CVE-2025-13856 is a stored cross-site scripting vulnerability identified in the Extra Post Images plugin for WordPress, developed by michaelcole1991. The vulnerability stems from improper neutralization of input during web page generation (CWE-79), specifically in the handling of the 'id' parameter within the extra-images shortcode. All versions up to and including 1.0 are affected due to insufficient input sanitization and lack of output escaping. An attacker with authenticated Contributor-level privileges or higher can inject arbitrary JavaScript code into posts or pages. This malicious script is stored persistently and executes in the context of any user who views the compromised page, potentially allowing session hijacking, privilege escalation, or redirection to malicious sites. The vulnerability has a CVSS 3.1 base score of 6.4, reflecting medium severity, with an attack vector of network, low attack complexity, requiring privileges, no user interaction, and scope change. No public exploits or patches have been reported at the time of publication. The vulnerability is particularly concerning because Contributor-level users are often trusted content creators, making it easier for attackers to gain the necessary access. The persistent nature of the XSS increases the risk of widespread impact across site visitors. Detection requires monitoring for unusual script injections in posts or pages using the extra-images shortcode. Remediation will require the plugin developer to implement proper input validation and output encoding for the 'id' parameter, and site administrators to update the plugin once a patch is available.

For European organizations, this vulnerability poses a moderate risk primarily to websites running WordPress with the Extra Post Images plugin installed. Exploitation could lead to unauthorized script execution in the browsers of site visitors, potentially resulting in session hijacking, theft of sensitive information, defacement, or distribution of malware. Organizations relying on WordPress for content management, especially those with Contributor-level users who can add or edit content, are at risk. This can affect e-commerce sites, news outlets, government portals, and corporate blogs, undermining user trust and potentially causing reputational damage. The vulnerability does not directly impact system availability but compromises confidentiality and integrity of user sessions and data. Given the widespread use of WordPress across Europe and the common practice of granting Contributor access to multiple users, the attack surface is significant. However, the requirement for authenticated access limits exploitation to insiders or compromised accounts, reducing the likelihood of mass automated attacks but increasing the risk of targeted insider threats or account takeover scenarios.

European organizations should immediately audit their WordPress installations to identify the presence of the Extra Post Images plugin and determine if it is in use. Restrict Contributor-level access to trusted users only and monitor user activity for suspicious behavior. Implement web application firewalls (WAFs) with rules to detect and block malicious script injections targeting the 'id' parameter in the extra-images shortcode. Regularly scan website content for unexpected JavaScript code or anomalies in posts and pages. Backup website data frequently to enable quick restoration if defacement occurs. Engage with the plugin developer or community to track the release of security patches and apply updates promptly once available. Consider temporarily disabling or removing the plugin if it is not essential to reduce exposure. Educate content creators about the risks of XSS and the importance of secure content practices. Employ Content Security Policy (CSP) headers to limit the execution of unauthorized scripts on affected sites. Finally, implement multi-factor authentication (MFA) for all WordPress user accounts to reduce the risk of account compromise.