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BREW-fastmcp-CVE-2025-69196 — FastMCP OAuth Proxy token reuse across MCP servers

Affected: Homebrew: fastmcp

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: JWTIssuer = JWTIssuer( issuer=str(self.base_url), audience=f"{str(self.base_url).rstrip('/')}/mcp", signing_key=jwt_signing_key, ) ```

Since the issued access and refresh tokens do not include information about the resource the token was issued for, it is impossible for the MCP server to properly verify whether the token was issued for it, hence violating the requirement of doing so demanded by the [specification](https://mcp.mintlify.app/specification/2025-11-25/basic/authorization#token-audience-binding-and-validation). Being able to verify whether the token was issued for the target MCP server enforces the protections offered by the steps proposed by the specification and the Resource Indicators OAuth extension.

Therefore, this misconfiguration exposes all MCP server setups using the FastMCP OAuth Proxy to an attack where an adversary creates a malicious MCP server that advertises the benign OAuth Proxy authorization server as its own authorization server. Once a vic…

EUVD-2026-337716

fkie_cve-2025-69196

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.

GHSA-5h2m-4q8j-pqpj — FastMCP OAuth Proxy token reuse across MCP servers

Affected: PyPI: fastmcp

**Affected File:** *https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L828*

**Affected Code:** ```python self._jwt_issuer: JWTIssuer = JWTIssuer( issuer=str(self.base_url), audience=f"{str(self.base_url).rstrip('/')}/mcp", signing_key=jwt_signing_key, ) ```

PYSEC-2026-2474 — FastMCP OAuth Proxy token reuse across MCP servers

Affected: PyPI: fastmcp

**Affected File:** *https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L828*

**Affected Code:** ```python self._jwt_issuer: JWTIssuer = JWTIssuer( issuer=str(self.base_url), audience=f"{str(self.base_url).rstrip('/')}/mcp", signing_key=jwt_signing_key, ) ```

RHSA-2026:36350 — Red Hat Security Advisory: satellite/foreman-mcp-server-rhel9 container image available as a Technology Preview

fastmcp: FastMCP: Improper token issuance due to incorrect resource parameter handling python-diskcache: python-diskcache: Arbitrary code execution via insecure pickle deserialization fastmcp: FastMCP: Authenticated Server-Side Request Forgery via path traversal in OpenAPI path parameters urllib3: urllib3: Information disclosure via cross-origin redirects forwarding sensitive headers urllib3: urllib3: Denial of Service due to excessive HTTP response decompression python-pyjwt: PyJWT: Authentication bypass due to forged JSON Web Tokens