Affected: Homebrew: fastmcp

## 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 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.

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## Vulnerable Code

**File:** `fastmcp/utilities/openapi/director.py`

```python def _build_url( self, path_template: str, path_params: dict[str, Any], base_url: str ) -> str: # Direct string substitution without encoding url_path = path_template for param_name, param_value in path_params.items(): placeholder = f"{{{param_name}}}" if placeholder in url_path: url_path = url_path.replace(placeholder, str(param_value))

# urljoin resolves ../ escape sequences return urljoin(base_url.rstrip("/") + "/", url_path.lstrip("/")) ```

### Root Cause

1. Pat…

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.

Affected: PyPI: fastmcp

## 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 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.

---

## Vulnerable Code

**File:** `fastmcp/utilities/openapi/director.py`

```python def _build_url( self, path_template: str, path_params: dict[str, Any], base_url: str ) -> str: # Direct string substitution without encoding url_path = path_template for param_name, param_value in path_params.items(): placeholder = f"{{{param_name}}}" if placeholder in url_path: url_path = url_path.replace(placeholder, str(param_value))

# urljoin resolves ../ escape sequences return urljoin(base_url.rstrip("/") + "/", url_path.lstrip("/")) ```

### Root Cause

1. Pat…

Affected: PyPI: fastmcp

## 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 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.

--- ## Vulnerable Code

**File:** `fastmcp/utilities/openapi/director.py`

```python def _build_url( self, path_template: str, path_params: dict[str, Any], base_url: str ) -> str: # Direct string substitution without encoding url_path = path_template for param_name, param_value in path_params.items(): placeholder = f"{{{param_name}}}" if placeholder in url_path: url_path = url_path.replace(placeholder, str(param_value))

# urljoin resolves ../ escape sequences return urljoin(base_url.rstrip("/" ) + "/", url_path.lstrip("/")) ```

### Root Cause

1.…

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