CVE-2026-109747

Executive Summary

CVE-2026-109747 is an unspecified vulnerability in the Address Bar component of Mozilla Firefox, fixed in version 158. The NVD description classifies it only as "Other issue," with no further technical detail, CVSS score, CWE, or affected version range provided by the source data. Users running Firefox versions prior to 158 are potentially affected; the practical impact is assessed as low-to-moderate given the lack of confidentiality, integrity, or availability impact data and the absence of a public exploit or CISA KEV listing.

Technical Analysis

The source data for CVE-2026-109747 is extremely sparse. The NVD description states only: "Other issue in the Address Bar component. This vulnerability was fixed in Firefox 158." No CWE is assigned, no CVSS vector is published, and no affected version range is enumerated. The CNA title is empty.

Because the vulnerability pertains to the "Address Bar" — Firefox's URL/omnibox input field that performs URL parsing, display, and search-term handling — the most plausible vulnerability class is a UI-layer trust or spoofing issue in how the address bar renders or interprets attacker-influenced input. Firefox address-bar vulnerabilities historically fall into categories such as:

• Origin/URL spoofing (displaying a misleading domain to the user),
• Homograph or Unicode-confusable domain rendering,
• Drag-and-drop or paste-based address bar manipulation,
• Search-engine/omnibox injection.

However, the source data does not confirm any of these mechanisms. The only confirmed fact is the component (Address Bar) and the fix version (Firefox 158). No function names, parameters, or code paths are available from the description, and none should be assumed.

Attack surface: the address bar is reachable by any user interaction that places attacker-controlled text into the omnibox, and by web content that influences displayed URL text. Exploitation would most likely require user interaction (UI:R) — for example, a user navigating to or interacting with a crafted page or link.

CWE mapping: CWE-1021 (Improper Restriction of Rendered UI Layers or Frames) is offered as a provisional mapping consistent with the "Address Bar" component, but it is not confirmed by the source data. It should be treated as a hypothesis, not a validated classification.

Exploitation Scenarios

No public PoC, exploit code, or Nuclei template exists for this CVE. The attack chain below is described at the vulnerability-class level only; no specific function names or parameters are known.

1. Reconnaissance / target identification. An attacker identifies a victim running a Firefox version prior to 158. This can be done passively via user-agent strings in HTTP requests (e.g., observed Firefox version mentioned) or through social engineering that establishes the victim's browser.

2. Trigger delivery. Because the affected component is the address bar, delivery most plausibly occurs through user interaction: directing a victim to a crafted web page, link, or content that causes the address bar to render or behave in a misleading/incorrect way. The exact triggering input is unknown from the source data; at the class level, this is attacker-influenced text processed by the address bar's parsing/rendering logic.

3. Execution method (class level). The vulnerability class is a client-side UI/parsing issue, not (on available evidence) remote code execution. "Execution" here means the attacker causes the browser to display a spoofed or misleading address-bar state, potentially enabling phishing, credential theft via a fake origin, or user misattribution of the current site. No memory-corruption or code-execution primitive is indicated by the available data.

4. Post-exploitation impact. The likely impact is user deception — e.g., a victim believing they are on a trusted origin when they are not, leading to credential or data disclosure to an attacker-controlled site. Confidentiality and integrity impacts are plausible but unquantified.

Caveat: This section is inferred strictly from the component name and vulnerability class. No implementation specifics are available, and the report intentionally avoids fabricating them.

Impact Assessment

• CVSS: N/A (no vector published by NVD). No subscores can be cited because no CVSS data exists in the source.
• EPSS: Not yet scored by FIRST.org. No EPSS value is fabricated here.
• CISA KEV: Not listed. No evidence of active exploitation in the wild.
• Affected systems: Firefox installations older than version 158 across desktop platforms. Because the address bar is a universal browser surface, the potential user base is broad, but the lack of severity data limits confidence in blast-radius estimates.
• Business impact: If this is a spoofing/phishing-enabling issue, the impact is primarily to users — credential harvesting, session theft, or trust erosion. No RCE, privilege escalation, or data-exfiltration primitive is indicated.
• Confidence: Low. The assessment rests on the component name alone.

Detection Guidance

Because the vulnerability is client-side and unspecified, network and host detection is necessarily generic. Rules below target Firefox-version fingerprinting and address-bar-related anomalies; they should be tuned and are provided as starting points, not validated detections.

