CVE-2026-109759

Executive Summary

CVE-2026-109759 is a site isolation vulnerability in the Networking component of Mozilla Firefox. This flaw allows a malicious website to bypass the browser's site isolation protections, potentially enabling cross-origin data leakage or code execution in the context of another site. All users running Firefox versions prior to 158 and Firefox ESR versions prior to 153.5 are affected. No public exploit code or proof-of-concept currently exists, and the vulnerability has not been observed in active exploitation.

Technical Analysis

Site isolation is a core security architecture in modern browsers designed to ensure that content from different origins is rendered in separate processes, preventing a compromised renderer from accessing data belonging to another origin. CVE-2026-109759 represents a failure in this isolation mechanism within the Networking component of Firefox. The specific technical mechanism—such as whether the flaw resides in process allocation for network requests, cross-origin resource handling, or IPC validation—is not detailed in the available NVD description. The vulnerability class (CWE-693: Protection Mechanism Failure) indicates that the browser's enforcement of origin boundaries was incomplete or bypassable. Successful exploitation would allow a malicious page to interact with resources from a different origin, breaking the same-origin policy at the process level. The attack surface is the networking layer that handles HTTP requests, response routing, and process assignment for web content. Without vendor-provided technical details, the precise trigger condition, affected code path, or memory-safety implications remain unspecified.

Exploitation Scenarios

No public PoC or exploit code is available; the attack chain is inferred from the vulnerability class (site isolation bypass).

1. Reconnaissance/target identification: An attacker identifies a target using a vulnerable Firefox version (< 158 or ESR < 153.5) through user-agent fingerprinting or passive browsing behavior.
2. Triggering the flaw: The attacker hosts a malicious webpage that causes the Networking component to misassign or fail to isolate a cross-origin request. The exact HTTP response headers, request patterns, or navigation sequences required are not specified in available data. This likely involves a crafted sequence of network requests that confuses the browser's origin-to-process mapping.
3. Payload/execution method: At the vulnerability-class level, a successful site isolation bypass allows the attacker's page to read or manipulate data from a different origin. This could be leveraged to steal session tokens, CSRF tokens, or sensitive DOM content from a targeted site (e.g., webmail, banking portal). If the flaw permits script execution in the victim origin's context, it escalates to full cross-site scripting with the privileges of the targeted domain.
4. Post-exploitation impact: The attacker could exfiltrate authenticated session data, perform actions on behalf of the victim in a trusted origin, or pivot to further attacks such as delivering malware via compromised web sessions. The blast radius is limited to the browser's security context but affects all sites the user visits during the vulnerable session.

Impact Assessment

Business impact includes potential unauthorized access to sensitive web application data, session hijacking, and data exfiltration from enterprise SaaS or internal web tools accessed via Firefox. Affected systems are any endpoint running Firefox < 158 or Firefox ESR < 153.5 on Windows, macOS, Linux, or Android. The CVSS vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N) indicates a network-accessible, low-complexity attack requiring user interaction (visiting a malicious page). The scope is changed (S:C), meaning the vulnerable component (browser networking) impacts resources beyond its security authority (other origins). Confidentiality and integrity impacts are high; availability impact is none. No EPSS score is available, and the vulnerability is not listed in CISA KEV.

Detection Guidance

Sigma Rule

title: Firefox Site Isolation Bypass Attempt - Suspicious Cross-Origin Requests
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects potential exploitation of CVE-2026-109759 via anomalous cross-origin network activity from Firefox processes
author: Threat Intelligence
date: 2026-01-15
logsource:
  category: network_connection
  product: windows
detection:
  selection:
    Image|endswith: '\firefox.exe'
    DestinationPort:
      - 80
      - 443
    Initiated: 'true'
  filter_legitimate:
    DestinationIp:
      - '192.168.0.0/16'
      - '10.0.0.0/8'
  condition: selection and not filter_legitimate
fields:
  - Image
  - DestinationIp
  - DestinationPort
  - User
falsepositives:
  - Normal web browsing to external sites
level: low
tags:
  - attack.t1189
  - cve.2026.109759

YARA Rule

rule CVE_2026_109759_Firefox_Site_Isolation_Bypass
{
    meta:
        description = "Detects Firefox binaries associated with vulnerable versions prior to 158 or ESR 153.5"
        author = "Threat Intelligence"
        date = "2026-01-15"
        cve = "CVE-2026-109759"
        severity = "high"
    strings:
        $version_1 = "Mozilla Firefox 15" ascii wide
        $version_2 = "Mozilla Firefox 14" ascii wide
        $version_3 = "Firefox ESR 15" ascii wide
        $component = "Networking" ascii wide
    condition:
        (uint16(0) == 0x5A4D) and
        (any of ($version_*) or $component) and
        filesize < 200MB
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109759 Firefox Site Isolation Bypass Attempt"; flow:to_server,established; content:"User-Agent|3a| Mozilla/5.0"; http_header; content:"Firefox/15"; http_header; content:"Firefox/14"; http_header; distance:0; within:20; classtype:attempted-user; sid:2026109759; rev:1; metadata:affected_product Firefox, cve CVE-2026-109759;)

Elastic Detection

network where process.name == "firefox.exe" and
  destination.port in (80, 443) and
  not cidr_match(destination.ip, "10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16") and
  event.action == "connection_attempted"

Splunk SPL Query

index=network sourcetype=firefox:proxy OR sourcetype=pan:traffic
| search "Mozilla/5.0" AND ("Firefox/1" OR "Firefox/15" OR "Firefox/14")
| rex field=user_agent "Firefox/(?<firefox_version>\d+\.\d+)"
| where firefox_version < 158
| stats count by src_ip, dest_ip, dest_port, firefox_version
| where count > 10
| sort - count

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109759,">
  <rule id="100759" level="10">
    <decoded_as>json</decoded_as>
    <field name="Image">\.*firefox\.exe</field>
    <field name="DestinationPort">^443$</field>
    <field name="UserAgent">Mozilla/5.0.*Firefox/(1[0-4][0-9]|[0-9])\.</field>
    <description>CVE-2026-109759: Possible Firefox site isolation bypass attempt detected</description>
    <mitre>
      <id>T1189</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

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

1. Immediate mitigations: For environments that cannot upgrade immediately, enforce strict network segmentation and DNS filtering to block known malicious domains. Disable JavaScript for untrusted sites via browser policies or extensions. Consider using a different browser for sensitive web applications until an upgrade is possible.
2. Patch/upgrade steps: The vulnerability is fixed in Firefox 158 and Firefox ESR 153.5. Administrators should upgrade to these versions or later. For enterprise deployments, use the Mozilla Firefox Enterprise deployment guide or package managers (e.g., apt, yum, winget) to push the updated version. Verify version after upgrade via firefox --version or the about dialog.
3. Long-term hardening: Enable automatic updates for Firefox. Implement browser isolation or remote browser isolation (RBI) for high-risk browsing. Monitor for future Mozilla security advisories and subscribe to the Mozilla Security Blog. Apply the principle of least privilege to browser processes and restrict extensions that can modify networking behavior.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109759
• Mozilla Security Advisories: https://www.mozilla.org/en-US/security/advisories/
• Mozilla Foundation Security Advisory (hypothetical reference for Firefox 158/ESR 153.5): https://www.mozilla.org/security/advisories/mfsa2026-XX/
• CWE-693: Protection Mechanism Failure: https://cwe.mitre.org/data/definitions/693.html
• ATT&CK T1189: Drive-by Compromise: https://attack.mitre.org/techniques/T1189/ No known threat actor campaigns or exploit-in-the-wild evidence is available for this CVE.