CVE-2025-4513 is a medium-severity security vulnerability identified in the Catalyst User Key Authentication Plugin version 20220819 used within Moodle, a widely adopted open-source learning management system. The vulnerability resides in the logout functionality, specifically in the /auth/userkey/logout.php file. It involves improper handling of the 'return' argument, which can be manipulated to perform an open redirect attack. An open redirect occurs when an application accepts untrusted input that causes it to redirect users to a malicious external site. This vulnerability can be exploited remotely without requiring authentication or privileges, and only requires user interaction to follow the crafted redirect link. The CVSS 4.0 base score is 5.3, indicating a medium severity level. The attack vector is network-based with low attack complexity and no privileges or user authentication required, but user interaction is necessary. The vulnerability does not impact confidentiality or availability directly but can affect integrity by redirecting users to malicious sites, potentially facilitating phishing, credential theft, or malware distribution. The vendor was notified but has not responded or issued a patch, and no known exploits have been reported in the wild yet. Given Moodle's widespread use in educational institutions, this vulnerability could be leveraged by attackers to trick users into visiting malicious websites under the guise of legitimate logout redirection, undermining user trust and potentially leading to further compromise through social engineering or drive-by downloads.

For European organizations, especially educational institutions and universities that extensively use Moodle, this vulnerability poses a significant risk. The open redirect can be exploited to conduct phishing campaigns targeting students, faculty, and staff by redirecting them to malicious sites that mimic legitimate services. This can lead to credential compromise, unauthorized access to sensitive academic or personal data, and potential malware infections. Since Moodle is often integrated with other educational tools and identity providers, a successful attack could cascade into broader access issues. The lack of vendor response and patch availability increases the window of exposure. Additionally, GDPR and other data protection regulations in Europe impose strict requirements on protecting user data and preventing unauthorized access, so exploitation of this vulnerability could result in regulatory penalties and reputational damage for affected organizations.

European organizations using the affected Catalyst User Key Authentication Plugin version 20220819 should take immediate steps to mitigate risk. First, implement strict input validation and sanitization on the 'return' parameter at the web application firewall (WAF) or reverse proxy level to block or rewrite suspicious redirect URLs. Organizations should consider disabling or restricting the use of the User Key Authentication Plugin logout functionality until a vendor patch or update is available. Educate users about the risks of clicking on unexpected logout links and encourage verification of URLs before following redirects. Monitoring web server logs for unusual redirect patterns can help detect exploitation attempts. If feasible, organizations can deploy custom patches or overrides to enforce safe redirect destinations (e.g., only allowing internal URLs). Finally, maintain up-to-date backups and incident response plans to quickly address any compromise resulting from phishing or malware infections stemming from this vulnerability.