KIA Subtitle <= 4.0.1 - [Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')]

This research plan outlines the process for analyzing and demonstrating the Stored Cross-Site Scripting (XSS) vulnerability in the KIA Subtitle plugin (CVE-2026-7509).

1. Vulnerability Summary

The KIA Subtitle plugin is vulnerable to Stored Cross-Site Scripting via the [the-subtitle] shortcode. The vulnerability exists because the plugin's shortcode handler accepts before and after attributes from the user and outputs them to the page without proper sanitization or escaping (e.g., using esc_html() or wp_kses()). An authenticated attacker with Contributor permissions can create a post containing this shortcode with malicious JavaScript in the attributes, which will execute in the context of any user viewing the post.

2. Attack Vector Analysis

• Endpoint: WordPress Post Editor (via post.php or REST API)
• Vulnerable Component: Shortcode callback function for [the-subtitle]
• Payload Location: Shortcode attributes before and/or after
• Authentication Level: Contributor or higher (any role capable of using shortcodes in post content)
• Precondition: The post must have a subtitle value set (via the kia_subtitle meta field) for the shortcode to generate output.

3. Code Flow

1. Registration: In kia-subtitle.php, the shortcode is registered in the constructor:
   add_shortcode( 'the-subtitle', array( $this, 'shortcode' ) ); (Line 126).
2. Attribute Handling: The shortcode() method (inferred) likely uses shortcode_atts() to extract before and after.
3. Core Function Call: The shortcode method likely calls the helper function the_subtitle() (documented in readme.txt) or get_the_subtitle().
4. The Sink: The the_subtitle() function (or the shortcode handler itself) concatenates the $before and $after strings with the subtitle content and returns/echoes the result. Because these strings are not passed through escaping functions before being returned to the WordPress content filter, the injected HTML/JavaScript is rendered by the browser.

4. Nonce Acquisition Strategy

Since the vulnerability is a Stored XSS triggered by a Contributor-level user saving a post, we need to acquire the standard WordPress nonces required for post creation.

1. Authentication: Authenticate as a Contributor user using wp-login.php.
2. Access Editor: Navigate to wp-admin/post-new.php.
3. Extract Nonce: Use browser_eval to extract the _wpnonce from the page source or the wp-nonce required for the REST API if using the Block Editor.
   • For the Classic Editor (used by default if Gutenberg is disabled): document.querySelector('#_wpnonce').value.
   • For the REST API (Gutenberg): The nonce is typically found in wpApiSettings.nonce.

5. Exploitation Strategy

The goal is to create a post as a Contributor that executes an alert when viewed by an Admin.

1. Setup Phase:
   • Use wp-cli to create a Contributor user and a post.
   • Crucially, set a value for the kia_subtitle meta key so the shortcode produces output.
2. Execution Phase (HTTP):
   • Step 1: Log in as the Contributor.
   • Step 2: Update a post's content to include the malicious shortcode.
   • Payload: [the-subtitle before='<script>alert("XSS_BEFORE")</script>' after='<script>alert("XSS_AFTER")</script>']
3. HTTP Request Details:
   • Method: POST
   • URL: http://localhost:8080/wp-admin/post.php
   • Headers: Content-Type: application/x-www-form-urlencoded
   • Body Parameters:
     • action: editpost
     • post_ID: [TARGET_POST_ID]
     • _wpnonce: [EXTRACTED_NONCE]
     • content: [the-subtitle before='<script>alert(1)</script>']
     • post_title: XSS Test

6. Test Data Setup

1. Plugin: Install and activate kia-subtitle version 4.0.1.
2. User: Create a user attacker with the contributor role.
3. Target Post:
   • wp post create --post_author=[ATTACKER_ID] --post_title="XSS Test" --post_status="publish" --post_content="Shortcode here"
   • Important: wp post meta set [POST_ID] kia_subtitle "Valid Subtitle" (The shortcode only renders if a subtitle exists).

7. Expected Results

When any user (specifically an Administrator) navigates to the permalink of the created post:

1. The WordPress engine parses the [the-subtitle] shortcode.
2. The shortcode handler injects the raw <script> tags into the page output.
3. The browser executes the JavaScript, resulting in an alert box showing 1.

8. Verification Steps

1. Check Meta: Confirm the meta is set: wp post meta get [POST_ID] kia_subtitle.
2. Verify Rendering: Use http_request (GET) on the post URL and check if the raw payload exists in the source:
   • grep "<script>alert(1)</script>"
3. Confirm Sink: Ensure the script is not encoded as <script>.

9. Alternative Approaches

If the before/after attributes are filtered by the WordPress KSES system during saving (unlikely for shortcodes), try bypassing via attribute breakout:

• Payload: [the-subtitle before='<img src=x onerror=alert(1)>']
• If the site uses the Block Editor, try injecting the payload via the kia/post-subtitle block's attributes if they are mirrored in the shortcode implementation. However, the CVE specifically identifies the shortcode as the primary vector.