 2026_01_01, cve CVE_2026_109766, author valtersit.com;)

Elastic Detection

file where file.path : ("*\\distribution\\policies.json", "*\\Mozilla\\Firefox\\policies.json") and event.action in ("creation", "modification", "rename")

Splunk SPL Query

index=endpoint (EventCode=11 OR EventCode=4663)
| search TargetFilename="*policies.json"
| search TargetFilename="*Firefox*" OR TargetFilename="*Mozilla*"
| stats count min(_time) as first_seen max(_time) as last_seen by host, user, TargetFilename, EventCode
| where count > 0
| sort - last_seen

Wazuh OSSEC Rule

<group name="firefox,cve_2026_109766,">
  <rule id="109766" level="10">
    <if_sid>550</if_sid>
    <field name="file">policies.json</field>
    <description>CVE-2026-109766: Firefox enterprise policy file modified - possible mitigation bypass attempt</description>
    <mitre>
      <id>T1562</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations. Upgrade all managed Firefox installations to version 158 (standard release) or Firefox ESR 153.5, which the NVD description identifies as the fixed releases. Until fleet-wide upgrade completes, prioritize endpoints that depend on enterprise policy enforcement as a security control. Audit existing enterprise policy configuration to confirm that intended restrictions are actually taking effect on currently deployed versions, since a bypass may manifest as a policy that appears configured but is not enforced. Where policy integrity is a concern, restrict local write access to policy configuration locations to administrative accounts only, limiting the ability of a lower-privileged local user to influence the enforcement state.

2. Patch / upgrade steps. Transition managed fleets from affected versions (below 158 standard; below 153.5 ESR) to the fixed releases. Use existing software distribution or management tooling to push upgrades and verify installed version post-deployment. Confirm that enterprise policy enforcement remains intact after upgrading, and re-validate that previously configured policies apply as intended.

3. Long-term hardening. Treat enterprise policy enforcement as a monitored control rather than a set-once configuration: detect and alert on unexpected modification of policy files and on endpoints where expected policies are missing. Maintain an inventory of security-relevant policies and periodically validate their enforcement on live endpoints. Adopt the ESR release channel for managed deployments to receive consistent security backports, and track Mozilla security advisories to correlate future fixes with deployed versions. Because no vendor advisory reference, workarounds, or affected-version list was provided in the source data, verification against Mozilla's own advisories is required to confirm the exact scope and any interim guidance.

References

• NVD — CVE-2026-109766: https://nvd.nist.gov/vuln/detail/CVE-2026-109766
• Mozilla Foundation Security Advisories (verify the advisory corresponding to the Firefox 158 / Firefox ESR 153.5 release): https://www.mozilla.org/security/advisories/
• Mozilla Firefox Security Updates: https://www.mozilla.org/en-US/firefox/security/
• CISA Known Exploited Vulnerabilities Catalog (checked — CVE-2026-109766 is NOT listed as of this report): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (no score published for this CVE as of this report): https://www.first.org/epss/

No known threat actor campaign or exploit-in-the-wild evidence is associated with CVE-2026-109766 in the available intelligence.