CVE-2026-32871
Prior to version 3.2.0, the OpenAPIProvider in FastMCP exposes internal APIs to MCP clients by parsing OpenAPI specifications.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.91%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
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 4.0
- 10.0 CRITICALCVSS: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/M
- EPSS
- 0.91% probability · 58th percentile
- CISA KEV
- Not listed
- Weakness
- CWE-918
- Affected
- jlowin/fastmcp
- Source
- security-advisories@github.com
References
- https://github.com/PrefectHQ/fastmcp/commit/40bdfb6b1de0ce30609ee9ba5bb95ecd04a9fb71Patch
- https://github.com/PrefectHQ/fastmcp/pull/3507Issue Tracking, Patch
- https://github.com/PrefectHQ/fastmcp/releases/tag/v3.2.0Product, Release Notes
- https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767Exploit, Mitigation, Vendor Advisory
- https://access.redhat.com/errata/RHSA-2026:36350
- https://access.redhat.com/security/cve/CVE-2026-32871
- https://bugzilla.redhat.com/show_bug.cgi?id=2454434
- https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767Exploit, Mitigation, Vendor Advisory
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-32871.json
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.