CVE-2025-62800 is a reflected cross-site scripting (XSS) vulnerability classified under CWE-79, affecting jlowin's FastMCP framework versions earlier than 2.13.0. FastMCP is a widely used framework for building MCP applications, and the vulnerability specifically resides in the OAuth client callback page (oauth_callback.py). The flaw occurs because the application inserts user-controlled input directly into the HTML response without proper escaping or sanitization. This improper neutralization of input allows attackers to craft malicious URLs that, when visited by a user, execute arbitrary JavaScript code within the context of the vulnerable server's origin. The vulnerability does not require authentication (AV:N) and has low attack complexity (AC:L), but it does require user interaction (UI:P), such as clicking a malicious link. The impact is limited to client-side script execution, which can lead to session hijacking, credential theft, or redirection to malicious sites. The CVSS 4.0 base score is 5.3, reflecting medium severity with limited scope and no direct impact on confidentiality, integrity, or availability of the server itself. No known exploits have been reported in the wild, but the vulnerability is publicly disclosed and fixed in FastMCP version 2.13.0. Organizations using vulnerable versions should prioritize patching and review their OAuth callback implementations to ensure proper input validation and output encoding.

For European organizations, this vulnerability poses a risk primarily to web application users interacting with OAuth authentication flows built on FastMCP versions prior to 2.13.0. Successful exploitation could allow attackers to execute arbitrary JavaScript in users' browsers, potentially leading to session hijacking, theft of OAuth tokens, or unauthorized actions performed on behalf of the user. This can undermine user trust, lead to data breaches, and facilitate further attacks such as phishing or lateral movement within networks. While the vulnerability does not directly compromise server confidentiality or availability, the client-side impact can cascade into broader organizational risks, especially in sectors relying heavily on OAuth for authentication, such as finance, healthcare, and government services. The medium severity rating suggests a moderate risk level, but the ease of exploitation combined with the widespread use of OAuth means that unpatched systems could be attractive targets. European organizations must consider the regulatory implications of any data compromise under GDPR and related privacy laws.

The primary mitigation is to upgrade all FastMCP deployments to version 2.13.0 or later, where the vulnerability is fixed. Organizations should audit their OAuth callback implementations to ensure that all user-supplied input is properly validated and escaped before being included in HTML responses. Implementing Content Security Policy (CSP) headers can help reduce the impact of XSS by restricting the execution of unauthorized scripts. Additionally, employing security-focused code reviews and automated scanning tools to detect improper input handling in web applications is recommended. User education about the risks of clicking suspicious links in OAuth workflows can also reduce exploitation likelihood. For critical systems, consider deploying Web Application Firewalls (WAFs) with rules to detect and block reflected XSS payloads targeting OAuth endpoints. Finally, monitor application logs and user reports for signs of suspicious activity related to OAuth authentication flows.