CVE-2026-109782

Executive Summary

CVE-2026-109782 is a mitigation bypass vulnerability in the File Handling component of Mozilla Firefox ESR, fixed in version 153.5. The CNA title and NVD description provide only a high-level classification without specifying the exact mechanism, affected functions, or file paths. Organizations running Firefox ESR versions prior to 153.5 are affected and should treat the vulnerability as requiring user interaction to exploit, with potential impact limited to data confidentiality and integrity of locally handled files.

Technical Analysis

The NVD description states: "Mitigation bypass in the File Handling component. This vulnerability was fixed in Firefox ESR 153.5." No CWE identifier, CVSS score, or detailed technical write-up is provided in the supplied data. The vulnerability class is a mitigation bypass — a failure of a protective mechanism — which in browser file-handling contexts typically involves circumventing a security control such as download protection prompts, file type restrictions, or content-disposition enforcement. Because the description references "File Handling" specifically, the most plausible attack surface is the download/file-save path exposed to web content. However, no function names, API parameters, file paths, or module identifiers are present in the provided description, so the exact code-level mechanism cannot be stated without fabrication. Analysts should consult Mozilla's security advisories for Firefox ESR 153.5 for the authoritative technical detail and bug reference.

Exploitation Scenarios

Based strictly on the vulnerability class (mitigation bypass in File Handling), the attack chain is inferred as follows:

1. Reconnaissance: An attacker identifies a target that uses a pre-153.5 Firefox ESR build. Detection can be performed passively via User-Agent strings presented to attacker-controlled or observed web endpoints, or via Shodan lookups for exposed Firefox-branded web interfaces.
2. Trigger: The victim navigates to attacker-controlled content that initiates a file-handling operation. The specific flaw class is a bypass of a browser mitigation, meaning the security control that would normally restrict or warn about the file operation is defeated.
3. Payload/execution: At the vulnerability-class level, the bypass enables the file operation to proceed without the expected mitigation — for example, writing or saving a file in a manner the browser would otherwise block or warn about. No specific URL endpoints, function names, or parameters are available in the provided description; therefore no implementation-level payload can be described.
4. Post-exploitation: Impact is bounded by the bypassed control. Given the CVSS vector assessed (C:L/I:L/A:N) and the requirement for user interaction, likely outcomes are limited disclosure of local file data or limited integrity impact, not arbitrary code execution. No public PoC is available; the attack chain is inferred from the vulnerability class only.

Impact Assessment

Affected systems: installations of Mozilla Firefox ESR earlier than 153.5. Blast radius is limited to users who browse attacker-influenced content with a vulnerable ESR build. Business impact: user-interaction-required, limited confidentiality and integrity exposure; availability is not implicated. The CVSS vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N yields a base score of 5.4 (Medium) under the CVSS 3.1 specification, consistent with a mitigation bypass requiring user interaction and producing limited impact. No CVSS score was provided by the NVD data, so the subscore references above should be treated as analyst-assigned and validated against the vendor advisory.

Detection Guidance

Sigma Rule

title: Suspicious Firefox ESR File Handling Activity (CVE-2026-109782)
id: 4d2c9e1a-8b7f-4c3d-9e2a-7f1b6a5c8d34
status: experimental
description: Detects anomalous file writes by Firefox ESR processes that may indicate a mitigation bypass in file handling.
logsource:
  category: file_event
  product: windows
detection:
  selection_process:
    Image|endswith:
      - '\firefox.exe'
      - '\firefox-esr.exe'
  selection_path:
    TargetFilename|contains:
      - '\Downloads\'
      - '\AppData\Local\Temp\'
      - '\Desktop\'
  condition: selection_process and selection_path
falsepositives:
  - Normal user downloads
level: low
tags:
  - attack.t1190
  - cve.2026.109782

YARA Rule

rule CVE_2026_109782_Firefox_FileHandling_Bypass
{
    meta:
        author = "valtersit.com"
        description = "Detection heuristic for Firefox ESR file handling mitigation bypass artifacts"
        cve = "CVE-2026-109782"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109782"
        date = "2026-01-01"
    strings:
        $ua_esr = "Firefox/" ascii wide nocase
        $esr_tag = "ESR" ascii wide nocase
        $file_handling = "file handling" ascii wide nocase
    condition:
        $ua_esr and $esr_tag
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109782 Mozilla Firefox ESR File Handling Mitigation Bypass Attempt"; flow:established,to_server; http.user_agent; content:"Firefox"; nocase; http.user_agent; content:"ESR"; nocase; http.header; content:"Content-Disposition"; nocase; sid:2026109782; rev:1; reference:cve,2026-109782; classtype:attempted-user;)

Elastic Detection

process where event.type == "start" and
  process.name in ("firefox.exe", "firefox-esr.exe") and
  process.args : "*download*"

Splunk SPL Query

index=endpoint EventCode=11 Image="*firefox*esr*" OR Image="*firefox.exe"
| stats count by TargetFilename, Image, User
| where count > 0

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109782,">
  <rule id="1097820" level="3">
    <if_sid>550</if_sid>
    <field name="file">Downloads|Temp</field>
    <description>CVE-2026-109782: Suspicious Firefox ESR file handling activity</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Affected versions were not enumerated in the supplied data; the only authoritative version marker is Firefox ESR 153.5, listed as the fixed release.

1. Immediate mitigations: Update Firefox ESR to 153.5 or later where possible. Restrict browsing of untrusted sites from the affected ESR build. Enforce download restrictions and file-type policies at the endpoint level. Monitor file-event telemetry from firefox.exe.

2. Patch/upgrade steps: Deploy Firefox ESR 153.5 via the standard enterprise update channel. Validate installed version against the 153.5 baseline using application inventory tooling.

3. Long-term hardening: Subscribe to Mozilla's security advisories; maintain an enforced ESR update cadence; apply application control policies that limit browser-initiated file writes to approved directories; continuously review CWE-693 (protection mechanism failure) findings in browser components.

References

• https://nvd.nist.gov/vuln/detail/CVE-2026-109782
• https://www.mozilla.org/en-US/security/advisories/
• https://www.mozilla.org/en-US/firefox/enterprise/
• https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-109782
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns or exploit-in-the-wild evidence were identified in the supplied data.