CVE-2025-24559: WP Mailster Reflected XSS Vulnerability

CVE-2025-24559 Overview

CVE-2025-24559 is a reflected Cross-Site Scripting (XSS) vulnerability [CWE-79] affecting the WP Mailster plugin for WordPress developed by brandtoss. The flaw stems from improper neutralization of user-supplied input during web page generation. Attackers can craft malicious URLs that execute arbitrary JavaScript in a victim's browser when the victim interacts with the link. The vulnerability affects WP Mailster versions up to and including 1.8.15.0.

Critical Impact
Successful exploitation enables attackers to execute arbitrary JavaScript in the context of the victim's browser session, potentially leading to session hijacking, credential theft, or targeted phishing against WordPress administrators.

Affected Products

WP Mailster plugin for WordPress
All versions from n/a through 1.8.15.0
WordPress sites running the vulnerable plugin

Discovery Timeline

2025-02-03 - CVE-2025-24559 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-24559

Vulnerability Analysis

The vulnerability is a reflected Cross-Site Scripting flaw in the WP Mailster WordPress plugin. The plugin fails to properly sanitize or encode user-controlled input before including it in HTML responses. When a victim follows a crafted link containing malicious script payloads, the input is reflected back into the rendered page and executed by the browser.

Exploitation requires user interaction, as noted in the attack characteristics. The scope is changed, meaning the injected script can affect resources beyond the vulnerable component's authorization scope. Both confidentiality and integrity impacts are limited, with no direct impact on availability. According to the Exploit Prediction Scoring System, the vulnerability has a probability of 0.305% (percentile 22.614).

Root Cause

The root cause is improper neutralization of input during web page generation. The plugin echoes request parameters into HTML output without applying context-appropriate escaping such as esc_html(), esc_attr(), or wp_kses(). This allows HTML and JavaScript syntax within input to be interpreted rather than rendered as literal text.

Attack Vector

An attacker crafts a URL containing a malicious payload targeting a vulnerable WP Mailster endpoint. The attacker delivers the URL to a victim through phishing emails, forum posts, or malicious redirects. When the authenticated WordPress user clicks the link, the payload executes in their browser session under the trust context of the WordPress site. The attacker can then steal session cookies, perform actions on behalf of the user, or pivot to further attacks against site administrators.

See the Patchstack WP Mailster XSS Vulnerability advisory for additional technical detail.

Detection Methods for CVE-2025-24559

Indicators of Compromise

Suspicious URL parameters containing HTML tags such as <script>, onerror=, or javascript: targeting WP Mailster endpoints
Web server access logs showing encoded payloads (%3Cscript%3E, %22%3E) directed at plugin request handlers
Unexpected outbound requests from administrator browsers to attacker-controlled domains following clicks on external links
Session anomalies such as concurrent logins from disparate geolocations for WordPress administrator accounts

Detection Strategies

Deploy a Web Application Firewall (WAF) with rules that block reflected XSS payload patterns in query strings and POST bodies
Enable Content Security Policy (CSP) reporting to capture attempted script executions from unauthorized sources
Audit WordPress plugin inventory to identify sites running WP Mailster 1.8.15.0 or earlier
Correlate referrer headers with administrator session activity to identify social engineering entry points

Monitoring Recommendations

Monitor WordPress wp-admin sessions for unusual actions performed immediately after external referrer navigation
Alert on HTTP requests to WP Mailster endpoints containing characters commonly used in XSS payloads
Track plugin version drift across managed WordPress deployments to identify unpatched installations
Log and review CSP violation reports for the WordPress domain

How to Mitigate CVE-2025-24559

Immediate Actions Required

Identify all WordPress sites running the WP Mailster plugin at version 1.8.15.0 or earlier
Update WP Mailster to a version released after 1.8.15.0 that addresses the reflected XSS flaw
If a patched version is not yet available, deactivate the plugin until a fix is released
Rotate WordPress administrator sessions and reset credentials if suspicious activity is detected

Patch Information

Refer to the Patchstack advisory for the latest patch status and vendor guidance. Apply the fixed release through the WordPress plugin management interface and verify the installed version after upgrade.

Workarounds

Deploy WAF rules that block requests containing script tags, event handlers, or javascript: URIs targeting the plugin
Enforce a strict Content Security Policy that disallows inline scripts on WordPress administrative pages
Restrict WordPress administrator access to trusted networks using IP allowlists
Educate administrators to avoid clicking untrusted links while authenticated to WordPress

bash

# Example WAF rule pattern (ModSecurity) to block common reflected XSS payloads
SecRule ARGS "@rx (?i)(<script|onerror=|onload=|javascript:)" \
    "id:1002459,phase:2,deny,status:403,msg:'Blocked reflected XSS attempt against WP Mailster'"