CVE-2025-69196: FastMCP OAuth Proxy token reuse across MCP servers

What it is

While testing the OAuth Proxy implementation, it was noticed that the server does not properly respect the resource parameter submitted by the client in the authorization and token request. Instead of issuing the token explicitly for this MCP server, the token is issued for the base_url passed to the OAuthProxy during initialization.

Affected File: https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L828

Since the issued access and refresh tokens do not include information about the resource the token was issued for, it is impossible for the MCP server to properly verify whether the token was issued for it, hence violating the requirement of doing so demanded by the specification. Being able to verify whether the token was issued for the target MCP server enforces the protections offered by the steps proposed by the specification and the Resource Indicators OAuth extension.

How to check

1. Affected: fastmcp (pip) < 2.14.2.
2. Compare your installed version with the range above (command below).

How to fix

1. Upgrade fastmcp to 2.14.2 or later.

pip show fastmcp | grep Version

Sources

• GitHub Security Advisory GHSA-5h2m-4q8j-pqpj ↗
• NVD: CVE-2025-69196 ↗