CVE-2026-32871
10.0 · CRITICALOverview
CVE-2026-32871 is a critical-severity vulnerability affecting jlowin fastmcp. It was published on April 2, 2026 and has a CVSS 3.1 base score of 10.0 (CRITICAL).
This vulnerability has a CVSS 3.1 base score of 10.0, rated CRITICAL. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.
Technical 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.
Remediation
Check the references section for vendor advisories and patches from jlowin. Update fastmcp to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.
Affected Products
| Vendor | Product | Versions | Status |
|---|---|---|---|
| jlowin | fastmcp | >= 0, < 3.2.0 | Affected |
References
Frequently Asked Questions
What is CVE-2026-32871?
CVE-2026-32871 is a critical-severity vulnerability affecting jlowin fastmcp. It was published on April 2, 2026 and has a CVSS 3.1 base score of 10.0 (CRITICAL).
How severe is CVE-2026-32871?
This vulnerability has a CVSS 3.1 base score of 10.0, rated CRITICAL. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.
How do I fix or remediate CVE-2026-32871?
Check the references section for vendor advisories and patches from jlowin. Update fastmcp to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.
How can CyberStrike help with CVE-2026-32871?
CyberStrike's AI-powered security agents can automatically detect CVE-2026-32871 across your infrastructure using autonomous pentesting, DAST scanning, and HackBrowser. The platform continuously monitors for known vulnerabilities and provides actionable remediation guidance prioritized by real-world exploitability.
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