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CVE-2026-32871

Jlowin Fastmcp server-side request forgery (SSRF)

10.0 CRITICALPublic exploitEPSS 1.37%
  1. Published2 Apr 2026

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeChanged
ConfidentialityHigh
IntegrityHigh
AvailabilityHigh
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Affected products

Jlowin Fastmcp< 3.2.0

Remediation

Fixed in
Jlowin Fastmcp≥ 3.2.0

References

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Frequently asked

Is CVE-2026-32871 being actively exploited?

CVE-2026-32871 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 1.37%.

How severe is CVE-2026-32871?

CVE-2026-32871 has a CVSS 3.1 base score of 10.0, rated critical. EPSS estimates a 1.37% probability of exploitation.

Which products are affected by CVE-2026-32871?

Affected products include Jlowin Fastmcp.

How do I fix CVE-2026-32871?

Upgrade affected software to a fixed version, for example Jlowin Fastmcp 3.2.0 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-32871 against the real software inventory on every managed device, as each hourly inventory arrives. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.