FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server

影響: pip:fastmcp >= 4.0.0a1, < 4.1.2 → 4.1.2, pip:fastmcp-slim >= 4.0.0a1, < 4.1.2 → 4.1.2

Maintainer assessment and remediation This report is accepted as a FastMCP credential-binding vulnerability. Affected versions of the OAuth client credentials providers do not expose the authorization-server issuer that owns pre-registered credentials. An untrusted or compromised MCP server can therefore advertise a different authorization server and receive the client secret or a signed assertion. Signing keys themselves remain local; assertion replayability depends on the assertion source and…

Read on GitHub Advisory Database

On 10/24/2026, 06:31 AM, GitHub Advisory Database published "FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server" (type: Advisory). Severity is "Medium" (medium). Related technologies: Python.