CVE-2025-31077

Executive Summary

CVE-2025-31077 is a DOM-Based Cross-Site Scripting (XSS) vulnerability in the Ultimate Blocks WordPress plugin, affecting versions up to and including 3.2.7. This flaw allows an attacker to inject malicious scripts into a web page’s DOM (Document Object Model) via unsanitized user input, which then executes in the victim’s browser within the context of the vulnerable WordPress site. The vulnerability is classified as Improper Neutralization of Input During Web Page Generation (CWE-79). Given that this is a WordPress plugin issue, the attack surface is limited to sites running the vulnerable plugin, and exploitation requires user interaction (e.g., clicking a crafted link). The CVSS score of 6.5 (Medium) reflects the need for user interaction and the partial impact on confidentiality and integrity. No public exploit code or patch is currently available, making proactive detection and mitigation critical.

Supply Chain & Package Security

This vulnerability resides in a WordPress plugin distributed via the official WordPress plugin repository. As such, it is part of the WordPress ecosystem and does not involve NPM, PyPI, Go, or Ruby packages. The supply chain risk is limited to the plugin’s update mechanism; site administrators should monitor for official patches from the plugin vendor.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
• Base Score: 6.5 (Medium)
• Attack Vector (AV): Network – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low – No special conditions are required.
• Privileges Required (PR): None – No authentication needed.
• User Interaction (UI): Required – The victim must click a crafted link or visit a malicious page.
• Scope (S): Changed – The impact extends beyond the vulnerable component.
• Confidentiality (C): Low – Limited data exposure possible.
• Integrity (I): Low – Limited ability to modify content.
• Availability (A): None – No impact on availability.

Threat Level: Medium. Exploitation requires user interaction, but the attack surface is broad due to the plugin’s popularity. No active exploitation has been reported, but the lack of a patch increases risk over time.

Deep Dive & Mechanics

The vulnerability stems from improper sanitization of user-supplied input that is later processed by client-side JavaScript within the Ultimate Blocks plugin. Specifically, the plugin’s JavaScript code manipulates the DOM using data from URL parameters, local storage, or other client-side sources without proper validation. An attacker can craft a malicious URL containing JavaScript payloads (e.g., <script>alert('XSS')</script> or event handlers like onload). When a victim visits the crafted URL, the plugin’s script reads the malicious input and injects it into the DOM, causing the payload to execute.

Root Cause: The plugin fails to use safe DOM manipulation methods (e.g., textContent instead of innerHTML) and does not sanitize input via functions like DOMPurify or WordPress’s built-in wp_sanitize functions on the client side.

Attack Flow:

1. Attacker crafts a URL: https://victim-site.com/?block_data=<script>alert(document.cookie)</script>
2. Victim clicks the link.
3. The Ultimate Blocks plugin’s JavaScript reads the block_data parameter.
4. The script injects the payload into the DOM using innerHTML or similar.
5. The browser executes the script, leading to cookie theft, redirection, or other malicious actions.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code pattern is described generically. The vulnerable plugin likely contains JavaScript similar to:

// Vulnerable code (Ultimate Blocks <= 3.2.7)
var userInput = new URLSearchParams(window.location.search).get('block_data');
document.getElementById('dynamic-content').innerHTML = userInput;

A patched version would sanitize the input before DOM insertion:

// Patched code (hypothetical)
var userInput = new URLSearchParams(window.location.search).get('block_data');
var sanitized = DOMPurify.sanitize(userInput);
document.getElementById('dynamic-content').textContent = sanitized;
// Or use textContent to avoid HTML injection

Exploitation & Lab Setup

Red Team (Proof of Concept)

Since no public exploit exists, a basic PoC can be constructed. This demonstrates the vulnerability but should only be used in authorized environments.

PoC URL:

https://victim-site.com/?block_data=<img src=x onerror=alert('XSS')>

Step-by-step:

1. Identify a WordPress site running Ultimate Blocks <= 3.2.7.
2. Craft a URL with a malicious block_data parameter.
3. Send the link to a victim (e.g., via phishing email).
4. When the victim clicks, the payload executes in their browser.

Note: The exact parameter name may vary; reconnaissance is needed to identify the vulnerable input vector.

Blue Team (Detection & Hardening)

Detection:

• Monitor web logs for suspicious URL parameters containing <script>, onerror, onload, or base64-encoded payloads.
• Use browser-based XSS filters (e.g., X-XSS-Protection header, though deprecated).
• Deploy a Web Application Firewall (WAF) with rules to block common XSS patterns.

Hardening:

• Disable the Ultimate Blocks plugin until a patch is released.
• Implement Content Security Policy (CSP) headers to restrict script execution.
• Use WordPress security plugins (e.g., Wordfence) to add virtual patching.

Indicators of Compromise (IOCs)

• URL Patterns: Requests containing block_data= (or similar parameter) with HTML/JavaScript payloads.
• Log Entries: HTTP requests with parameters like ?block_data=<script>, ?block_data=<img, etc.
• Client-Side Artifacts: Injected DOM elements in the browser’s developer tools (e.g., unexpected <script> tags).
• Network Traffic: Outbound connections to attacker-controlled domains (if payload exfiltrates data).

Detection Engineering

YARA Rule

rule CVE_2025_31077_DOM_XSS {
    meta:
        description = "Detects DOM-based XSS attempts targeting Ultimate Blocks plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-31077"
    strings:
        $payload1 = /block_data=.*<script/i
        $payload2 = /block_data=.*onerror/i
        $payload3 = /block_data=.*onload/i
        $payload4 = /block_data=.*javascript:/i
    condition:
        any of them
}

Sigma Rule

title: CVE-2025-31077 DOM-Based XSS in Ultimate Blocks
id: 8a4b3c2d-1e5f-6a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects HTTP requests with XSS payloads targeting the Ultimate Blocks plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        c-uri|contains:
            - 'block_data=<script'
            - 'block_data=<img'
            - 'block_data=javascript:'
    condition: selection
falsepositives:
    - Legitimate use of block_data parameter with safe content
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-31077 DOM-Based XSS in Ultimate Blocks"; flow:to_server,established; content:"block_data="; http_uri; pcre:"/block_data=.*(<script|<img|onerror|onload|javascript:)/i"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Disable the Ultimate Blocks plugin if not critical to operations.
2. Patch: Await an official update from the plugin vendor. Monitor the WordPress plugin repository for version > 3.2.7.
3. Virtual Patching: Use a WAF to block requests containing block_data= with suspicious patterns.
4. CSP Implementation: Add the following header to your web server configuration:

   Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval'; object-src 'none';
   

   (Note: unsafe-inline may be needed for plugin functionality; adjust carefully.)
5. User Education: Train users to avoid clicking suspicious links, especially those with query parameters.
6. Monitoring: Continuously monitor web logs for XSS patterns and review plugin updates.

Valters IT