CVE-2026-57423: Contact Form 7 Message Filter XSS Flaw

CVE-2026-57423 Overview

CVE-2026-57423 is a reflected Cross-Site Scripting (XSS) vulnerability in the Kofi Mokome Message Filter for Contact Form 7 WordPress plugin. The flaw affects all versions up to and including 1.6.3.8. Attackers can craft URLs containing malicious JavaScript that executes in a victim's browser when clicked. The vulnerability is tracked under CWE-79 for improper neutralization of input during web page generation.

Critical Impact
Successful exploitation allows attackers to execute arbitrary JavaScript in the context of a victim's authenticated session, enabling session theft, credential harvesting, or forced administrative actions on WordPress sites running the plugin.

Affected Products

Kofi Mokome Message Filter for Contact Form 7 (plugin slug: cf7-message-filter)
All plugin versions from initial release through 1.6.3.8
WordPress installations using the vulnerable plugin release

Discovery Timeline

2026-07-13 - CVE-2026-57423 published to NVD
2026-07-13 - Last updated in NVD database

Technical Details for CVE-2026-57423

Vulnerability Analysis

The vulnerability stems from improper input neutralization in the Message Filter for Contact Form 7 plugin. User-supplied input reaches the rendered web page without proper HTML encoding or sanitization. This allows attackers to inject arbitrary script content that executes in the browser of any user who visits a crafted URL.

Because the flaw is reflected, it requires user interaction. A victim must click an attacker-controlled link or visit a page that triggers the malicious request. The scope-changed component of the vulnerability indicates that impact can extend beyond the vulnerable plugin, affecting the broader WordPress session and browser context.

Exploitation does not require authentication. Any unauthenticated attacker can craft a payload and distribute it through phishing, social media, or malicious advertising. The CWE-79 classification confirms the root category of Cross-Site Scripting.

Root Cause

The plugin fails to apply output escaping functions such as esc_html(), esc_attr(), or wp_kses() on request parameters before echoing them into HTML responses. WordPress provides these sanitization APIs specifically to prevent XSS, but the affected code paths in cf7-message-filter do not invoke them consistently through version 1.6.3.8.

Attack Vector

An attacker constructs a URL containing a JavaScript payload in a query parameter that the plugin reflects into the response. The victim clicks the link while authenticated to the WordPress site. The injected script runs with the victim's privileges, allowing the attacker to steal cookies, hijack sessions, or perform actions on behalf of administrators.

The vulnerability manifests when the plugin renders unsanitized request parameters into HTML output. Refer to the Patchstack WordPress Vulnerability Report for technical specifics.

Detection Methods for CVE-2026-57423

Indicators of Compromise

HTTP requests to WordPress sites containing URL-encoded <script> tags, javascript: schemes, or event handlers such as onerror= and onload= in query strings targeting cf7-message-filter endpoints.
Web server access logs showing referrer chains from phishing domains to plugin pages with abnormally long query parameters.
Browser console errors or unexpected outbound requests to attacker-controlled domains originating from WordPress admin sessions.

Detection Strategies

Deploy a Web Application Firewall (WAF) rule that inspects query strings and POST bodies for common XSS patterns before requests reach the plugin.
Enable WordPress audit logging to record suspicious parameter values submitted to plugin URLs.
Correlate outbound network telemetry from administrator workstations with WordPress login sessions to identify script-driven exfiltration.

Monitoring Recommendations

Monitor plugin version inventories to identify installations of cf7-message-filter at or below 1.6.3.8.
Track HTTP response bodies for reflected content matching request parameters, which indicates candidate XSS sinks.
Alert on Content Security Policy (CSP) violation reports generated by browsers rendering unauthorized inline scripts.

How to Mitigate CVE-2026-57423

Immediate Actions Required

Update the Message Filter for Contact Form 7 plugin to a version released after 1.6.3.8 as soon as the vendor publishes a fix.
If no patch is available, deactivate and remove the plugin until a fixed version is released.
Force-reset administrator sessions and rotate credentials for sites that may have been targeted by phishing campaigns.

Patch Information

At the time of publication, the Patchstack advisory confirms all versions up to and including 1.6.3.8 are affected. Site owners should monitor the plugin's WordPress.org page and vendor channels for a security release addressing CVE-2026-57423.

Workarounds

Deploy a WAF such as Wordfence, Patchstack, or Cloudflare with XSS signature rules covering the affected plugin paths.
Implement a strict Content Security Policy that blocks inline script execution and restricts script sources to trusted origins.
Restrict administrative access using IP allowlists and require multi-factor authentication to limit the value of stolen sessions.

bash

# Example NGINX configuration to block common XSS payloads targeting the plugin
location ~* /wp-content/plugins/cf7-message-filter/ {
    if ($query_string ~* "(<|%3C).*script.*(>|%3E)") {
        return 403;
    }
    if ($query_string ~* "(javascript:|onerror=|onload=)") {
        return 403;
    }
}

# Add a Content Security Policy header site-wide
add_header Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none';" always;