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Response snapshot Oct 27, 2026, 01:46 UTC

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VulnerabilityCVE-2026-32871

FastMCP is a Pythonic way to build MCP servers and clients.

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.

At a glance

Exploited
Not known
Severity
10.0 Critical (CVSS 4.0)
Chance of exploitation
0.9% in the next 30 days
Sources
2 independent sources

What to do

Check the vendor advisory for affected versions and fixes.

Read NVD CVE-2026-32871 (opens a new tab)

What happened

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.

Scores and how they were measured

CVSS base severity

10.0

Version 4.0As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.9%

Scored Percentile 59.7%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    NVD CVE-2026-32871 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  2. Primary source

    Vendor advisory (via NVD references) (opens a new tab)

    The vendor advisory referenced by the CVE record, for affected versions and fixes.

    Published Date not suppliedChecked

  3. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  4. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  5. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-32871, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 26). FastMCP is a Pythonic way to build MCP servers and clients. (CVE-2026-32871). https://kevwire.com/record/CVE-2026-32871

MLA

"FastMCP is a Pythonic way to build MCP servers and clients. (CVE-2026-32871)." KEVWire, 26 Oct. 2026, kevwire.com/record/CVE-2026-32871.

Chicago

KEVWire. "FastMCP is a Pythonic way to build MCP servers and clients. (CVE-2026-32871)." Last modified October 26, 2026. https://kevwire.com/record/CVE-2026-32871.

Evidence summary (plain text)

FastMCP is a Pythonic way to build MCP servers and clients. (Apr 2, 2026).
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 (version 4.0; evidence as of Oct 26, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-32871).
EPSS 0.9% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-32871&date=2026-10-26).
Sources:
NVD CVE-2026-32871: https://nvd.nist.gov/vuln/detail/CVE-2026-32871
Vendor advisory (via NVD references): https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vv7q-7jx5-f767
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-10-26T20:37:18.143Z&lastModEndDate=2026-10-26T22:10:56.995Z&resultsPerPage=500&startIndex=0
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-10-26T20:05:59.720Z&lastModEndDate=2026-10-26T21:35:56.549Z&resultsPerPage=500&startIndex=0
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-32871&date=2026-10-26
Record ID: CVE-2026-32871.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

NVD status: Modified. Publication in the NVD is not evidence of exploitation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

Record JSON

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Record details

Record detail