Ninja Forms Plugin Flaw Exploited to Hack WordPress Sites

Overview

A stored Cross-Site Scripting (XSS) vulnerability within the Ninja Forms WordPress plugin is currently under active exploitation. Threat actors are leveraging this flaw to compromise WordPress websites, primarily by installing backdoors and establishing rogue administrative user accounts. This poses a critical risk to the integrity and security of affected web properties.

Technical Analysis

The vulnerability is identified as a stored Cross-Site Scripting (XSS) flaw in the Ninja Forms plugin. This type of vulnerability allows attackers to inject malicious scripts into web applications, which are then stored on the server and executed when a user (often an administrator) views the compromised content. In this exploitation, the XSS payload facilitates:

• Backdoor Installation: Malicious files are uploaded or created on the server, providing persistent access to the compromised WordPress environment.
• Rogue Admin Account Creation: Unauthorized user accounts with administrative privileges are created, granting attackers full control over the WordPress site.

The specific affected versions of the Ninja Forms plugin are not detailed in the source material, but active exploitation indicates that unpatched installations are vulnerable. The attack vector typically involves submitting crafted input through a form managed by Ninja Forms, where the input is not properly sanitized before being stored and subsequently displayed.

Detection

Defenders can identify potential exploitation through the following methods:

• Web Server Access Logs: Monitor for unusual POST requests to WordPress admin endpoints (/wp-admin/, /wp-json/, /wp-content/plugins/ninja-forms/) containing script tags (<script>), javascript: URIs, or common XSS event handlers (onerror=, onload=).
• File Integrity Monitoring (FIM): Look for newly created or modified .php files in sensitive WordPress directories such as wp-content/uploads/, wp-content/plugins/, or wp-includes/, especially if created by the web server process.
• WordPress Database Auditing: Review wp_users and wp_options tables for new, unauthorized administrative user accounts or suspicious entries indicative of backdoor code.
• User Activity Logs: Monitor for the creation of new user accounts with administrator privileges, particularly if not initiated by a legitimate administrator.
• WAF Logs: Analyze Web Application Firewall logs for blocked XSS attempts targeting WordPress form submission endpoints.

Sigma Detection Rules

WordPress Backdoor File Creation (Sysmon)

title: WordPress Backdoor File Creation (Sysmon)
id: e6d5f4c3-b2a1-4d0e-8c7f-6a5b4d3c2b1a
status: experimental
description: Detects suspicious file creation in common WordPress directories, potentially indicating a backdoor upload via a compromised web server process.
logsource:
  product: windows
  service: sysmon
detection:
  selection:
    EventID: 11
    TargetFilename|contains:
      - '\wp-content\uploads\'
      - '\wp-content\plugins\'
      - '\wp-includes\'
    TargetFilename|endswith:
      - '.php'
      - '.phtml'
      - '.asp'
      - '.aspx'
    Image|endswith:
      - '\w3wp.exe'
      - '\httpd.exe'
      - '\nginx.exe'
  condition: selection
level: high

WordPress Rogue Admin Account Creation (Web Server Logs)

title: WordPress Rogue Admin Account Creation (Web Server Logs)
id: a1b2c3d4-e5f6-7890-1234-567890abcdef
status: experimental
description: Detects attempts to create new administrative user accounts in WordPress via web server access logs, which could indicate post-exploitation activity.
logsource:
  product: webserver
detection:
  selection:
    cs-method: 'POST'
    cs-uri-stem|contains: '/wp-admin/user-new.php'
    cs-uri-query|contains: 'role=administrator'
  condition: selection
level: high

Mitigations

Prioritize the following actions to mitigate the risk and impact of this vulnerability:

1. Update Ninja Forms Plugin: Immediately update the Ninja Forms plugin to the latest available version. Consult the official plugin repository or vendor for patch availability.
2. Review User Accounts: Audit all WordPress user accounts for unauthorized administrators or suspicious new users. Remove any accounts that cannot be legitimate.
3. Scan for Backdoors: Perform a comprehensive scan of the WordPress installation using reputable security plugins or tools to identify and remove any installed backdoors or malicious files.
4. Implement Web Application Firewall (WAF): Deploy and configure a WAF to detect and block XSS attempts and other common web exploits.
5. Regular Backups: Ensure regular, verified backups of your WordPress site and database are maintained to facilitate recovery in case of compromise.

References

• https://www.bleepingcomputer.com/news/security/ninja-forms-plugin-flaw-exploited-to-hack-wordpress-sites/

Indicators of Compromise

No public IOCs available at time of writing.

MITRE ATT&CK

• T1190 — Exploit Public-Facing Application
• T1136.001 — Local Account
• T1505.003 — Web Shell
• T1059.004 — Unix Shell