vulnerability / NVI-00009062 / Revision 2
FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability
GitHub lists this reviewed advisory for fastmcp. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
10 critical
GitHub source severity; exploitability and local consequence require separate evidence.
As of · GitHub Advisory Database
0.925%
FIRST EPSS model v2026.06.15; exploitation probability over 30 days, percentile 0.60464. Missing scores are unknown.
As of · FIRST EPSS
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
fastmcp
source listed< 3.2.0
## 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.
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. FIRST EPSS
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-vv7q-7jx5-f767 ↗
GitHub Advisory Database · vendor advisory
Observed . GitHub Advisory Database; CC-BY-4.0. Each advisory links its original attribution. Null normalizes fields.
- CVE-2026-32871 ↗
FIRST EPSS · official catalog
Observed . FIRST EPSS; probability of exploitation in the next 30 days, not local compromise. Dataset date/model retained; FIRST services terms apply.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 2 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.