CVE-2026-109777

Executive Summary

CVE-2026-109777 is an information disclosure vulnerability residing in the Storage: IndexedDB component of Mozilla Firefox. An attacker could potentially exploit this flaw to leak sensitive user data stored within the browser's client-side database, leading to privacy breaches or session hijacking. The vulnerability was addressed in Firefox version 158 and Firefox ESR 153.5; all prior versions are considered affected. There is currently no public proof-of-concept or evidence of active exploitation in the wild.

Technical Analysis

The vulnerability is classified as an Information Disclosure (CWE-200) issue within the Storage: IndexedDB component. IndexedDB is a low-level API for client-side storage of significant amounts of structured data, including files/blobs. It is used extensively by web applications to store user data, application state, and sometimes authentication tokens or cached sensitive information.

Based strictly on the available description, the flaw likely stems from an improper access control or a "leak" where data from one origin (website) could be read by another, or where a browser-internal process improperly exposes stored data. This class of vulnerability typically involves:

1. Cross-Origin Leakage: A failure in the Same-Origin Policy (SOP) enforcement within the IndexedDB implementation. This could allow a malicious website to read the contents of an IndexedDB database belonging to a different origin.
2. Privilege Escalation: A leak where a lower-privileged context (e.g., a web content process) can access data belonging to a higher-privileged context (e.g., browser UI or system principal).
3. Memory Leakage: Uninitialized memory or use-after-free conditions within the IndexedDB storage engine that exposes residual data from other operations.

Since the CNA title and NVD description do not specify the exact bug class (e.g., use-after-free, logic error), the attack surface is limited to the context of the browser's handling of IndexedDB operations, specifically during data retrieval or database management. The fix in Firefox 158 and ESR 153.5 suggests a code-level correction to the access control mechanisms or memory handling within this component.

Exploitation Scenarios

No public PoC or exploit code is available. The following chain is inferred strictly from the vulnerability class (Information Disclosure) and the affected component (IndexedDB).

1. Reconnaissance/Target Identification: An attacker identifies a victim who is using an affected version of Firefox (<158 or <153.5). The attacker would target users who visit a malicious website or interact with a malicious advertisement. The attacker does not need to be on the same network; this is a client-side attack requiring user interaction to visit a webpage.

2. Triggering the Flaw: The attacker crafts a website that interacts with the IndexedDB API. The specific mechanism of the flaw is unknown, but the exploitation would involve the attacker's script executing specific IndexedDB transactions. This could involve:

   • Attempting to open a database with a name that triggers a collision or race condition.
   • Sending malformed requests to the IndexedDB API that bypass origin checks.
   • Waiting for the victim to interact with a legitimate site (e.g., via an iframe) and then reading residual data from the shared process memory or database. The attack does not require any special privileges beyond the ability to run JavaScript in the victim's browser.

3. Payload/Execution Method: The payload is a malicious script (JavaScript) embedded in the attacker's webpage. When the victim visits the page, the script executes and attempts to read data from the vulnerable IndexedDB component. The script would contain logic to exfiltrate the gathered data to an attacker-controlled server (e.g., via fetch or XMLHttpRequest). There is no specific API endpoint or parameter name available to name here; the attack operates at the API level of the browser itself.

4. Post-Exploitation Impact: Upon successful exploitation, the attacker gains access to sensitive information stored in the victim's IndexedDB. This could include:

   • Authentication tokens or session cookies stored by web apps.
   • Personal user data (emails, messages, documents) cached by web applications.
   • Data from other websites the victim has visited, if the origin isolation is broken. The attacker can exfiltrate this data to their server, leading to account takeover, privacy violations, and further targeted attacks.

Impact Assessment

The CVSS score is N/A, but based on the CWE-200 classification and the nature of the component, the impact is primarily on Confidentiality (High) . There is no impact on Integrity or Availability. The attack vector is Network (AV:N) because the attacker delivers the exploit via a webpage, but it requires User Interaction (UI:R) as the victim must visit the malicious page. The attack complexity is likely Low (AC:L) for the attacker, as no special conditions beyond a crafted webpage are known.

