FastMCP: Post-transform component authorization is skipped on direct calls

## Maintainer assessment and remediation This report is accepted as a FastMCP authorization vulnerability. In supported configurations, a server transform can change the metadata used by a component's authorization policy. Direct lookups previously checked the original component but returned the transformed component without rechecking its policy. A client that knows the component identifier can invoke a tool or retrieve a resource or prompt that the configured policy denies. The fixes authori

FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server

## Maintainer assessment and remediation This report is accepted as a FastMCP credential-binding vulnerability. Affected versions of the OAuth client credentials providers do not expose the authorization-server issuer that owns pre-registered credentials. An untrusted or compromised MCP server can therefore advertise a different authorization server and receive the client secret or a signed assertion. Signing keys themselves remain local; assertion replayability depends on the assertion source

FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities

## Summary 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 Details An adversary can initi

FastMCP has a Command Injection vulnerability - Gemini CLI

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: ```python from fastmcp import FastMCP mcp = FastMCP(name="test&calc") @mcp.tool def rol

FastMCP OAuth Proxy token reuse across MCP servers

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:** ```python self._jwt_issuer: