FastMCP is a framework for building MCP applications. The vulnerability lies in the OAuthProxy initialization, where the authorization and token requests ignore the resource parameter supplied by the client. As a result, tokens issued are tied to the base_url rather than the intended MCP server, allowing tokens issued for one server to be reused against another. This can enable an adversary to access resources or perform actions on other MCP servers without proper authorization. The weakness corresponds to CWE-1220 (Improper Authorization) and CWE-863 (Missing Authorization Check).

Affected vendor: jlowin:fastmcp. All releases prior to 2.14.2 are impacted. The issue is fixed in FastMCP 2.14.2 and later. The relevant CPE is cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:*.

The CVSS score of 7.4 indicates high severity, while an EPSS of less than 1% suggests exploitation is currently rare. The vulnerability is not listed in the KEV catalog. An attacker likely needs network access to a client that can request tokens and supply fabricated resource parameters; no privileged access is required, making it relatively simple if such access is available. The impact of unauthorized cross‑server access is significant, resulting in a high overall risk.