auth: add OAuth authenticating middleware for server (#261)

Add auth.RequireBearerToken and associated types.

This piece of middleware authenticates clients using OAuth 2.0, as
specified in the MCP spec.

For #237.

Usage:
```
st := mcp.NewStreamableServerTransport(...)
http.Handle(path, auth.RequireBearerToken(verifier, nil)(st.ServeHTTP))
```

auth/auth.go

@@ -0,0 +1,96 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package auth
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"slices"
+	"strings"
+	"time"
+)
+
+type TokenInfo struct {
+	Scopes     []string
+	Expiration time.Time
+}
+
+type TokenVerifier func(ctx context.Context, token string) (*TokenInfo, error)
+
+type RequireBearerTokenOptions struct {
+	Scopes              []string
+	ResourceMetadataURL string
+}
+
+var ErrInvalidToken = errors.New("invalid token")
+
+type tokenInfoKey struct{}
+
+// RequireBearerToken returns a piece of middleware that verifies a bearer token using the verifier.
+// If verification succeeds, the [TokenInfo] is added to the request's context and the request proceeds.
+// If verification fails, the request fails with a 401 Unauthenticated, and the WWW-Authenticate header
+// is populated to enable [protected resource metadata].
+//
+// [protected resource metadata]: https://datatracker.ietf.org/doc/rfc9728
+func RequireBearerToken(verifier TokenVerifier, opts *RequireBearerTokenOptions) func(http.Handler) http.Handler {
+	// Based on typescript-sdk/src/server/auth/middleware/bearerAuth.ts.
+
+	return func(handler http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			tokenInfo, errmsg, code := verify(r.Context(), verifier, opts, r.Header.Get("Authorization"))
+			if code != 0 {
+				if code == http.StatusUnauthorized || code == http.StatusForbidden {
+					if opts != nil && opts.ResourceMetadataURL != "" {
+						w.Header().Add("WWW-Authenticate", "Bearer resource_metadata="+opts.ResourceMetadataURL)
+					}
+				}
+				http.Error(w, errmsg, code)
+				return
+			}
+			r = r.WithContext(context.WithValue(r.Context(), tokenInfoKey{}, tokenInfo))
+			handler.ServeHTTP(w, r)
+		})
+	}
+}
+
+func verify(ctx context.Context, verifier TokenVerifier, opts *RequireBearerTokenOptions, authHeader string) (_ *TokenInfo, errmsg string, code int) {
+	// Extract bearer token.
+	fields := strings.Fields(authHeader)
+	if len(fields) != 2 || strings.ToLower(fields[0]) != "bearer" {
+		return nil, "no bearer token", http.StatusUnauthorized
+	}
+
+	// Verify the token and get information from it.
+	tokenInfo, err := verifier(ctx, fields[1])
+	if err != nil {
+		if errors.Is(err, ErrInvalidToken) {
+			return nil, err.Error(), http.StatusUnauthorized
+		}
+		// TODO: the TS SDK distinguishes another error, OAuthError, and returns a 400.
+		// Investigate how that works.
+		// See typescript-sdk/src/server/auth/middleware/bearerAuth.ts.
+		return nil, err.Error(), http.StatusInternalServerError
+	}
+
+	// Check scopes.
+	if opts != nil {
+		// Note: quadratic, but N is small.
+		for _, s := range opts.Scopes {
+			if !slices.Contains(tokenInfo.Scopes, s) {
+				return nil, "insufficient scope", http.StatusForbidden
+			}
+		}
+	}
+
+	// Check expiration.
+	if tokenInfo.Expiration.IsZero() {
+		return nil, "token missing expiration", http.StatusUnauthorized
+	}
+	if tokenInfo.Expiration.Before(time.Now()) {
+		return nil, "token expired", http.StatusUnauthorized
+	}
+	return tokenInfo, "", 0
+}

auth/auth_test.go

@@ -0,0 +1,70 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package auth
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestVerify(t *testing.T) {
+	ctx := context.Background()
+	verifier := func(_ context.Context, token string) (*TokenInfo, error) {
+		switch token {
+		case "valid":
+			return &TokenInfo{Expiration: time.Now().Add(time.Hour)}, nil
+		case "invalid":
+			return nil, ErrInvalidToken
+		case "noexp":
+			return &TokenInfo{}, nil
+		case "expired":
+			return &TokenInfo{Expiration: time.Now().Add(-time.Hour)}, nil
+		default:
+			return nil, errors.New("unknown")
+		}
+	}
+
+	for _, tt := range []struct {
+		name     string
+		opts     *RequireBearerTokenOptions
+		header   string
+		wantMsg  string
+		wantCode int
+	}{
+		{
+			"valid", nil, "Bearer valid",
+			"", 0,
+		},
+		{
+			"bad header", nil, "Barer valid",
+			"no bearer token", 401,
+		},
+		{
+			"invalid", nil, "bearer invalid",
+			"invalid token", 401,
+		},
+		{
+			"no expiration", nil, "Bearer noexp",
+			"token missing expiration", 401,
+		},
+		{
+			"expired", nil, "Bearer expired",
+			"token expired", 401,
+		},
+		{
+			"missing scope", &RequireBearerTokenOptions{Scopes: []string{"s1"}}, "Bearer valid",
+			"insufficient scope", 403,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			_, gotMsg, gotCode := verify(ctx, verifier, tt.opts, tt.header)
+			if gotMsg != tt.wantMsg || gotCode != tt.wantCode {
+				t.Errorf("got (%q, %d), want (%q, %d)", gotMsg, gotCode, tt.wantMsg, tt.wantCode)
+			}
+		})
+	}
+}