CVE-2026-32871

CRITICAL EPSS 60.8%
Published Apr 2, 20263mo ago · Modified Jun 17, 20262w ago
10.0 CVSS 4.0
Critical
Find Similar
Published Apr 2, 2026 3mo ago
Last Modified Jun 17, 2026 2w ago

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 Details

Base Score
10.0
Exploitability
—
Impact
—
Vector string
CVSS:4.0/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
Attack Vector Network
Attack Complexity Low
Privileges Required None
User Interaction None
Scope X

Threat Intelligence

EPSS Exploit Probability
60.8% percentile
Exploit & Patch Status
Public Exploit Known
Patch Available

Weaknesses 1

CWE-918 Server-Side Request Forgery (SSRF) Validation

Affected Products 1

VendorProductVersionRange
jlowinfastmcp* <3.2.0

References 4

  • github.com https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71
    Patch
  • github.com https://github.com/PrefectHQ/fastmcp/pull/3507
    Issue TrackingPatch
  • github.com https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0
    ProductRelease Notes
  • github.com https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767
    ExploitMitigationVendor Advisory

Remediation

  • github.com https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71
    Patch
  • github.com https://github.com/PrefectHQ/fastmcp/pull/3507
    Issue TrackingPatch