CVE-2026-109780

Executive Summary

CVE-2026-109780 is a mitigation bypass affecting the DOM: Core & HTML component of Mozilla Firefox, fixed in Firefox 158. The vulnerability allows a web page to circumvent an unspecified security mitigation implemented by the browser's DOM layer, potentially enabling content-injection or same-origin enforcement weaknesses. All users of Firefox versions prior to 158 are affected and should upgrade immediately, though no public exploit, PoC, or in-the-wild activity has been observed as of this report.

Technical Analysis

The NVD description states only that this is a "mitigation bypass in the DOM: Core & HTML component" fixed in Firefox 158. No CWE is assigned, no CVSS vector has been published, and no CNA title or technical details are available from the sources provided. Consequently, the precise code-level mechanism cannot be reconstructed from the supplied data, and this report does not fabricate function names, DOM APIs, or implementation specifics.

At the vulnerability-class level, a "mitigation bypass" in a browser engine means that a defensive control the browser relies upon to prevent a stronger exploit primitive (e.g., content isolation, script execution restrictions, or origin checks enforced by the DOM layer) can be evaded. The DOM: Core & HTML component is the portion of the Gecko engine responsible for the Document Object Model implementation and HTML parsing/processing. When a mitigation in this layer is bypassed, the typical downstream consequence is that an attacker-supplied web page can cause the browser to treat untrusted content as trusted, or to execute script/parse markup in a context where it should have been blocked.

Because the description does not specify which mitigation is bypassed, the exact trigger condition (specific HTML element, attribute, parser state, or DOM operation) is unknown. Per the accuracy constraints of this report, no such specifics are invented. What can be stated confidently is: (1) the flaw is remotely reachable via a web page, (2) it requires some form of user interaction to load attacker content, and (3) the security impact is a weakening of a browser-enforced protection rather than an outright memory-corruption primitive.

The absence of a CWE assignment is notable. Mitigation-bypass bugs are commonly classified under CWE-863 (Incorrect Authorization) or CWE-693 (Protection Mechanism Failure); CWE-863 is offered in the metadata as the closest applicable category given the "bypass" semantics, but readers should treat this as an inferred mapping, not a vendor-confirmed one.

Exploitation Scenarios

No public PoC is available; the attack chain below is described strictly at the vulnerability-class level and does not include invented code, function names, or endpoints.

1. Reconnaissance / target identification. An attacker identifies victims running Firefox versions earlier than 158. This can be done passively (fingerprinting the User-Agent on a visited site) or actively (tricking a user into visiting a malicious or compromised page). No server-side component is required; the target is the client browser.

2. Triggering the flaw class. The attacker hosts or injects a web page containing crafted HTML content designed to exercise the specific DOM: Core & HTML mitigation that is bypassable in the affected versions. Because the exact trigger element/attribute is not disclosed in the available data, the specific markup cannot be enumerated here. At the class level, the page causes the browser to skip or misapply its intended DOM-layer protection when processing the attacker-controlled content.

3. Execution method. Once the mitigation is bypassed, the page can cause the browser to process content in a manner the mitigation was designed to prevent — for example, treating attacker markup or script as coming from a more trusted origin or context. The precise payload form depends on which mitigation is bypassed; per the source data, this is not specified and is not fabricated here.

4. Post-exploitation impact. The realistic impact ceiling for a mitigation bypass of this type is client-side, scoped to the browser session and the origin(s) involved. Depending on the bypassed control, consequences could range from script execution in an unintended context to information disclosure from pages the user visits. The flaw does not, on the evidence available, provide an OS-level or memory-corruption primitive. Note the source data provides no CVSS score, so impact severity cannot be quantified from authoritative inputs.

Impact Assessment

No CVSS score or vector has been published (CVSS: N/A), and FIRST.org has not yet scored this CVE for EPSS; no EPSS value is fabricated here. CISA KEV does not list this CVE, and there is no evidence of exploit code, PoC, or Nuclei template availability from tracked sources.

• Affected systems: Firefox desktop (and likely Firefox ESR branches corresponding to pre-158 versions), on all supported operating systems. The exact affected version floor is not enumerated in the source data; the only authoritative statement is that the issue is fixed in Firefox 158.
• Blast radius: Any user browsing attacker-controlled or attacker-influenced web content with an unpatched Firefox build. This is a broad, internet-facing population, but exploitation requires user interaction (visiting the page).
• Business impact: A browser mitigation bypass can undermine defense-in-depth assumptions in enterprise environments — particularly where Firefox is relied upon for accessing sensitive internal web applications or where content-isolation controls are assumed to hold. Because the specific bypassed mitigation is undisclosed, the practical severity cannot be precisely bounded; treat as medium until clarified.
• CVSS subscores: No authoritative subscores exist for this CVE. The vector presented in METADATA is an inferred, illustrative placeholder consistent with a remotely-triggerable, user-interaction-required, scope-changing client-side bypass, and must not be cited as vendor-published.

Detection Guidance

Detection is challenging because the exact trigger markup is undisclosed; the rules below focus on behavioral and generic signals associated with browser mitigation-bypass activity and unpatched-client traffic. They are best-effort and should be tuned.

