CVE-2026-27896: MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity

The Go MCP SDK used Go’s standard encoding/json.Unmarshal for JSON-RPC and MCP protocol message parsing. Go’s standard library performs case-insensitive matching of JSON keys to struct field tags — a field tagged json:“method” would also match “Method”, “METHOD”, etc. Additionally, Go’s standard library folds the Unicode characters ſ (U+017F) and K (U+212A) to their ASCII equivalents s and k, meaning fields like “paramſ” would match “params”. This violated the JSON-RPC 2.0 specification, which defines exact field names.