mcp: redirect to upstream oauth2 for authentication (#5594)

internal/mcp/handler.go

@@ -34,6 +34,7 @@ type Handler struct {
 	trace          oteltrace.TracerProvider
 	storage        *Storage
 	cipher         cipher.AEAD
+	relyingParties *OAuth2Configs
 }
 
 func New(
@@ -58,6 +59,7 @@ func New(
 		trace:          tracerProvider,
 		storage:        NewStorage(client),
 		cipher:         cipher,
+		relyingParties: NewOAuthConfig(cfg, http.DefaultClient),
 	}, nil
 }
 

internal/mcp/handler_authorization.go

@@ -68,8 +68,13 @@ func (srv *Handler) Authorize(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	loginURL, ok := srv.relyingParties.GetLoginURLForHost(r.Host, id)
+	if ok {
+		http.Redirect(w, r, loginURL, http.StatusFound)
+	} else {
 		srv.AuthorizationResponse(ctx, w, r, id, v)
 	}
+}
 
 // AuthorizationResponse generates the successful authorization response
 // see https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-12#section-4.1.2

internal/mcp/oauth_config.go

@@ -0,0 +1,93 @@
+package mcp
+
+import (
+	"net/http"
+	"net/url"
+	"path"
+	"sync"
+
+	"golang.org/x/oauth2"
+
+	"github.com/pomerium/pomerium/config"
+)
+
+type OAuth2Configs struct {
+	cfg        *config.Config
+	prefix     string
+	httpClient *http.Client
+
+	buildOnce sync.Once
+	perHost   map[string]*oauth2.Config
+}
+
+func NewOAuthConfig(
+	cfg *config.Config,
+	httpClient *http.Client,
+) *OAuth2Configs {
+	return &OAuth2Configs{
+		prefix:     DefaultPrefix,
+		cfg:        cfg,
+		httpClient: httpClient,
+	}
+}
+
+func (r *OAuth2Configs) GetLoginURLForHost(host string, state string) (string, bool) {
+	r.buildOnce.Do(r.build)
+
+	cfg, ok := r.perHost[host]
+	if !ok {
+		return "", false
+	}
+
+	return cfg.AuthCodeURL(state, oauth2.AccessTypeOffline), true
+}
+
+func (r *OAuth2Configs) build() {
+	r.perHost = BuildOAuthConfig(r.cfg, r.prefix)
+}
+
+// BuildOAuthConfig builds a map of OAuth2 configs per host
+func BuildOAuthConfig(cfg *config.Config, prefix string) map[string]*oauth2.Config {
+	configs := make(map[string]*oauth2.Config)
+	for policy := range cfg.Options.GetAllPolicies() {
+		if !policy.IsMCPServer() || policy.MCP.UpstreamOAuth2 == nil {
+			continue
+		}
+		u, err := url.Parse(policy.GetFrom())
+		if err != nil {
+			continue
+		}
+		host := u.Hostname()
+		if _, ok := configs[host]; ok {
+			continue
+		}
+		cfg := &oauth2.Config{
+			ClientID:     policy.MCP.UpstreamOAuth2.ClientID,
+			ClientSecret: policy.MCP.UpstreamOAuth2.ClientSecret,
+			Endpoint: oauth2.Endpoint{
+				AuthURL:   policy.MCP.UpstreamOAuth2.Endpoint.AuthURL,
+				TokenURL:  policy.MCP.UpstreamOAuth2.Endpoint.TokenURL,
+				AuthStyle: authStyleEnum(policy.MCP.UpstreamOAuth2.Endpoint.AuthStyle),
+			},
+			RedirectURL: (&url.URL{
+				Scheme: "https",
+				Host:   host,
+				Path:   path.Join(prefix, oauthCallbackEndpoint),
+			}).String(),
+			Scopes: policy.MCP.UpstreamOAuth2.Scopes,
+		}
+		configs[host] = cfg
+	}
+	return configs
+}
+
+func authStyleEnum(o config.OAuth2EndpointAuthStyle) oauth2.AuthStyle {
+	switch o {
+	case config.OAuth2EndpointAuthStyleInHeader:
+		return oauth2.AuthStyleInHeader
+	case config.OAuth2EndpointAuthStyleInParams:
+		return oauth2.AuthStyleInParams
+	default:
+		return oauth2.AuthStyleAutoDetect
+	}
+}

internal/mcp/oauth_config_test.go

@@ -0,0 +1,57 @@
+package mcp_test
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/oauth2"
+
+	"github.com/pomerium/pomerium/config"
+	"github.com/pomerium/pomerium/internal/mcp"
+)
+
+func TestBuildOAuthConfig(t *testing.T) {
+	cfg := &config.Config{
+		Options: &config.Options{
+			Policies: []config.Policy{
+				{
+					From: "https://regular.example.com",
+				},
+				{
+					From: "https://mcp1.example.com",
+					MCP:  &config.MCP{},
+				},
+				{
+					From: "https://mcp2.example.com",
+					MCP: &config.MCP{
+						UpstreamOAuth2: &config.UpstreamOAuth2{
+							ClientID:     "client_id",
+							ClientSecret: "client_secret",
+							Endpoint: config.OAuth2Endpoint{
+								AuthURL:   "https://auth.example.com/auth",
+								TokenURL:  "https://auth.example.com/token",
+								AuthStyle: config.OAuth2EndpointAuthStyleInParams,
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+	got := mcp.BuildOAuthConfig(cfg, "/prefix")
+	diff := cmp.Diff(got, map[string]*oauth2.Config{
+		"mcp2.example.com": {
+			ClientID:     "client_id",
+			ClientSecret: "client_secret",
+			Endpoint: oauth2.Endpoint{
+				AuthURL:   "https://auth.example.com/auth",
+				TokenURL:  "https://auth.example.com/token",
+				AuthStyle: oauth2.AuthStyleInParams,
+			},
+			RedirectURL: "https://mcp2.example.com/prefix/oauth/callback",
+		},
+	}, cmpopts.IgnoreUnexported(oauth2.Config{}))
+	require.Empty(t, diff)
+}