feat: add header-based authentication for HTTP transports (#57)

Adds optional token auth via AUTH_HEADER and AUTH_VALUE env vars.
When configured, requests without the correct header receive 401.

This is supposed to be used in tests.

Author: jhrozek

cmd/yardstick-server/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"flag"
 	"fmt"
 	"log"
@@ -28,11 +29,33 @@ type EchoResponse struct {
 var alphanumericRegex = regexp.MustCompile(`^[a-zA-Z0-9]+$`)
 var transport string
 var port int
+var authHeader string
+var authValue string
 
 func validateAlphanumeric(input string) bool {
 	return alphanumericRegex.MatchString(input)
 }
 
+func checkAuth(r *http.Request) error {
+	if authHeader == "" {
+		return nil
+	}
+	if r.Header.Get(authHeader) != authValue {
+		return errors.New("unauthorized")
+	}
+	return nil
+}
+
+func authWrapper(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if err := checkAuth(r); err != nil {
+			http.Error(w, err.Error(), http.StatusUnauthorized)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
 func echoHandler(_ context.Context, _ *mcp.CallToolRequest, params EchoRequest) (*mcp.CallToolResult, EchoResponse, error) {
 	if !validateAlphanumeric(params.Input) {
 		return &mcp.CallToolResult{
@@ -98,7 +121,7 @@ func main() {
 		}, nil)
 
 		// Mount the SSE handler at /sse - it will handle both GET (SSE stream) and POST (messages) requests
-		http.Handle("/sse", handler)
+		http.Handle("/sse", authWrapper(handler))
 
 		// Create server with timeouts to address G114 gosec issue
 		srv := &http.Server{
@@ -116,7 +139,7 @@ func main() {
 			return server
 		}, nil)
 
-		http.Handle("/mcp", handler)
+		http.Handle("/mcp", authWrapper(handler))
 
 		// Create server with timeouts to address G114 gosec issue
 		srv := &http.Server{
@@ -150,4 +173,7 @@ func parseConfig() {
 			port = intValue
 		}
 	}
+
+	authHeader = os.Getenv("AUTH_HEADER")
+	authValue = os.Getenv("AUTH_VALUE")
 }

cmd/yardstick-server/main_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"net/http"
 	"testing"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
@@ -111,3 +112,79 @@ func TestEchoResponseCreation(t *testing.T) {
 	response := EchoResponse{Output: "test123"}
 	assert.Equal(t, "test123", response.Output)
 }
+
+func TestCheckAuth_HeaderAuth(t *testing.T) {
+	// Save original values
+	origHeader := authHeader
+	origValue := authValue
+	defer func() {
+		authHeader = origHeader
+		authValue = origValue
+	}()
+
+	// Set auth config
+	authHeader = "X-Auth-Token"
+	authValue = "secret123"
+
+	// Create request with correct header
+	req, err := http.NewRequest(http.MethodGet, "/test", nil)
+	assert.NoError(t, err)
+	req.Header.Set("X-Auth-Token", "secret123")
+
+	// Should pass authentication
+	err = checkAuth(req)
+	assert.NoError(t, err)
+}
+
+func TestCheckAuth_HeaderAuth_Fail(t *testing.T) {
+	// Save original values
+	origHeader := authHeader
+	origValue := authValue
+	defer func() {
+		authHeader = origHeader
+		authValue = origValue
+	}()
+
+	// Set auth config
+	authHeader = "X-Auth-Token"
+	authValue = "secret123"
+
+	// Test with wrong header value
+	req, err := http.NewRequest(http.MethodGet, "/test", nil)
+	assert.NoError(t, err)
+	req.Header.Set("X-Auth-Token", "wrongvalue")
+
+	err = checkAuth(req)
+	assert.Error(t, err)
+	assert.Equal(t, "unauthorized", err.Error())
+
+	// Test with missing header
+	req2, err := http.NewRequest(http.MethodGet, "/test", nil)
+	assert.NoError(t, err)
+
+	err = checkAuth(req2)
+	assert.Error(t, err)
+	assert.Equal(t, "unauthorized", err.Error())
+}
+
+func TestCheckAuth_Disabled(t *testing.T) {
+	// Save original values
+	origHeader := authHeader
+	origValue := authValue
+	defer func() {
+		authHeader = origHeader
+		authValue = origValue
+	}()
+
+	// Auth disabled when authHeader is empty
+	authHeader = ""
+	authValue = ""
+
+	// Create request without any auth header
+	req, err := http.NewRequest(http.MethodGet, "/test", nil)
+	assert.NoError(t, err)
+
+	// Should pass since auth is disabled
+	err = checkAuth(req)
+	assert.NoError(t, err)
+}