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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:

FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name

### Summary 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 ### Details 1. generate_cursor_deeplink(server_name, …) embeds server_name verbatim in a cursor://…?name= query string. 2. open_deeplink() is invoked with shell=True only on Windows. That calls cmd.exe /c start <deeplink>. 3. Any cmd metacharacter inside server_name (&, |, >, ^, …) escapes the start comma

FastMCP vulnerable to reflected XSS in client's callback page

### Summary 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. ### Details The affected code is located in *https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/client/oauth_callback.py*, which embeds all values passed to the `create_callback_html` function via the `message` parameter it into the callback pag