---
id: CVE-2026-32871
title: FastMCP is a Pythonic way to build MCP servers and clients
summary: >-
  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
  const…
severity: critical
cvss: 10
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H'
cwe:
  - CWE-918
vendor: jlowin
product: fastmcp
affected:
  - fastmcp < 3.2.0
patched:
  - fastmcp 3.2.0
published: '2026-04-02'
updated: '2026-07-08'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-32871'
references:
  - url: >-
      https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71
    label: security-advisories@github.com
  - url: 'https://github.com/PrefectHQ/fastmcp/pull/3507'
    label: security-advisories@github.com
  - url: 'https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0'
    label: security-advisories@github.com
  - url: >-
      https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767
    label: security-advisories@github.com
  - url: 'https://access.redhat.com/errata/RHSA-2026:36350'
    label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
  - url: 'https://access.redhat.com/security/cve/CVE-2026-32871'
    label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2454434'
    label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
  - url: >-
      https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-32871.json
    label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-32871'
  - url: 'https://github.com/PrefectHQ/fastmcp'
tags:
  - nvd
  - osv
  - pip
epss: 0.01369
epssPercentile: 0.70688
ingestedAt: '2026-07-08T13:51:10.446Z'
aliases:
  - GHSA-vv7q-7jx5-f767
  - PYSEC-2026-338
ecosystem: pip
---

## Overview

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

- `fastmcp < 3.2.0`

## Remediation

Upgrade past the affected range:

- `fastmcp 3.2.0`

## Package advisory (CVE-2026-32871)

Affected packages:

- `fastmcp < 3.2.0`

Patched in:

- `fastmcp 3.2.0`

Source: https://osv.dev/vulnerability/GHSA-vv7q-7jx5-f767