fix: Implicit Grant discovery

Signed-off-by: m.nabokikh <maksim.nabokikh@flant.com>

server/oauth2.go

@@ -128,6 +128,7 @@ const (
 const (
 	grantTypeAuthorizationCode = "authorization_code"
 	grantTypeRefreshToken      = "refresh_token"
+	grantTypeImplicit          = "implicit"
 	grantTypePassword          = "password"
 	grantTypeDeviceCode        = "urn:ietf:params:oauth:grant-type:device_code"
 )

server/server.go

@@ -213,20 +213,27 @@ func newServer(ctx context.Context, c Config, rotationStrategy rotationStrategy)
 		c.SupportedResponseTypes = []string{responseTypeCode}
 	}
 
+	supportedGrant := []string{grantTypeAuthorizationCode, grantTypeRefreshToken, grantTypeDeviceCode} // default
 	supportedRes := make(map[string]bool)
+
 	for _, respType := range c.SupportedResponseTypes {
 		switch respType {
-		case responseTypeCode, responseTypeIDToken, responseTypeToken:
+		case responseTypeCode, responseTypeIDToken:
+			// continue
+		case responseTypeToken:
+			// response_type=token is an implicit flow, let's add it to the discovery info
+			// https://datatracker.ietf.org/doc/html/rfc6749#section-4.2.1
+			supportedGrant = append(supportedGrant, grantTypeImplicit)
 		default:
 			return nil, fmt.Errorf("unsupported response_type %q", respType)
 		}
 		supportedRes[respType] = true
 	}
 
-	supportedGrant := []string{grantTypeAuthorizationCode, grantTypeRefreshToken, grantTypeDeviceCode} // default
 	if c.PasswordConnector != "" {
 		supportedGrant = append(supportedGrant, grantTypePassword)
 	}
+
 	sort.Strings(supportedGrant)
 
 	webFS := web.FS()

server/server_test.go

@@ -1735,3 +1735,42 @@ func TestOAuth2DeviceFlow(t *testing.T) {
 		}
 	}
 }
+
+func TestServerSupportedGrants(t *testing.T) {
+	tests := []struct {
+		name      string
+		config    func(c *Config)
+		resGrants []string
+	}{
+		{
+			name:      "Simple",
+			config:    func(c *Config) {},
+			resGrants: []string{grantTypeAuthorizationCode, grantTypeRefreshToken, grantTypeDeviceCode},
+		},
+		{
+			name:      "With password connector",
+			config:    func(c *Config) { c.PasswordConnector = "local" },
+			resGrants: []string{grantTypeAuthorizationCode, grantTypePassword, grantTypeRefreshToken, grantTypeDeviceCode},
+		},
+		{
+			name:      "With token response",
+			config:    func(c *Config) { c.SupportedResponseTypes = append(c.SupportedResponseTypes, responseTypeToken) },
+			resGrants: []string{grantTypeAuthorizationCode, grantTypeImplicit, grantTypeRefreshToken, grantTypeDeviceCode},
+		},
+		{
+			name: "All",
+			config: func(c *Config) {
+				c.PasswordConnector = "local"
+				c.SupportedResponseTypes = append(c.SupportedResponseTypes, responseTypeToken)
+			},
+			resGrants: []string{grantTypeAuthorizationCode, grantTypeImplicit, grantTypePassword, grantTypeRefreshToken, grantTypeDeviceCode},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, srv := newTestServer(context.TODO(), t, tc.config)
+			require.Equal(t, srv.supportedGrantTypes, tc.resGrants)
+		})
+	}
+}