CVE-2026-109767

Executive Summary

CVE-2026-109767 is an information disclosure vulnerability affecting the Networking component of Mozilla Firefox and Firefox ESR. The flaw, tracked by Mozilla's internal security process, was corrected in Firefox 158 and Firefox ESR 153.5. No public proof-of-concept, exploit code, or Nuclei template is known to exist, and the CVSS score remains unassigned in the provided dataset, meaning prioritization must rely on the affected product's ubiquity rather than a numeric severity rating.

Technical Analysis

The vulnerability resides in the Networking component of Mozilla Firefox. The NVD description is intentionally minimal — "Information disclosure in the Networking component" — and Mozilla's convention of withholding technical detail until a majority of the user base has upgraded limits what can be stated with confidence. No CWE identifier, CVSS vector, or affected function name is disclosed in the source data.

At the vulnerability-class level, "information disclosure in the Networking component" refers to a fault in Firefox's HTTP stack, connection handling, or resource-loading pipeline that causes data the browser is supposed to protect to be exposed. In Firefox's architecture, the "Networking" component encompasses the Necko networking library, which implements HTTP/HTTPS transport, DNS resolution, connection pooling, caching, and proxy handling. Disclosure faults in this layer typically fall into several recognizable classes:

• Cross-origin data leakage: A networking-layer logic error allows content fetched for one origin to be read or observed by another, bypassing the same-origin policy. This could involve connection reuse, cache keying, or redirect handling.
• Response/resource leakage through caching: Incorrect cache partitioning or cache-key computation may cause a response from a privileged or authenticated context to be served to an unprivileged context.
• Metadata/credential leakage: Failure to strip sensitive headers, cookies, or authentication material when following redirects or transitioning between schemes (e.g., HTTPS to HTTP, or through proxies).
• Cross-site leak via browser side channels: Networking components that expose timing, size, or error-state information can facilitate XS-Leaks (cross-site leaks) that reveal whether a resource exists or is accessible to the victim.

Because the description explicitly names only the component and the fix versions, the precise root cause cannot be reconstructed from the available data. Analysts should treat all of the above as candidate classes rather than confirmed mechanisms. No function names, file paths, or API parameters are provided in the CNA title or NVD description, and none are invented here.

The absence of a CWE assignment in the provided data is itself notable: Mozilla advisories for information-disclosure issues sometimes map to CWE-200 (Exposure of Sensitive Information) when the issue is a general leakage, CWE-346 (Origin Validation Error) for cross-origin faults, or CWE-524 (Use of Cache Containing Sensitive Information) for cache-partitioning bugs. The report applies CWE-200 as the most general and defensible mapping.

Exploitation Scenarios

No public proof-of-concept or exploit code exists for CVE-2026-109767, and no exploitation has been observed in the wild. The attack chain below is therefore inferred at the vulnerability-class level from the NVD description ("information disclosure in the Networking component") and must not be read as a reconstruction of a specific, verified exploit. It is equally possible the bug is a low-severity internal finding rather than a remotely triggerable issue.

1. Reconnaissance / target identification. An attacker operating a malicious or compromised web origin identifies the victim's browser version through the User-Agent string, feature detection, or behavior probing. Targets running Firefox before 158 (or Firefox ESR before 153.5) are potentially affected. The attacker does not require network position: the typical delivery vector for a browser information-disclosure bug is a web page the victim visits.

2. Triggering the flaw class. Whatever the underlying networking-layer fault, exploitation requires the victim to load attacker-controlled content that causes the browser to enter the vulnerable networking state. In the cross-origin or cache-leakage classes, this means the attacker page initiates requests (images, fetches, subresources, or navigations) that interact with a sensitive target origin — for example a webmail, banking, or intranet endpoint the victim is authenticated to. The attacker's page then observes a side channel or receives data that the victim's browser should have shielded.

3. Payload / execution method. At the class level, the "payload" is not executable code but a carefully constructed web page that induces the vulnerable request pattern. No specific API, endpoint, or URL scheme can be named from the available data. If the flaw is a timing or size side channel, the attacker measures response characteristics; if it is a direct cross-origin read, the attacker reads the leaked bytes directly. In neither case does the public record confirm the mechanism.

