CVE-2025-13889 identifies a stored Cross-Site Scripting (XSS) vulnerability in the Simple Nivo Slider plugin for WordPress, maintained by tmus. This vulnerability exists in all versions up to and including 0.5.6 due to insufficient sanitization and escaping of the 'id' shortcode parameter during web page generation. An attacker with at least Contributor-level privileges can inject arbitrary JavaScript code into pages by manipulating this parameter. Because the malicious script is stored persistently, it executes every time a user accesses the infected page, potentially compromising user sessions, stealing cookies, or performing actions on behalf of the victim. The vulnerability is classified under CWE-79, highlighting improper neutralization of input during web page generation. The CVSS v3.1 base score of 6.4 reflects network attack vector, low attack complexity, requiring privileges but no user interaction, and impacts confidentiality and integrity with no availability impact. No patches or fixes are currently linked, and no known exploits have been reported in the wild. The vulnerability's scope is confined to WordPress sites using this plugin, but given WordPress's widespread use, the risk is significant. The flaw emphasizes the importance of secure coding practices, especially input validation and output encoding in plugins that generate dynamic content.

For European organizations, this vulnerability poses a moderate risk primarily to websites running WordPress with the Simple Nivo Slider plugin installed. Exploitation can lead to unauthorized script execution, enabling attackers to hijack user sessions, deface websites, or conduct phishing attacks by injecting malicious content. This can damage organizational reputation, lead to data leakage, and potentially facilitate further internal attacks if administrative users are targeted. Since the attack requires authenticated access at Contributor level or above, organizations with lax access controls or many contributors are at higher risk. The vulnerability does not affect availability directly but compromises confidentiality and integrity of web content and user data. Given the prevalence of WordPress in Europe, especially among SMEs and public sector websites, the impact could be widespread if unaddressed. Additionally, GDPR considerations mean that data breaches resulting from such attacks could lead to regulatory penalties. The absence of known exploits currently provides a window for proactive mitigation before active exploitation emerges.

1. Immediately audit WordPress sites for the presence of the Simple Nivo Slider plugin and verify the version in use. 2. If an updated, patched version becomes available, prioritize upgrading to eliminate the vulnerability. 3. Until a patch is released, restrict Contributor-level and higher privileges to trusted users only, minimizing the risk of malicious shortcode injection. 4. Implement Web Application Firewall (WAF) rules to detect and block suspicious shortcode parameters or script injection attempts targeting the 'id' parameter. 5. Employ Content Security Policy (CSP) headers to limit the execution of unauthorized scripts on affected pages. 6. Regularly monitor logs and site content for unexpected shortcode usage or injected scripts. 7. Educate content contributors about secure content practices and the risks of injecting untrusted code. 8. Consider disabling or replacing the plugin with a more secure alternative if patching is delayed. 9. Conduct periodic security assessments focusing on plugin vulnerabilities and user privilege management. 10. Backup website data regularly to enable quick restoration in case of compromise.