CVE-2026-109763

Executive Summary

CVE-2026-109763 is a mitigation bypass vulnerability in the File Handling component of Mozilla Firefox, fixed in Firefox 158 and Firefox ESR 153.5. The flaw allows an attacker to circumvent security controls implemented in the browser's file handling logic, potentially leading to code execution or unauthorized file access when a user interacts with crafted content. Organizations running unpatched Firefox or Firefox ESR versions are affected; because no CVSS score, EPSS value, or public exploit is currently available, the practical risk is driven by the ease of mitigation bypass classes and the ubiquity of Firefox in enterprise endpoints.

Technical Analysis

The vulnerability is classified as a mitigation bypass within the File Handling component. Mozilla's "mitigation bypass" category typically refers to flaws that defeat a browser-enforced security boundary (e.g., Safe Browsing, download protection, sandbox restrictions, or content-type enforcement). Because the NVD description does not disclose the specific function, file path, or mechanism, the analysis must remain at the vulnerability-class level: an attacker can cause the browser to treat a malicious file in a way that evades the intended control, enabling the payload to be processed outside the expected security policy.

The File Handling component is the attack surface responsible for deciding how downloaded or opened files are checked, labeled, and launched. A mitigation bypass here means the browser's protective logic — whatever check it performs on file type, origin, or content — fails or can be deliberately avoided. This is a client-side issue: the user must typically be enticed to download or open a file or navigate to attacker-controlled content. The absence of a CWE identifier, vendor advisory text, and patch commit details prevents deeper code-level attribution.

No public proof-of-concept, Nuclei template, or exploit code is known. The attack mechanism is therefore inferred from the vulnerability class (mitigation bypass in file handling), not from disclosed implementation details.

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred from the vulnerability class and the affected component.

1. Reconnaissance / Target Identification: The attacker identifies users or endpoints running Firefox versions prior to 158, or Firefox ESR versions prior to 153.5. This can be done through user-agent fingerprinting on a malicious or compromised website.

2. Triggering the Flaw: The victim is lured to a page or download that serves a crafted file. Because the flaw is a mitigation bypass in File Handling, the browser's protective check on that file is evaded, and the file is handled in a manner the security control was designed to prevent.

3. Payload / Execution: The crafted file is delivered and then processed or opened by the user or by the file-handling path, bypassing the intended mitigation. The precise execution primitive is not described in the available data and is not fabricated here.

4. Post-Exploitation Impact: Depending on the mitigation that is bypassed, consequences range from unauthorized file access to client-side code execution in the user's context. Business impact includes endpoint compromise and potential lateral movement from the victim workstation.

Impact Assessment

CVSS score, subscores, and vector are not available (NVD: N/A), so impact must be assessed qualitatively. A mitigation bypass in a browser's file-handling path is significant because it undermines a defense the vendor explicitly relies on to protect users. Affected systems are Firefox < 158 and Firefox ESR < 153.5 on all supported desktop platforms. The blast radius is broad: Firefox ESR is widely deployed in enterprises, so an unpatched fleet represents many potential victims. Exploitation requires user interaction (opening/downloading a file), which limits mass exploitation but does not reduce severity for targeted attacks. Because no EPSS or KEV data exists, there is no known active exploitation; however, unpatched browser client vulnerabilities are routinely targeted.

Detection Guidance

Sigma Rule

title: Firefox Mitigation Bypass Exploitation Attempt - CVE-2026-109763
id: 7b1f3c2e-9a4d-4e6b-8c21-3f5e9d0a1b2c
status: experimental
description: Detects suspicious file download and execution patterns consistent with a Firefox file handling mitigation bypass.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109763
author: valtersit.com
date: 2026-06-01
logsource:
  category: process_creation
  product: windows
detection:
  selection_browser:
    ParentImage|endswith:
      - '\firefox.exe'
  selection_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
      - '\rundll32.exe'
  condition: selection_browser and selection_child
falsepositives:
  - Legitimate browser-launched helper applications
level: medium

YARA Rule

rule CVE_2026_109763_Firefox_Mitigation_Bypass
{
    meta:
        description = "Detects potential Firefox file handling mitigation bypass artifacts"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109763"
        date = "2026-06-01"
        severity = "high"
    strings:
        $ff1 = "Mozilla Firefox" ascii wide nocase
        $ff2 = "firefox.exe" ascii wide nocase
        $ver = "ESR 153" ascii wide
    condition:
        uint16(0) == 0x5A4D and 2 of ($ff1, $ff2) and not $ver
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109763 Firefox File Handling Mitigation Bypass Download Attempt"; flow:established,to_server; http.user_agent; content:"Firefox"; http.header; content:".exe"; threshold:type both,track by_src,count 3,seconds 60; classtype:attempted-user; sid:1097630; rev:1;)

Elastic Detection

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

Splunk SPL Query

index=endpoint source=*Sysmon* EventCode=1 ParentImage="*\\firefox.exe"
| where match(Image, "(?i)(cmd|powershell|wscript|cscript|mshta|rundll32)\\.exe$")
| stats count by ComputerName, User, ParentImage, Image, CommandLine
| where count > 0

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109763,">
  <rule id="109763" level="10">
    <if_sid>60000</if_sid>
    <field name="win.eventdata.parentImage">firefox.exe</field>
    <field name="win.eventdata.image">\.(exe|dll)$</field>
    <description>CVE-2026-109763: Possible Firefox file handling mitigation bypass - child process spawned from Firefox</description>
    <mitre>
      <id>T1203</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Note: the NVD description states the issue was fixed in Firefox 158 and Firefox ESR 153.5, but no patch source, commit, or vendor advisory has been confirmed by tracked data.

1. Immediate Mitigations:

• Update all Firefox installations to version 158 or later, and Firefox ESR installations to 153.5 or later.
• Where immediate upgrade is not possible, enforce strict download policies, disable automatic file opening, and restrict untrusted file execution via application control (e.g., AppLocker, WDAC).
• Monitor for and block suspicious child processes spawned by firefox.exe.

2. Patch / Upgrade Steps:

• Deploy the current Firefox / Firefox ESR releases through your software management tooling.
• Validate version compliance across all managed endpoints; prioritize internet-facing and high-value user workstations.
• Reboot or restart the browser to ensure the updated binary is running.

3. Long-Term Hardening:

• Maintain a rapid browser patch cadence, as browser client flaws are frequently exploited.
• Deploy endpoint detection for process lineage anomalies involving browser parent processes.
• Restrict user ability to execute downloaded files from commonly abused paths (Downloads, Temp).
• Enable browser sandboxing and content-type enforcement where available, and monitor bypass attempts.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109763
• Mozilla Security Advisories: https://www.mozilla.org/en-US/security/advisories/
• Mozilla Firefox Release Notes: https://www.mozilla.org/en-US/firefox/releases/
• CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog (no entry for CVE-2026-109763 at time of writing)
• MITRE ATT&CK T1203 — Exploitation for Client Execution: https://attack.mitre.org/techniques/T1203/

No known threat actor campaigns or exploit-in-the-wild evidence for CVE-2026-109763 is available as of this report; no public PoC or Nuclei template exists.