4. Post-exploitation impact. The end result is unauthorized exposure of information to the attacker's origin. Depending on what the victim was accessing, this could include authenticated web content, intranet resources, account metadata, or — in the worst case for a networking-layer bug — session tokens or credentials. Because no CVSS score, EPSS score, or exploit evidence is available, the realized impact cannot be quantified and is contingent on the true root cause.

Impact Assessment

No CVSS base score is assigned in the provided data, so the vector supplied above (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N) is an illustrative default for a network-reachable, user-interaction-driven, confidentiality-only browser flaw — not an authoritative score. It should not be quoted as the official rating. Key assessment points:

• Confidentiality: The only impacted CVSS property is confidentiality. If the disclosed data includes session material or private web content, the confidentiality impact is High; if it is limited to low-value metadata, it is Low.
• Integrity / Availability: No basis exists in the description for integrity or availability impact.
• Attack vector / complexity: A Networking-component disclosure in a browser is typically network-reachable with some user interaction (victim visits a page or loads a resource). Complexity is unknown.
• Scope: Likely unchanged — the leaked data belongs to the same security context as the victim's browsing session.
• Blast radius: Firefox is a mass-market browser with desktop, mobile (Android/iOS), and ESR (long-term support) channels. Enterprises standardizing on Firefox ESR — common in regulated and air-gapped-adjacent environments — are exposed until ESR 153.5 is deployed. Because the bug is unpatched in the dataset's tracking terms (zero patch-confidence sources) but Mozilla has shipped fixed releases, the true exposure is the gap between release and fleet-wide upgrade.
• Exploitation likelihood: Low-to-moderate at present. There is no PoC, no KEV entry, and no EPSS score. However, browser information-disclosure bugs are routinely weaponized in targeted-attack and surveillance contexts, and the absence of public tooling does not preclude private exploitation.

Detection Guidance

Detection for this CVE is challenging because the underlying mechanism is undisclosed and an information-disclosure fault in a browser's networking stack generally does not produce distinctive host- or network-level artifacts. The rules below are therefore behavioral and heuristic, aimed at surfacing the reconnaissance and anomalous-request patterns consistent with browser information-disclosure campaigns rather than at signature-matching a known exploit. Analysts must tune thresholds to their environment.

Sigma Rule

title: Potential Firefox Networking Component Information Disclosure - Version Recon
id: 8f3c1a72-1097-4c7e-9b21-1a6f3d109767
status: experimental
description: Detects web server-side reconnaissance patterns consistent with fingerprinting Firefox versions prior to 158 / ESR 153.5. Heuristic only - not a signature of CVE-2026-109767.
author: Threat Intelligence
date: 2026/01/01
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109767
logsource:
  category: webserver
detection:
  selection_ua:
    cs-useragent|contains:
      - 'Firefox/15'
      - 'Firefox/14'
      - 'Firefox/13'
      - 'Firefox/12'
  selection_paths:
    cs-uri|contains:
      - '/api/'
      - '/user/'
      - '/account/'
      - '/redirect'
  condition: selection_ua and selection_paths
falsepositives:
  - Legitimate older Firefox clients
  - Normal authenticated browsing
level: low

YARA Rule

rule CVE_2026_109767_Firefox_Networking_InfoDisclosure
{
    meta:
        description = "Heuristic detection for artifacts related to Firefox Networking component information disclosure (CVE-2026-109767). Not a verified exploit signature."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109767"
        date = "2026-01-01"
        author = "Threat Intelligence"
        severity = "informational"

    strings:
        $fx_net = "nsHttpChannel" ascii nocase
        $fx_socket = "nsSocketTransport" ascii nocase
        $fx_conn = "nsHttpConnection" ascii nocase
        $cve = "CVE-2026-109767" ascii
        $ver = "Firefox ESR" ascii nocase

    condition:
        2 of them
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109767 Possible Firefox Networking Info-Disclosure Recon"; flow:established,to_server; http.user_agent; content:"Firefox/1"; http.user_agent; pcre:"/Firefox\/(1[0-4][0-9]|15[0-7])\b/"; http.uri; content:"redirect"; nocase; threshold:type both, track by_src, count 20, seconds 60; classtype:attempted-recon; sid:1097670; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=5m
  [network where network.protocol == "http" and user_agent.original like~ "Firefox/1*" and process.name == "firefox"]
  [network where network.protocol == "http" and url.path like~ ("*redirect*", "*account*", "*user*")]

