WS-2026-0196
Published:October 23, 2026
Updated:November 13, 2026
fastmcp / fastmcp-slim 4.0.0a1 through 4.1.1: the MCP client's machine-to-machine ClientCredentialsOAuthProvider (fastmcp/client/auth/client_credentials.py) is built without binding to an expected issuer and trusts the authorization server the MCP server advertises, so a malicious or compromised MCP server can point the client at an attacker-controlled token endpoint and receive the client_secret. Fixed in 4.1.2, which adds an issuer parameter and namespaces cached tokens by issuer.
Affected Packages
fastmcp (CONDA):
Affected version(s) >=4.0.3 <4.1.2Fix Suggestion:
Update to version 4.1.2fastmcp-slim (CONDA):
Affected version(s) >=4.0.3 <4.1.2Fix Suggestion:
Update to version 4.1.2https://github.com/PrefectHQ/fastmcp.git (GITHUB):
Affected version(s) >=v4.0.0a1 <v4.1.2Fix Suggestion:
Update to version v4.1.2fastmcp (PYTHON):
Affected version(s) >=4.0.0a1 <4.1.2Fix Suggestion:
Update to version 4.1.2fastmcp-slim (PYTHON):
Affected version(s) >=4.0.0a1 <4.1.2Fix Suggestion:
Update to version 4.1.2Related Resources (5)
Do you need more information?
Contact UsCVSS v4
Base Score:
8.2
Attack Vector
NETWORK
Attack Complexity
HIGH
Attack Requirements
NONE
Privileges Required
NONE
User Interaction
NONE
Vulnerable System Confidentiality
HIGH
Vulnerable System Integrity
NONE
Vulnerable System Availability
NONE
Subsequent System Confidentiality
NONE
Subsequent System Integrity
NONE
Subsequent System Availability
NONE
CVSS v3
Base Score:
5.9
Attack Vector
NETWORK
Attack Complexity
HIGH
Privileges Required
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
HIGH
Integrity
NONE
Availability
NONE