The Go MCP SDK used Go's standard encoding/json. Prior to version 1.4.1, the Go SDK's Streamable HTTP transport accepted browser-generated cross-site POST requests without validating the Origin header and without requiring Content-Type: application/json. In deployments without Authorization, especially stateless or sessionless configurations, this allows an arbitrary website to send MCP requests to a local server and potentially trigger tool execution. Version 1.4.1 contains a patch for the issue.
Exploitability
AV:NAC:LPR:NUI:RScope
S:UImpact
C:NI:HA:L7.1/CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:LAuthentication