CVE-2025-62800 identifies a reflected cross-site scripting (XSS) vulnerability in the FastMCP framework, a standard platform for building MCP applications. The vulnerability exists in versions prior to 2.13.0 within the OAuth client callback page (oauth_callback.py). The root cause is the improper neutralization of user-supplied input, which is inserted into the generated HTML without adequate escaping or sanitization. This allows an attacker to craft malicious URLs that, when visited by a user, execute arbitrary JavaScript code in the context of the callback server's origin. The vulnerability is classified under CWE-79, reflecting improper input handling during web page generation. The CVSS 4.0 vector indicates the attack can be performed remotely over the network (AV:N) with low attack complexity (AC:L), requires no privileges (PR:N), but does require user interaction (UI:P). The impact is limited to the confidentiality and integrity of user sessions and data accessible via the browser, with no direct impact on availability. No known exploits have been reported in the wild as of the publication date, but the risk remains significant for applications relying on FastMCP for OAuth workflows. The vulnerability is resolved in version 2.13.0 by properly escaping user inputs before rendering them in the HTML response. Given the nature of OAuth callback pages, exploitation could lead to session hijacking, phishing, or unauthorized actions performed on behalf of users.

For European organizations, the impact of this vulnerability primarily concerns web applications utilizing the FastMCP framework for OAuth authentication flows. Successful exploitation could allow attackers to execute arbitrary scripts in users' browsers, potentially leading to session hijacking, theft of OAuth tokens, or redirection to malicious sites. This compromises user confidentiality and integrity, undermining trust in authentication mechanisms. Organizations handling sensitive user data or providing critical services via FastMCP-based applications face increased risk of data breaches or account compromise. Additionally, regulatory frameworks such as GDPR impose strict requirements on protecting user data, and exploitation of this vulnerability could lead to compliance violations and reputational damage. The reflected XSS nature means attacks require user interaction, often through phishing or social engineering, which could be leveraged in targeted campaigns against European enterprises. The medium CVSS score reflects moderate risk, but the widespread use of OAuth in modern web services elevates the potential impact on identity and access management systems.

European organizations should immediately upgrade all FastMCP instances to version 2.13.0 or later to apply the official fix. In parallel, implement strict input validation and output encoding on all user-controllable parameters, especially those involved in OAuth callback URLs. Deploy Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS attacks. Conduct thorough security testing of OAuth workflows, including fuzzing and penetration testing focused on injection points. Educate users and administrators about phishing risks associated with malicious URLs exploiting this vulnerability. Monitor web server logs for suspicious callback requests containing unusual or encoded payloads. Where possible, implement multi-factor authentication (MFA) to mitigate the impact of session hijacking. Finally, maintain an incident response plan tailored to web application attacks to rapidly detect and respond to exploitation attempts.