FastMCP's direct component paths can return a transformed component before its applicable AuthCheck, allowing low-privilege callers to execute hidden protected behavior. I reproduced this against FastMCP 4.1.0 and commit e2fcc412001ff7414095c5dc282f94c70299a612 using only an in-process FastMCP server. The attacker does not have server-side access and does not control FastMCP configuration.
fastmcp's ClientCredentialsOAuthProvider and PrivateKeyJWTOAuthProvider do not expose or validate OAuth issuer binding when connecting to MCP servers. A client secret configured for a legitimate authorization server can be transmitted to an attacker-controlled authorization server if the MCP server URL is attacker-supplied or compromised. This is the same vulnerability class as GHSA-qx49-fqc8-xw99 (Python MCP SDK) and GHSA-6qxp-vccf-f47h (TypeScript MCP SDK), applied to fastmcp's wrapper layer.
While testing the GitHubProvider OAuth integration, which allows authentication to a FastMCP MCP server via a FastMCP OAuthProxy using GitHub OAuth, it was discovered that the FastMCP OAuthProxy does not properly validate the user's consent upon receiving the authorization code from GitHub. In combination with GitHub’s behavior of skipping the consent page for previously authorized clients, this introduces a Confused Deputy vulnerability.
Technical Description The OpenAPIProvider in FastMCP exposes internal APIs to MCP clients by parsing OpenAPI specifications. The RequestDirector class is responsible for constructing HTTP requests to the backend service. A critical vulnerability exists in the _build_url() method. When an OpenAPI operation defines path parameters (e.g., /api/v1/users/{user_id}), the system directly substitutes parameter values into the URL template string without URL-encoding. Subsequently, urllib.parse.urljoin() resolves the final URL. Since urljoin() interprets ../ sequences …
Server names containing shell metacharacters (e.g., &) can cause command injection on Windows when passed to fastmcp install claude-code or fastmcp install gemini-cli. These install paths use subprocess.run() with a list argument, but on Windows the target CLIs often resolve to .cmd wrappers that are executed through cmd.exe, which interprets metacharacters in the flattened command string. PoC: from fastmcp import FastMCP mcp = FastMCP(name="test&calc") @mcp.tool def roll_dice(n_dice: int) -> list[int]: …
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 Affected Code: self._jwt_issuer: JWTIssuer = JWTIssuer( issuer=str(self.base_url), audience=f"{str(self.base_url).rstrip('/')}/mcp", signing_key=jwt_signing_key, )
There was a recent CVE report on MCP: https://nvd.nist.gov/vuln/detail/CVE-2025-66416. FastMCP does not use any of the affected components of the MCP SDK directly. However, FastMCP versions prior to 2.14.0 did allow MCP SDK versions <1.23 that were vulnerable to CVE-2025-66416. Users should upgrade to FastMCP 2.14.0 or later.
A command-injection vulnerability lets any attacker who can influence the server_name field of an MCP execute arbitrary OS commands on Windows hosts that run fastmcp install cursor
While setting up an oauth client, it was noticed that the callback page hosted by the client during the flow embeds user-controlled content without escaping or sanitizing it. This leads to a reflected Cross-Site-Scripting vulnerability.
FastMCP documentation covers the scenario where it is possible to use Entra ID or other providers for authentication. In this context, because Entra ID does not support Dynamic Client Registration (DCR), the FastMCP-hosted MCP server is acting as the authorization provider, as declared in the Protected Resource Metadata (PRM) document hosted on the server. For example, on a local MCP server, it may be hosted here: http://localhost:8000/.well-known/oauth-protected-resource And the JSON …