CVE-2025-0863

Executive Summary

CVE-2025-0868 is a Stored Cross-Site Scripting (XSS) vulnerability in the Flexmls® IDX Plugin for WordPress, affecting all versions up to and including 3.14.27. The flaw resides in the idx_frame shortcode, where user-supplied attributes are not properly sanitized or escaped. An authenticated attacker with contributor-level access or higher can inject arbitrary web scripts into pages. When a victim (including administrators) visits the compromised page, the injected script executes in their browser, potentially leading to session hijacking, credential theft, or defacement.

This vulnerability is specific to the WordPress ecosystem and does not involve Apache or Nginx misconfigurations. The CVSS score of 6.4 (Medium) reflects the need for authentication but also the potential for cross-site scripting in a WordPress context.

Supply Chain & Package Security

This is a WordPress plugin vulnerability. The plugin is distributed via the WordPress plugin repository and directly installed by site administrators. No supply chain compromise of the plugin’s source code repository has been reported. The risk is limited to sites running the vulnerable plugin version.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
• Base Score: 6.4 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) – requires contributor-level access
• User Interaction: None (UI:N)
• Scope: Changed (S:C) – the vulnerable component impacts resources beyond its security scope
• Confidentiality Impact: Low (C:L) – limited data exposure
• Integrity Impact: Low (I:L) – limited modification of content
• Availability Impact: None (A:N)

Threat Level: Medium. Exploitation requires authenticated access, but the impact can be severe if an attacker targets an administrator session.

Deep Dive & Mechanics

The vulnerability exists in the idx_frame shortcode provided by the Flexmls® IDX Plugin. Shortcodes in WordPress allow users to embed dynamic content via attributes. The plugin fails to properly sanitize and escape user-supplied attributes before rendering them in the HTML output.

Root Cause: Insufficient input sanitization and output escaping on shortcode attributes. Specifically, the idx_frame shortcode likely passes attribute values directly into HTML attributes or JavaScript contexts without proper encoding.

Attack Flow:

1. An authenticated user with contributor-level access creates or edits a post/page.
2. The attacker inserts the [idx_frame] shortcode with a malicious attribute, e.g., [idx_frame src="javascript:alert('XSS')"] or [idx_frame onmouseover="alert(1)"].
3. The plugin renders the shortcode without escaping the attribute value, injecting the malicious script into the page.
4. When any user (including administrators) views the page, the script executes in their browser.

WordPress Context: This vulnerability is specific to the Flexmls plugin and does not affect core WordPress. The attack surface is limited to sites using the vulnerable plugin.

Vulnerable vs. Patched Code

Since no official patch is available, the vulnerable code pattern is described below.

Vulnerable Code (Conceptual):

function idx_frame_shortcode($atts) {
    $src = $atts['src'];
    return '<iframe src="' . $src . '"></iframe>';
}

The $src attribute is directly concatenated into the HTML without escaping.

Patched Code (Conceptual):

function idx_frame_shortcode($atts) {
    $src = esc_url($atts['src']);
    return '<iframe src="' . $src . '"></iframe>';
}

Using esc_url() or esc_attr() ensures the attribute is safe for HTML output.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• WordPress site with Flexmls® IDX Plugin <= 3.14.27
• Authenticated user with contributor-level access

Exploitation Steps:

1. Log in as a contributor.
2. Create a new post or page.
3. Insert the following shortcode in the content editor:

   [idx_frame src="javascript:alert('XSS')"]
   

   Or use an event handler:

   [idx_frame onmouseover="alert(1)"]
   

4. Publish the post.
5. When any user views the post, the script executes.

Proof-of-Concept (PoC):

<!-- No public exploit code available. The above shortcode injection is the PoC. -->

Blue Team (Detection & Hardening)

Detection:

• Monitor for posts/pages containing the [idx_frame] shortcode with suspicious attributes (e.g., javascript:, on* events).
• Review user activity logs for contributors creating posts with unusual shortcode usage.

Hardening:

• Disable the plugin until a patch is released.
• Restrict contributor-level access to trusted users only.
• Use a Web Application Firewall (WAF) to block XSS payloads in shortcode attributes.
• Implement Content Security Policy (CSP) headers to mitigate script execution.

Indicators of Compromise (IOCs)

• File Indicators:

  • Presence of the flexmls-idx plugin in /wp-content/plugins/flexmls-idx/
  • Modified posts/pages containing [idx_frame] shortcode with non-standard attributes

• Network Indicators:

  • Outbound connections to attacker-controlled domains from compromised pages
  • HTTP requests containing [idx_frame] with javascript: or event handler attributes

• Log Indicators:

  • WordPress audit logs showing creation/editing of posts by contributors with shortcode usage
  • Web server logs showing requests to pages with the vulnerable shortcode

Detection Engineering

YARA Rule

rule flexmls_idx_xss {
    meta:
        description = "Detects stored XSS payloads in Flexmls IDX shortcode attributes"
        author = "Valters IT"
        date = "2025-02-20"
        reference = "CVE-2025-0868"
    strings:
        $shortcode = /\[idx_frame\s+[^\]]*?(?:src|on\w+)\s*=\s*["'][^"']*?(?:javascript|alert|on\w+)/i
    condition:
        $shortcode
}

Sigma Rule

title: Stored XSS via Flexmls IDX Shortcode
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects creation of posts/pages containing malicious Flexmls IDX shortcode attributes
author: Valters IT
date: 2025-02-20
logsource:
    category: application
    product: wordpress
detection:
    selection:
        post_content|contains: '[idx_frame'
        post_content|re: '\[idx_frame\s+[^\]]*?(?:src|on\w+)\s*=\s*["\'][^"\' ]*?(?:javascript|alert|on\w+)'
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in shortcode attributes (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-0868 - Flexmls IDX Stored XSS Attempt"; flow:to_server,established; content:"[idx_frame"; http_client_body; pcre:"/\[idx_frame\s+[^\]]*?(?:src|on\w+)\s*=\s*["\'][^"\' ]*?(?:javascript|alert|on\w+)/i"; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patched version is released.
2. Audit existing posts/pages for malicious shortcode usage and remove them.
3. Review user roles and revoke contributor access for untrusted users.

Long-Term Fix:

• Apply the official patch from Flexmls when available.
• Implement input validation and output escaping for all shortcode attributes.

Workarounds:

• Use a WordPress security plugin to block XSS payloads.
• Add a custom function to sanitize shortcode attributes:

  add_shortcode('idx_frame', 'custom_idx_frame_shortcode');
  function custom_idx_frame_shortcode($atts) {
      $atts = shortcode_atts(array('src' => ''), $atts);
      $src = esc_url($atts['src']);
      return '<iframe src="' . $src . '"></iframe>';
  }
  

Valters IT Signature: This report was generated by Valters IT DevSecOps AI. All information is provided for educational and defensive purposes only.