CVE-2025-62800
📋 TL;DR
FastMCP versions before 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page. This allows attackers to inject malicious JavaScript that executes in the callback server's origin when users visit specially crafted URLs. Applications using FastMCP's OAuth functionality are affected.
💻 Affected Systems
- FastMCP
📦 What is this software?
Fastmcp by Jlowin
⚠️ Risk & Real-World Impact
Worst Case
Attackers could steal user session tokens, perform actions on behalf of users, or redirect users to malicious sites, potentially leading to account compromise and data theft.
Likely Case
Attackers craft malicious OAuth callback URLs that execute JavaScript in the victim's browser, potentially stealing OAuth tokens or session cookies.
If Mitigated
With proper input validation and output encoding, the vulnerability is prevented, though the attack surface remains if other similar issues exist.
🎯 Exploit Status
Exploitation requires tricking users into clicking malicious OAuth callback URLs. The vulnerability is reflected XSS, so the attack is client-side.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.13.0
Vendor Advisory: https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc
Restart Required: No
Instructions:
1. Update FastMCP to version 2.13.0 or later using pip: pip install fastmcp>=2.13.0. 2. Verify the update was successful. 3. No restart required as this is a library update.
🔧 Temporary Workarounds
Input Validation and Output Encoding
allImplement server-side validation and proper HTML encoding for all user-controlled values in the OAuth callback handler.
🧯 If You Can't Patch
- Implement a web application firewall (WAF) with XSS protection rules to block malicious payloads.
- Disable or restrict access to the OAuth callback functionality if not required.
🔍 How to Verify
Check if Vulnerable:
Check if FastMCP version is below 2.13.0 by examining the installed package version.Check Version:
pip show fastmcp | grep VersionVerify Fix Applied:
Verify that FastMCP version is 2.13.0 or higher and test the OAuth callback with malicious input to ensure it's properly encoded.📡 Detection & Monitoring
Log Indicators:
- Unusual OAuth callback requests with JavaScript payloads in query parameters
- Multiple failed OAuth authentication attempts from suspicious sources
Network Indicators:
- HTTP requests to OAuth callback endpoints containing script tags or JavaScript code in URL parameters
SIEM Query:
source="web_server" AND (url="*oauth_callback*" AND (url="*<script*" OR url="*javascript:*"))