CVE-2024-39646: Custom 404 Pro XSS Vulnerability

CVE-2024-39646 Overview

CVE-2024-39646 is a reflected cross-site scripting (XSS) vulnerability in the Kunal Custom 404 Pro WordPress plugin. The flaw affects all versions of custom-404-pro up to and including 3.11.1. The plugin fails to properly neutralize user-controlled input before reflecting it into a generated web page, allowing attackers to inject arbitrary JavaScript that executes in the victim's browser session. Successful exploitation requires user interaction, typically through a crafted link, and can lead to session theft, credential harvesting, or unauthorized actions performed on behalf of an authenticated administrator. The issue is classified under [CWE-79].

Critical Impact

Attackers can execute arbitrary JavaScript in an administrator's browser via a crafted link, enabling account takeover of WordPress sites running the vulnerable plugin.

Affected Products

• Kunal Custom 404 Pro (custom-404-pro) WordPress plugin
• All versions from initial release through 3.11.1
• WordPress installations with the plugin activated

Discovery Timeline

• 2024-08-01 - CVE-2024-39646 published to NVD
• 2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-39646

Vulnerability Analysis

The Custom 404 Pro plugin generates dynamic web page content that includes attacker-controllable input from HTTP request parameters. The plugin reflects this input into the response without sufficient output encoding or input sanitization, producing a reflected XSS condition [CWE-79]. When a victim clicks a crafted URL, the injected payload executes in the browser under the origin of the WordPress site.

Because the attack requires the victim to load the malicious URL, exploitation depends on social engineering. If the victim holds administrative privileges, the attacker can leverage the executed script to perform privileged actions, install malicious plugins, or exfiltrate authentication cookies. The scope change reflected in the CVSS vector indicates the impact extends beyond the vulnerable plugin's own security context to the broader WordPress application.

Root Cause

The root cause is missing or inadequate neutralization of user input during web page generation. The plugin does not apply WordPress escaping functions such as esc_html(), esc_attr(), or wp_kses() to request-derived data before rendering it. As a result, characters such as <, >, and quotation marks pass through into the HTML output and are interpreted by the browser as markup rather than literal text.

Attack Vector

Exploitation occurs over the network without authentication but requires user interaction. An attacker crafts a URL targeting a vulnerable endpoint of the Custom 404 Pro plugin with a JavaScript payload embedded in a reflected parameter. The attacker then delivers the URL through phishing, forum posts, or malicious advertisements. When a logged-in WordPress user follows the link, the payload executes in their browser context. See the Patchstack Vulnerability Report for additional technical detail.

Detection Methods for CVE-2024-39646

Indicators of Compromise

• Web server access logs showing requests to Custom 404 Pro endpoints containing URL-encoded <script>, onerror=, onload=, or javascript: tokens
• Unusual outbound requests from administrator browsers to attacker-controlled domains shortly after clicking external links
• Unexpected creation of WordPress administrator accounts or plugin installations following suspicious admin sessions
• Referer headers from external domains preceding sensitive admin panel activity

Detection Strategies

• Inspect HTTP query strings and POST bodies for HTML tags or JavaScript event handlers reflected in server responses
• Deploy a web application firewall rule set that matches common reflected XSS payloads targeting WordPress plugin parameters
• Correlate anomalous administrator activity with recent inbound clicks from external referrers

Monitoring Recommendations

• Enable verbose access logging on the WordPress front-end and forward logs to a centralized analytics platform for pattern analysis
• Alert on WordPress admin actions such as user creation, role changes, or plugin uploads that occur within seconds of an external referer
• Track the installed version of the custom-404-pro plugin across all managed WordPress sites and flag any instance at 3.11.1 or earlier

How to Mitigate CVE-2024-39646

Immediate Actions Required

• Identify all WordPress sites running the Custom 404 Pro plugin and confirm the installed version
• Update the plugin to a version later than 3.11.1 where the vendor has addressed the input handling flaw
• If no patched version is available for your environment, deactivate and remove the plugin until a fix ships
• Force a password reset and session invalidation for WordPress administrators who may have clicked untrusted links

Patch Information

Refer to the Patchstack Vulnerability Report for the current patch status and update guidance. Apply plugin updates from the official WordPress plugin repository. Verify the post-update version in the WordPress admin dashboard under Plugins.

Workarounds

• Deploy a web application firewall with rules that block reflected XSS payloads targeting plugin parameters
• Restrict WordPress admin access by IP address or through a VPN to reduce exposure to phishing-delivered links
• Enforce a strict Content Security Policy that disallows inline scripts and untrusted script sources
• Train administrators to avoid clicking untrusted links while authenticated to the WordPress admin panel

# Example Content Security Policy header for WordPress (add via server config or security plugin)
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none'; base-uri 'self'; frame-ancestors 'self'"