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CVE-2026-27124

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

FastMCP is the standard framework for building MCP applications. Prior to version 3.2.0, 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. This issue has been patched in version 3.2.0.

CVSS 4.0 · 8.2 High EPSS < 1% KEV · Not listed
Vendors
Jlowin
Products
Fastmcp
CVE-2025-64340

FastMCP has a Command Injection vulnerability - Gemini CLI

FastMCP is the standard framework for building MCP applications. Prior to version 3.2.0, 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. This issue has been patched in version 3.2.0.

CVSS 3.1 · 6.7 Medium EPSS < 1% KEV · Not listed
Vendors
Jlowin
Products
Fastmcp
CVE-2026-32871

FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability

FastMCP is a Pythonic way to build MCP servers and clients. Prior to version 3.2.0, 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 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 as directory traversal, an attacker controlling a path parameter can perform path traversal attacks to escape the intended API prefix and access arbitrary backend endpoints. This results in authenticated SSRF, as requests are sent with the authorization headers configured in the MCP provider. This issue has been patched in version 3.2.0.

CVSS 4.0 · 10 Critical EPSS < 1% KEV · Not listed
Vendors
Jlowin Prefecthq
Products
Fastmcp Fastmcp
CVE-2025-69196

FastMCP OAuth Proxy token reuse across MCP servers

FastMCP is the standard framework for building MCP applications. Prior to version 2.14.2, 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 the MCP server, the token is issued for the base_url passed to the OAuthProxy during initialization. This issue has been patched 2.14.2.

CVSS 4.0 · 7.4 High EPSS < 1% KEV · Not listed
Vendors
Jlowin
Products
Fastmcp
CVE-2025-62800

FastMCP vulnerable to reflected XSS in client's callback page

FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0.

CVSS 4.0 · 5.3 Medium EPSS < 1% KEV · Not listed
Vendors
Fastmcp Jlowin
Products
Fastmcp Fastmcp
CVE-2025-62801

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

FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0, 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. This vulnerability is fixed in 2.13.0.

CVSS 4.0 · 5.4 Medium EPSS < 1% KEV · Not listed
Vendors
Fastmcp Jlowin
Products
Fastmcp Fastmcp