CVE-2025-28917: Custom Smilies Plugin XSS Vulnerability

CVE-2025-28917 Overview

CVE-2025-28917 is a Stored Cross-Site Scripting (XSS) vulnerability affecting the Custom Smilies WordPress plugin developed by crazyloong. The vulnerability stems from improper neutralization of input during web page generation, allowing attackers to inject malicious scripts that persist in the application and execute in victims' browsers.

Critical Impact

Attackers can inject persistent malicious scripts that execute in the context of other users' sessions, potentially leading to session hijacking, credential theft, or unauthorized actions on behalf of authenticated users.

Affected Products

• Custom Smilies WordPress Plugin versions through 2.9.2
• WordPress installations using the custom-smilies-se plugin

Discovery Timeline

• 2025-03-26 - CVE-2025-28917 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2025-28917

Vulnerability Analysis

This Stored XSS vulnerability (CWE-79) occurs in the Custom Smilies WordPress plugin when user-supplied input is not properly sanitized before being stored and later rendered in web pages. Unlike reflected XSS, stored XSS payloads persist within the application's database, meaning every user who accesses the affected page will execute the malicious script without any additional interaction required from the attacker.

The vulnerability affects network-accessible WordPress installations and requires user interaction for successful exploitation. The scope is changed, meaning the vulnerable component impacts resources beyond its security scope, potentially affecting the entire WordPress installation and its users.

Root Cause

The root cause of this vulnerability is the failure to properly sanitize, validate, or encode user input before storing it in the database and subsequently rendering it in HTML output. The Custom Smilies plugin does not adequately implement output encoding or input validation, allowing HTML and JavaScript code to be stored and executed when the affected content is displayed to users.

Attack Vector

The attack vector is network-based, requiring an attacker to submit specially crafted input containing malicious JavaScript through the Custom Smilies plugin interface. The payload is then stored in the WordPress database and executed whenever a victim loads a page where the malicious smilie content is rendered. This could enable attackers to:

• Steal session cookies and authentication tokens
• Perform actions on behalf of authenticated administrators
• Redirect users to malicious websites
• Deface website content
• Deploy keyloggers to capture sensitive information

The vulnerability allows for confidentiality, integrity, and availability impacts due to the nature of stored XSS attacks in the context of WordPress administration interfaces.

Detection Methods for CVE-2025-28917

Indicators of Compromise

• Unexpected JavaScript code or HTML tags stored in smilie configurations or plugin database entries
• Unusual outbound connections from user browsers to external domains when accessing pages with smilies
• Reports of unexpected pop-ups, redirects, or behavior changes on pages using the Custom Smilies plugin
• Authentication anomalies or session hijacking incidents traced to XSS payload execution

Detection Strategies

• Implement Web Application Firewalls (WAF) with XSS detection rules targeting the Custom Smilies plugin endpoints
• Monitor WordPress database tables associated with the plugin for suspicious script tags or encoded JavaScript
• Review server access logs for POST requests to plugin endpoints containing potential XSS payloads
• Deploy browser-based Content Security Policy (CSP) violation reporting to detect script injection attempts

Monitoring Recommendations

• Enable WordPress security audit logging to track changes to plugin configurations
• Configure real-time alerts for database modifications to Custom Smilies plugin tables
• Implement Content Security Policy headers with strict script-src directives and violation reporting
• Regularly scan stored content for malicious JavaScript patterns using automated security tools

How to Mitigate CVE-2025-28917

Immediate Actions Required

• Audit existing smilie configurations for any suspicious or unexpected script content and remove malicious entries
• Consider temporarily disabling the Custom Smilies plugin until a patched version is available
• Implement Content Security Policy headers to mitigate the impact of any XSS exploitation
• Review WordPress user accounts for any unauthorized access or privilege changes

Patch Information

At the time of publication, versions through 2.9.2 of the Custom Smilies plugin remain vulnerable. Administrators should monitor the Patchstack Vulnerability Advisory for updates on patch availability. Consider removing the plugin entirely if it is not essential to site functionality, or evaluate alternative emoticon plugins with better security practices.

Workarounds

• Disable the Custom Smilies plugin until a security patch is released
• Restrict plugin access to only trusted administrator accounts
• Implement a Web Application Firewall with XSS filtering rules targeting the affected endpoints
• Add Content Security Policy headers to limit script execution sources and mitigate XSS impact

# WordPress wp-config.php - Add CSP headers as mitigation
# Add to your theme's functions.php or a security plugin
# This helps limit the impact of XSS attacks

# Example Apache .htaccess configuration
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline';"
Header set X-XSS-Protection "1; mode=block"
Header set X-Content-Type-Options "nosniff"