Go MCP SDK vulnerabilities

The Go MCP SDK used Go's standard encoding/json. Prior to version 1.4.1, the 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.

The Go MCP SDK prior to version 1.4.0 does not enable DNS rebinding protection by default for HTTP-based servers. When an MCP server runs on localhost without authentication using StreamableHTTPHandler or SSEHandler, a malicious website can exploit DNS rebinding to bypass same-origin policy and send requests to the local MCP server, potentially invoking tools or accessing resources.

In Go MCP SDK prior to version 1.3.1, Go's standard JSON unmarshaler performed case-insensitive matching of JSON keys to struct fields, violating the JSON-RPC 2.0 specification. A malicious MCP peer could send protocol messages with non-standard field casing that the SDK would silently accept, potentially bypassing intermediary inspection and causing cross-implementation inconsistency.