CVE-2026-32871
FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability
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.
INFO
Published Date :
April 2, 2026, 3:16 p.m.
Last Modified :
April 10, 2026, 3:58 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | CRITICAL | [email protected] | ||||
| CVSS 4.0 | CRITICAL | [email protected] |
Solution
- Update FastMCP to version 3.2.0 or later.
- Ensure path parameters are URL-encoded before use.
- Validate all user inputs for special characters.
Public PoC/Exploit Available at Github
CVE-2026-32871 has a 3 public
PoC/Exploit available at Github.
Go to the Public Exploits tab to see the list.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-32871.
| URL | Resource |
|---|---|
| https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71 | Patch |
| https://github.com/PrefectHQ/fastmcp/pull/3507 | Issue Tracking Patch |
| https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0 | Product Release Notes |
| https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767 | Exploit Mitigation Vendor Advisory |
| https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767 | Exploit Mitigation Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-32871 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-32871
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Claude skill: 12 reading modes for deep book extraction (NotebookLM + uploaded files)
claude-skill knowledge-extraction notebooklm reading zettelkasten
Este repo provee un **servidor MCP listo para produccion** en Python y TypeScript que implementa defensas contra los 10 errores criticos.
Dockerfile Python TypeScript Shell
NotebookLM MCP CLI - Access NotebookLM via Claude with security fixes and FastMCP 1.0 compatibility
Python
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-32871 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-32871 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Apr. 10, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H Added CPE Configuration OR *cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:* versions up to (excluding) 3.2.0 Added Reference Type GitHub, Inc.: https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71 Types: Patch Added Reference Type GitHub, Inc.: https://github.com/PrefectHQ/fastmcp/pull/3507 Types: Issue Tracking, Patch Added Reference Type GitHub, Inc.: https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0 Types: Product, Release Notes Added Reference Type GitHub, Inc.: https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767 Types: Exploit, Mitigation, Vendor Advisory Added Reference Type CISA-ADP: https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767 Types: Exploit, Mitigation, Vendor Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Apr. 02, 2026
Action Type Old Value New Value Added Reference https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767 -
New CVE Received by [email protected]
Apr. 02, 2026
Action Type Old Value New Value Added 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. Added CVSS V4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CWE CWE-918 Added Reference https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71 Added Reference https://github.com/PrefectHQ/fastmcp/pull/3507 Added Reference https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0 Added Reference https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767