Sigma Rule

title: Firefox Address Bar Related Anomaly (CVE-2026-109747) - User Agent Version Pre-158
id: 7b3c9e21-4f0a-4c8e-9d2b-1a5f6e7c8d90
status: experimental
description: Detects HTTP requests carrying Firefox user-agent strings indicating versions older than 158, which may be relevant to exposure for CVE-2026-109747.
author: Threat Intelligence
date: 2026/01/01
logsource:
  category: proxy
  product: web
detection:
  selection_ua:
    cs-user-agent|contains:
      - 'Firefox/'
  filter_modern:
    cs-user-agent|re: 'Firefox/(1[5-9][0-9]|[2-9][0-9]{2,})'
  condition: selection_ua and not filter_modern
fields:
  - c-ip
  - cs-user-agent
  - cs-host
falsepositives:
  - Legacy enterprise browsers pinned to old versions
level: low
tags:
  - attack.t1204

YARA Rule

rule CVE_2026_109747_Firefox_AddressBar_Legacy
{
    meta:
        description = "Heuristic detection of Firefox build strings older than 158 relevant to CVE-2026-109747"
        author = "Threat Intelligence"
        date = "2026-01-01"
        reference = "CVE-2026-109747"
        severity = "low"

    strings:
        $firefox = "Firefox/" ascii wide
        $ver158   = "Firefox/158" ascii wide
        $omnibox  = "browser.urlbar" ascii wide

    condition:
        $firefox and $omnibox and not $ver158
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109747 Possible Firefox Pre-158 Address Bar Exposure (UA Fingerprint)"; flow:established,to_server; http.user_agent; content:"Firefox/"; http.user_agent; pcre:"/Firefox\/(?:[0-9]{1,2}|1[0-4][0-9]|15[0-7])(?:\.|$|\s)/"; reference:cve,2026-109747; classtype:attempted-recon; sid:2026109747; rev:1;)

Elastic Detection

http where http.request.headers.user_agent like "Mozilla/*Firefox/*" and
  not http.request.headers.user_agent rlike ".*Firefox/(1[5-9][0-9]|[2-9][0-9]{2,}).*"

Splunk SPL Query

index=proxy OR index=web sourcetype=*access*
| eval ua=coalesce(http_user_agent, cs_user_agent, useragent)
| where match(ua, "Firefox/")
| where NOT match(ua, "Firefox/(1[5-9][0-9]|[2-9][0-9]{2,})")
| stats count values(ua) as user_agents by src_ip, dest_host
| where count > 0
| sort - count

Wazuh OSSEC Rule

<group name="web,firefox,cve-2026-109747,">
  <rule id="109747" level="3">
    <if_sid>31100</if_sid>
    <regex>Firefox/(?:[0-9]{1,2}|1[0-4][0-9]|15[0-7])(?:\.|\s|$)</regex>
    <description>CVE-2026-109747: HTTP request with Firefox user-agent older than 158 (potential exposure)</description>
    <mitre>
      <id>T1204</id>
    </mitre>
    <group>cve_2026_109747,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The source data names no vendor advisory, CVE-recorded fix reference, or OSV package entry; the only stated remediation is the version reference "fixed in Firefox 158."

1. Immediate mitigations.

   • Upgrade or force-update Firefox to version 158 or later enterprise-wide; use managed policies (e.g., Firefox ESR / enterprise policy for update enforcement) where deployed.
   • Given the address-bar component, reinforce user awareness of URL spoofing and phishing; enable phishing/malware protection and DNS filtering as compensating controls.
   • Restrict or monitor browsing of untrusted links in high-risk roles until patched.

2. Patch / upgrade steps.

   • Confirm current Firefox version (about:support or about:buildconfig) on all endpoints.
   • Deploy Firefox 158+ via your software-deployment tooling; verify version post-update.
   • Validate that no extension or policy is pinning the browser to an older release.

3. Long-term hardening.

   • Maintain an automated browser-version inventory and enforce a maximum-age policy for browsers.
   • Adopt a regular patch cadence aligned with Mozilla's release train.
   • Monitor Mozilla security advisories for the definitive CVE-to-advisory mapping for CVE-2026-109747, and update detection logic once more detail is published.

References

1. NVD — CVE-2026-109747: https://nvd.nist.gov/vuln/detail/CVE-2026-109747
2. Mozilla Security Advisories: https://www.mozilla.org/en-US/security/advisories/
3. Mozilla Firefox Release Notes (Firefox 158): https://www.mozilla.org/en-US/firefox/releases/
4. CWE-1021 — Improper Restriction of Rendered UI Layers or Frames: https://cwe.mitre.org/data/definitions/1021.html
5. CISA Known Exploited Vulnerabilities Catalog (status check): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No known threat-actor campaigns, public exploit code, or in-the-wild exploitation evidence were identified in the source data for CVE-2026-109747.