CVE-2023-51540
📋 TL;DR
This stored cross-site scripting (XSS) vulnerability in the WordPress Custom 404 Pro plugin allows attackers to inject malicious scripts into web pages. When users view affected pages, the scripts execute in their browsers, potentially stealing credentials or performing unauthorized actions. All WordPress sites using Custom 404 Pro versions up to 3.10.0 are affected.
💻 Affected Systems
- WordPress Custom 404 Pro plugin
📦 What is this software?
Custom 404 Pro by Kunalnagar
⚠️ Risk & Real-World Impact
Worst Case
Attackers could steal administrator credentials, take over WordPress sites, install backdoors, deface websites, or redirect visitors to malicious sites.
Likely Case
Attackers inject malicious JavaScript to steal user session cookies, perform actions as authenticated users, or redirect users to phishing pages.
If Mitigated
With proper input validation and output encoding, malicious scripts would be neutralized before reaching users' browsers.
🎯 Exploit Status
Stored XSS vulnerabilities are commonly exploited. While no public PoC is confirmed, the vulnerability type is well-understood and easily weaponized.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Versions after 3.10.0
Vendor Advisory: https://patchstack.com/database/vulnerability/custom-404-pro/wordpress-custom-404-pro-plugin-3-10-0-cross-site-scripting-xss-vulnerability?_s_id=cve
Restart Required: No
Instructions:
1. Log into WordPress admin panel. 2. Navigate to Plugins → Installed Plugins. 3. Find Custom 404 Pro and click 'Update Now'. 4. If no update is available, deactivate and delete the plugin immediately.
🔧 Temporary Workarounds
Disable Custom 404 Pro Plugin
allTemporarily deactivate the vulnerable plugin until patched version is available
wp plugin deactivate custom-404-pro
Implement Content Security Policy
allAdd CSP headers to restrict script execution sources
Add to .htaccess: Header set Content-Security-Policy "default-src 'self'; script-src 'self'"
Add to wp-config.php: header("Content-Security-Policy: default-src 'self'; script-src 'self'");
🧯 If You Can't Patch
- Disable the Custom 404 Pro plugin immediately
- Implement web application firewall (WAF) rules to block XSS payloads
🔍 How to Verify
Check if Vulnerable:
Check WordPress admin → Plugins → Installed Plugins for Custom 404 Pro version 3.10.0 or earlierCheck Version:
wp plugin get custom-404-pro --field=versionVerify Fix Applied:
Verify plugin version is higher than 3.10.0 in WordPress admin panel📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to WordPress admin-ajax.php or admin-post.php with script tags
- Multiple 404 errors followed by suspicious parameter values
Network Indicators:
- HTTP requests containing <script> tags in URL parameters or POST data
- Outbound connections to suspicious domains after visiting 404 pages
SIEM Query:
source="wordpress.log" AND ("custom-404-pro" OR "404-pro") AND ("<script>" OR "javascript:" OR "onerror=" OR "onload=")