Security Digest · Vulnerability record
CVE-2026-32871: FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability
FastMCP is a Pythonic way to build MCP servers and clients.
CVSS 10.0 Critical EPSS 1.4% · p72
Should you act?
Critical and remotely reachable
No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 1.4%, percentile 72: FIRST’s model estimates a 1.4% chance of exploitation activity in the next 30 days, higher than 72% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes
fastmcp(PyPI) < 3.2.0, fixed in 3.2.0. Check your lockfile, including transitive dependencies.
What it is
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 buildurl() method. When an OpenAPI operation defines path parameters (e.g., /api/v1/users/{userid}), 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.
From the GitHub advisory
Technical Description
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 critical vulnerability exists in the buildurl() method. When an OpenAPI operation defines path parameters (e.g., /api/v1/users/{userid}), 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.
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Weakness type: CWE-918.
Product: fastmcp (PyPI) · Assigned by: GitHub_M · Published: 2026-04-02
Affected and fixed versions
| Ecosystem | Package | Vulnerable | First fixed |
|---|---|---|---|
| PyPI | fastmcp | < 3.2.0 | 3.2.0 |
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| PrefectHQ | fastmcp | < 3.2.0 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| GitHub_M | 4.0 | 10 Critical | 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 |
| redhat-SADP | 3.1 | 8.5 High | CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H |
| GitHub | 4.0 | 10.0 | 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 |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Attack requirements: none
- Privileges required: none (no login)
- User interaction: none
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
- FIRST EPSS (model of 2026-10-21): 1.37% probability of exploitation activity in the next 30 days; percentile 72.
Timeline
- — GitHub-reviewed advisory GHSA-vv7q-7jx5-f767 published (critical)
- — CVE record published by GitHub_M
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- access.redhat.com access.redhat.com
Vulnerability records
- GitHub Advisory GHSA-vv7q-7jx5-f767 github.com
- CVE.org record CVE-2026-32871 cve.org
- NVD entry CVE-2026-32871 nvd.nist.gov
Other references from the CVE record and advisory
- github.com github.com
- github.com github.com
- github.com github.com
- github.com github.com
- access.redhat.com access.redhat.com
- RHBZ#2454434 bugzilla.redhat.com
- security.access.redhat.com security.access.redhat.com