Splunk SPL Query

index=proxy OR index=web
| eval ua=lower(coalesce(http_user_agent, useragent))
| where match(ua, "firefox/(1[0-4][0-9]|15[0-7])\\b")
| stats count min(_time) as first_seen max(_time) as last_seen values(uri) as uris by src_ip, ua
| where count > 50
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Wazuh OSSEC Rule

<group name="web,firefox,info_disclosure,">
  <rule id="109767" level="5">
    <if_sid>31100</if_sid>
    <match>Firefox/1</match>
    <description>CVE-2026-109767: Possible Firefox Networking information-disclosure reconnaissance - outdated Firefox user-agent accessing sensitive paths</description>
    <group>recon,pci_dss_10.6,</group>
  </rule>
  <rule id="109768" level="8">
    <if_sid>109767</if_sid>
    <regex>/redirect|/account|/user|/api</regex>
    <description>CVE-2026-109767: Outdated Firefox client requesting sensitive endpoint - investigate possible info-disclosure exploitation</description>
    <group>info_disclosure,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED per tracked sources — no patch evidence was returned from any tracked source, despite the NVD description naming fixed versions). The mismatch reflects a dataset gap, not an absence of a vendor fix; Mozilla has released fixed builds.

1. Immediate mitigations

   • Upgrade all Firefox channels to Firefox 158 or later, and Firefox ESR to 153.5 or later. This is the authoritative remediation and should be prioritized over any compensating control.
   • Where immediate upgrade is impractical, enable browser auto-update and enforce a maximum version-age policy; block or alert on Firefox versions below the fixed thresholds.
   • Harden the browsing environment: restrict or disable risky cross-origin behaviors via enterprise policies only if validated, understand that disabling the Networking component is not possible, and reduce the value of leaked data by segmenting sensitive intranet resources behind VPN/SSO rather than relying on origin isolation.
   • For high-risk users (executives, journalists, administrators), consider temporary use of an unaffiliated browser until Firefox is updated, and deploy network-level egress filtering.

2. Patch / upgrade steps

   • Confirm current Firefox/ESR version via about:support or the enterprise inventory solution.
   • Push Firefox 158 / ESR 153.5 through the enterprise deployment tool (e.g., policy-based MSI/PKG, package manager, or Mozilla's enterprise distribution channels).
   • Verify post-upgrade versions across the fleet and track compliance; treat any host still on an older version as exposed.
   • Re-baseline: no CVE-specific patch source is present in this dataset, so rely on the official Mozilla release and advisory as the patch reference.

3. Long-term hardening

   • Maintain continuous browser version visibility and enforce an N-day upgrade SLA for browsers, treating them as tier-0 user-facing software.
   • Adopt browser isolation or a cloud-browser proxy for risky external browsing by high-value users.
   • Deploy content-security and cross-origin protections server-side (COOP/COEP/CORP, SameSite cookies, Fetch Metadata) to blunt the impact of client-side disclosure bugs.
   • Monitor Mozilla security advisories as a standing feed and cross-check vendor fix data against internal NVD ingestion, since this record's patch confidence is inconsistent with the vendor's stated fix.

References

• NVD — CVE-2026-109767: https://nvd.nist.gov/vuln/detail/CVE-2026-109767
• Mozilla Security Advisories (Firefox / Firefox ESR): https://www.mozilla.org/security/advisories/
• Mozilla Foundation Security Advisories index: https://www.mozilla.org/en-US/security/advisories/
• CISA Known Exploited Vulnerabilities Catalog (no entry for CVE-2026-109767 as of this report): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (no score published for CVE-2026-109767 as of this report): https://www.first.org/epss/

Threat-actor / exploitation note: No public proof-of-concept, no Nuclei template, no exploit code, and no CISA KEV entry are associated with CVE-2026-109767 in the available data. No known threat-actor campaigns or in-the-wild exploitation have been reported. This section will require revision if Mozilla publishes an advisory with a CVE-to-bug mapping or if exploit tooling emerges.