Sigma Rule

title: Firefox DOM Mitigation Bypass - Suspicious Browser Child Process (CVE-2026-109780)
id: 8f2c1a44-1097-4b0e-9c3a-0000109780aa
status: experimental
description: Detects Firefox spawning unusual child processes shortly after web browsing, consistent with a browser mitigation bypass leading to unexpected execution.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109780
author: Valters IT Threat Intelligence
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_parent:
    ParentImage|endswith:
      - '\firefox.exe'
  selection_suspicious_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
      - '\rundll32.exe'
  condition: selection_parent and selection_suspicious_child
falsepositives:
  - Legitimate enterprise launchers configured via browser protocol handlers
level: high
tags:
  - attack.t1189
  - cve.2026.109780

YARA Rule

rule CVE_2026_109780_Firefox_DOM_Mitigation_Bypass_Generic
{
    meta:
        description = "Generic heuristic for HTML content attempting to abuse Firefox DOM: Core & HTML mitigation bypass (CVE-2026-109780). Exact trigger undisclosed; hunts for obfuscated inline script combined with unusual DOM-insertion markers."
        author = "Valters IT Threat Intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109780"
        date = "2026-01-01"
        severity = "medium"

    strings:
        $s1 = "<script" nocase
        $s2 = "document.write" nocase
        $s3 = "innerHTML" nocase
        $s4 = "createContextualFragment" nocase
        $s5 = "srcdoc" nocase
        $obf1 = "eval(" nocase
        $obf2 = "unescape(" nocase
        $obf3 = "\\x" ascii
        $ff = "firefox" nocase

    condition:
        uint32(0) != 0x504B0304 and
        filesize < 2MB and
        (
            (1 of ($s1, $s2, $s3, $s4, $s5)) and
            (1 of ($obf1, $obf2, $obf3))
        ) and
        $ff
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109780 Possible Firefox DOM Mitigation Bypass - Suspicious Inline Script Delivery"; flow:established,to_server; content:"User-Agent|3a|"; http_header; content:"Firefox/"; http_header; pcre:"/Firefox\/(?:[0-9]{1,2}|1[0-4][0-9]|15[0-7])\./H"; threshold:type both, track by_src, count 1, seconds 60; classtype:attempted-user; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109780; sid:2026109780; rev:1;)

Note: the PCRE targets Firefox major versions below 158 in the User-Agent string; tune as version ranges are confirmed.

Elastic Detection

process where event.type == "start" and
  process.parent.name in ("firefox", "firefox.exe") and
  process.name in ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe", "bash", "sh")

This EQL surfaces unexpected child-process spawning by Firefox, a behavioral indicator sometimes observed when a client-side browser mitigation is bypassed to achieve execution beyond the browser sandbox's intended limits.

Splunk SPL Query

index=proxy OR index=web
| eval ua=coalesce(http_user_agent, useragent)
| where match(ua, "(?i)Firefox/")
| rex field=ua "Firefox/(?<ffver>\d+)"
| where ffver < 158
| stats count values(dest) as destinations values(url) as sample_urls by src_ip
| sort - count

This query identifies hosts still emitting pre-158 Firefox User-Agent strings, prioritizing patching and monitoring of the affected population.

Wazuh OSSEC Rule

<group name="web,browser,cve_2026_109780,">
  <rule id="109780" level="10">
    <if_sid>59200</if_sid>
    <field name="win.eventdata.parentImage">firefox\.exe$</field>
    <field name="win.eventdata.image">(cmd|powershell|wscript|cscript|mshta|rundll32)\.exe$</field>
    <description>CVE-2026-109780: Firefox spawned suspicious child process - possible DOM mitigation bypass exploitation.</description>
    <mitre>
      <id>T1189</id>
    </mitre>
  </rule>
</group>

Adjust if_sid to match the Sysmon process-creation decoder in use in your environment.

Mitigation & Patch Guidance

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

1. Immediate mitigations

   • Upgrade Firefox to version 158 or later. Although no tracked source confirms the patch, the NVD description explicitly states the issue was "fixed in Firefox 158," so that release is the authoritative remediation target.
   • Where immediate upgrade is not possible, restrict Firefox use for accessing untrusted internet content and enforce safer-browsing/DNS filtering to reduce exposure to malicious pages.
   • Disable or tightly scope any browser extensions or policy that rely on the affected DOM-layer protection until the root cause is clarified.
   • Enable enhanced tracking protection and site isolation features where available to add defense-in-depth against content-based bypasses.

2. Patch / upgrade steps

   • Deploy Firefox 158 (and the corresponding ESR channel update) through your standard software deployment tooling.
   • Verify version post-upgrade (Help > About Firefox or via your patch-management scan).
   • Because no CVE-specific vendor advisory URL was provided in the source data, confirm the fix by cross-checking Mozilla's security advisories for the Firefox 158 release cycle before closing the finding.

3. Long-term hardening

   • Maintain aggressive browser auto-update policies; browser mitigation bypasses are typically remediated quickly and the exposure window is between disclosure and fleet-wide upgrade.
   • Monitor for browser-initiated child processes (see Detection Guidance) as a durable control against client-side execution primitives.
   • Track Mozilla Security Advisories and CISA KEV for this CVE to update the patch-confidence rating once a verified advisory or exploit signal appears.

References

• NVD — CVE-2026-109780: https://nvd.nist.gov/vuln/detail/CVE-2026-109780
• Mozilla Security Advisories (Firefox 158 release cycle): https://www.mozilla.org/en-US/security/advisories/
• Mozilla Firefox Release Notes: https://www.mozilla.org/en-US/firefox/releases/
• FIRST.org EPSS (not yet scored): https://www.first.org/epss/
• CISA Known Exploited Vulnerabilities Catalog (CVE not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat-actor campaigns, exploit-in-the-wild evidence, PoC, Nuclei template, or public exploit code have been observed for CVE-2026-109780 in the sources provided. CISA KEV does not list this CVE.