PrefectHQ fastmcp up to 3.1.x Template String /api/v1/users/ _build_url server-side request forgery

Summaryinfo

A vulnerability, which was classified as critical, has been found in PrefectHQ fastmcp up to 3.1.x. Impacted is the function _build_url of the file /api/v1/users/ of the component Template String Handler. The manipulation leads to server-side request forgery. This vulnerability is uniquely identified as CVE-2026-32871. The attack is possible to be carried out remotely. No exploit exists. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in PrefectHQ fastmcp up to 3.1.x. It has been rated as critical. This issue affects the function _build_url of the file /api/v1/users/ of the component Template String Handler. The manipulation with an unknown input leads to a server-side request forgery vulnerability. Using CWE to declare the problem leads to CWE-918. The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-32871 since 03/16/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 3.2.0 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Vendor

• PrefectHQ

Name

• fastmcp

Version

• 3.0
• 3.1

License

• open-source

Website

• Product: https://github.com/PrefectHQ/fastmcp/

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

Be aware that VulDB is the high quality source for vulnerability data.

Discussion