CVE-2026-32871FastMCP is a Pythonic way to build MCP servers and clients.
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No exploitation reported and no EPSS estimate yet.
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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
- 10.0 CRITICAL (v3.1, NVD)CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- Weakness (CWE)
- CWE-918
- Known exploited
- Not in CISA's KEV catalog
- EPSS (exploitation in the next 30 days)
- Not loaded for this CVE
- Assigned by
- Not given
- NVD analysis
- Modified
- Record last updated
- Jul 15, 2026
- First seen by InfoSec Signals
- Oct 9, 2026
Affected products
jlowin fastmcp jlowin:fastmcp | before 3.2.0 | NVD |
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://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-32871.json
The CVE record on cve.org NVD's entry
The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.