Business Impact:

• Privacy Breach: Exposure of personal user data stored in browser databases.
• Account Takeover: If authentication tokens are leaked, attackers can hijack user sessions on affected websites.
• Reputation Damage: For organizations relying on Firefox for internal or customer-facing applications, a data leak could erode trust.
• Blast Radius: The vulnerability affects individual users, but in an enterprise context, if a critical web application stores sensitive data in IndexedDB, the leak could affect many users across the organization.

Detection Guidance

Sigma Rule

title: Firefox IndexedDB Information Disclosure Attempt
id: 7b8c9d0e-1f2a-3b4c-5d6e-7f8a9b0c1d2e
status: experimental
description: Detects potential exploitation attempts of CVE-2026-109777 via unusual IndexedDB activity in Firefox.
author: Threat Intelligence Team
date: 2026-10-27
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\firefox.exe'
        CommandLine|contains: 
            - 'private'
            - 'profile'
    condition: selection
    falsepositives:
        - Legitimate Firefox usage with private browsing or custom profiles.
level: low
tags:
    - attack.t1190
    - cve.2026.109777

YARA Rule

rule CVE_2026_109777_Firefox_IndexedDB_Leak
{
    meta:
        description = "Detects Firefox versions affected by CVE-2026-109777 based on version strings in binaries or scripts"
        author = "Threat Intelligence Team"
        date = "2026-10-27"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109777"
        hash = "d41d8cd98f00b204e9800998ecf8427e"
    strings:
        $ver_158 = "Firefox/158" ascii wide nocase
        $ver_esr = "Firefox/153.5" ascii wide nocase
        $idx = "IndexedDB" ascii wide nocase
    condition:
        $idx and not ($ver_158 or $ver_esr)
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109777 Firefox IndexedDB Information Disclosure Exfiltration Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/"; http_uri; content:"IndexedDB"; http_client_body; nocase; classtype:attempted-user; sid:10000001; rev:1; metadata:created_at 2026_10_27, cve CVE_2026_109777;)

Elastic Detection

process where process.name == "firefox.exe" and process.command_line like~ "*IndexedDB*"

Splunk SPL Query

index=main source="WinEventLog:Security" OR source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational"
| search (Image="*\\firefox.exe" AND (CommandLine="*IndexedDB*" OR CommandLine="*CVE-2026-109777*"))
| stats count by ComputerName, User, CommandLine

Wazuh OSSEC Rule

<group name="web,appsec,">
  <rule id="100001" level="5">
    <decoded_as>json</decoded_as>
    <field name="http.request.body">IndexedDB</field>
    <description>CVE-2026-109777: Possible Firefox IndexedDB data leak attempt detected.</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

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

1. Immediate Mitigations:

   • Upgrade Mozilla Firefox to version 158 or later, or Firefox ESR to version 153.5 or later. This is the only definitive remediation.
   • If immediate upgrade is not possible, consider disabling JavaScript for untrusted websites via browser settings or extensions (e.g., NoScript), though this may break functionality.
   • Educate users on the risks of visiting untrusted websites and clicking on suspicious links.

2. Patch/Upgrade Steps:

   • For standard Firefox: Navigate to Help > About Firefox to trigger an automatic update, or download the installer from the official Mozilla website.
   • For Firefox ESR: Download the ESR 153.5 installer from the Mozilla ESR download page or use enterprise deployment tools (e.g., MSI, GPO) to push the update.
   • Verify the version after installation by checking about:support.

3. Long-Term Hardening:

   • Implement a browser update policy that ensures critical security patches are applied within a defined SLA (e.g., 48 hours for critical vulnerabilities).
   • Use enterprise browser management tools to enforce version compliance and restrict the use of outdated browsers.
   • Monitor for unusual data exfiltration patterns from browsers and consider network-level controls to block known malicious domains.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-109777
2. Mozilla Foundation Security Advisory (hypothetical reference for Firefox 158/ESR 153.5): https://www.mozilla.org/en-US/security/advisories/
3. Mozilla Firefox Release Notes: https://www.mozilla.org/en-US/firefox/releases/
4. CWE-200: Information Exposure: https://cwe.mitre.org/data/definitions/200.html
5. MITRE ATT&CK T1190: Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/