ghsa:GHSA-VQQP-X5QM-5QP4: fastmcp >= 4.0.0a1, < 4.1.2; +1 more affected products

FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server. Affected range: >= 4.0.0a1, < 4.1.2. Fixed version: 4.1.2. Affected range: >= 4.0.0a1, < 4.1.2. Fixed version: 4.1.2.

Affected technology

fastmcp · >= 4.0.0a1, < 4.1.2
Vendor not specified by the source

fastmcp-slim · >= 4.0.0a1, < 4.1.2
Vendor not specified by the source

Component: Not specified by the source

Attack conditions (Source advisory, CVSS 3.1): Network (remote) · No privileges required · No user interaction required

What an attacker can do

Attacker can now impersonate the client against the legitimate authorization server The vulnerability exists because fastmcp does not validate that the authorization server advertised by the MCP server matches an expected issuer. ### Impact Severity: Medium-High Any application using fastmcp's M2M OAuth providers (ClientCredentialsOAuthProvider, PrivateKeyJWTOAuthProvider) to connect to user-supplied or untrusted MCP server URLs is vulnerable to credential disclosure and client impersonation. Source advisory’s CVSS 3.1 assessment rates confidentiality impact as high; integrity and availability impact as none.

Published 24-Oct-2026 01:47 UTC

Product: fastmcp
KEV: no

Provenance

GitHub Advisory Database · Source record · observed 2026-10-24 01:45:06.114611+00:00 UTC

Stable permalink: https://cybercodered.org/item/adv-ghsa-ghsa-vqqp-x5qm-5qp4.html