CVE-2025-9816 is a high-severity stored Cross-Site Scripting (XSS) vulnerability affecting the WordPress plugin "WP Statistics – Simple, privacy-friendly Google Analytics alternative" developed by veronalabs. This plugin is widely used as a privacy-conscious alternative to Google Analytics for website traffic analysis. The vulnerability exists in all versions up to and including 14.5.4. It arises from improper input sanitization and output escaping of the User-Agent HTTP header, which is incorporated into web pages generated by the plugin. An unauthenticated attacker can exploit this flaw by crafting a malicious User-Agent string that contains arbitrary JavaScript code. When a user accesses a page where the injected User-Agent string is rendered, the malicious script executes in the context of the victim's browser. This stored XSS can lead to session hijacking, defacement, redirection to malicious sites, or other client-side attacks. The vulnerability has a CVSS 3.1 base score of 7.2, reflecting its network attack vector, low attack complexity, no privileges or user interaction required, and a scope change that affects confidentiality and integrity but not availability. Although no known exploits have been reported in the wild yet, the ease of exploitation and the widespread use of the plugin make this a significant threat. The vulnerability is categorized under CWE-79, indicating improper neutralization of input during web page generation. No official patches or mitigation links have been provided at the time of this report, increasing the urgency for administrators to take protective measures.

For European organizations, this vulnerability poses a substantial risk, especially for those relying on the WP Statistics plugin for website analytics. Exploitation could lead to unauthorized script execution in the browsers of site administrators or visitors, potentially compromising sensitive session tokens, user credentials, or other confidential data. This can result in unauthorized access to administrative interfaces, data leakage, or further pivoting within the organization's web infrastructure. Given the privacy-centric nature of the plugin, organizations that emphasize data protection and compliance with regulations such as GDPR could face reputational damage and regulatory scrutiny if user data is compromised. Additionally, the integrity of website content and analytics data could be undermined, affecting business decisions and trust. The vulnerability's ability to be exploited without authentication or user interaction increases the risk of automated mass exploitation campaigns targeting European websites. The potential for scope change (affecting confidentiality and integrity) means that the impact could extend beyond individual users to organizational assets and data.

Immediate mitigation steps include disabling or removing the WP Statistics plugin until a secure patched version is released. Administrators should monitor official channels from veronalabs for security updates and apply patches promptly once available. In the interim, implementing Web Application Firewall (WAF) rules to detect and block suspicious User-Agent headers containing script tags or other malicious payloads can reduce exposure. Employing Content Security Policy (CSP) headers that restrict script execution sources can mitigate the impact of injected scripts. Regularly auditing and sanitizing logs and analytics data to detect anomalous User-Agent strings is advisable. Organizations should also review user roles and permissions to limit the impact of potential session hijacking. Encouraging users to use browsers with up-to-date security features and enabling HTTP-only and secure flags on cookies can further reduce risk. Finally, conducting security awareness training for administrators to recognize signs of XSS exploitation and suspicious website behavior is recommended.