diff --git a/CHANGELOG.md b/CHANGELOG.md
index edc7c81b5..e0454a863 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ Add your entry here.
 - [#1715] Fix token introspection invalid request reason
 - [#1714] Fix `Doorkeeper::AccessToken.find_or_create_for` with empty scopes which raises NoMethodError
 - [#1712] Add `Pragma: no-cache` to token response
+- [#1726] Refactor token introspection class.
 
 ## 5.7.1
 
diff --git a/lib/doorkeeper/config.rb b/lib/doorkeeper/config.rb
index 277d427d0..d8d8ce88a 100644
--- a/lib/doorkeeper/config.rb
+++ b/lib/doorkeeper/config.rb
@@ -418,7 +418,7 @@ def configure_secrets_for(type, using:, fallback:)
            default: (lambda do |token, authorized_client, authorized_token|
              if authorized_token
                authorized_token.application == token&.application
-             elsif token.application
+             elsif token&.application
                authorized_client == token.application
              else
                true
diff --git a/lib/doorkeeper/oauth/token_introspection.rb b/lib/doorkeeper/oauth/token_introspection.rb
index 002c73cb0..4a88118fc 100644
--- a/lib/doorkeeper/oauth/token_introspection.rb
+++ b/lib/doorkeeper/oauth/token_introspection.rb
@@ -6,16 +6,15 @@ module OAuth
     #
     # @see https://datatracker.ietf.org/doc/html/rfc7662
     class TokenIntrospection
-      attr_reader :error, :invalid_request_reason
+      attr_reader :token, :error, :invalid_request_reason
 
       def initialize(server, token)
         @server = server
         @token = token
-
-        authorize!
       end
 
       def authorized?
+        authorize!
         @error.blank?
       end
 
@@ -37,7 +36,7 @@ def to_json(*)
 
       private
 
-      attr_reader :server, :token
+      attr_reader :server
 
       # If the protected resource uses OAuth 2.0 client credentials to
       # authenticate to the introspection endpoint and its credentials are
@@ -59,24 +58,38 @@ def to_json(*)
       def authorize!
         # Requested client authorization
         if server.credentials
-          @error = Errors::InvalidClient unless authorized_client
+          authorize_using_basic_auth!
         elsif authorized_token
-          # Requested bearer token authorization
-          #
-          #  If the protected resource uses an OAuth 2.0 bearer token to authorize
-          #  its call to the introspection endpoint and the token used for
-          #  authorization does not contain sufficient privileges or is otherwise
-          #  invalid for this request, the authorization server responds with an
-          #  HTTP 401 code as described in Section 3 of OAuth 2.0 Bearer Token
-          #  Usage [RFC6750].
-          #
-          @error = Errors::InvalidToken unless valid_authorized_token?
+          authorize_using_bearer_token!
         else
           @error = Errors::InvalidRequest
           @invalid_request_reason = :request_not_authorized
         end
       end
 
+      def authorize_using_basic_auth!
+        # Note that a properly formed and authorized query for an inactive or
+        # otherwise invalid token (or a token the protected resource is not
+        # allowed to know about) is not considered an error response by this
+        # specification. In these cases, the authorization server MUST instead
+        # respond with an introspection response with the "active" field set to
+        # "false" as described in Section 2.2.
+        @error = Errors::InvalidClient unless authorized_client
+      end
+
+      def authorize_using_bearer_token!
+        # Requested bearer token authorization
+        #
+        #  If the protected resource uses an OAuth 2.0 bearer token to authorize
+        #  its call to the introspection endpoint and the token used for
+        #  authorization does not contain sufficient privileges or is otherwise
+        #  invalid for this request, the authorization server responds with an
+        #  HTTP 401 code as described in Section 3 of OAuth 2.0 Bearer Token
+        #  Usage [RFC6750].
+        #
+        @error = Errors::InvalidToken unless valid_authorized_token?
+      end
+
       # Client Authentication
       def authorized_client
         @authorized_client ||= server.credentials && server.client
diff --git a/spec/controllers/tokens_controller_spec.rb b/spec/controllers/tokens_controller_spec.rb
index 9d029314a..08de1c189 100644
--- a/spec/controllers/tokens_controller_spec.rb
+++ b/spec/controllers/tokens_controller_spec.rb
@@ -272,7 +272,7 @@
         end
       end
 
-      it "responds with invalid_token error" do
+      it "responds with invalid_token error for bearer auth" do
         request.headers["Authorization"] = "Bearer #{access_token.token}"
 
         post :introspect, params: { token: token_for_introspection.token }
@@ -282,6 +282,16 @@
         expect(json_response).not_to include("active")
         expect(json_response).to include("error" => "invalid_token")
       end
+
+      it "responds with access_denied error for basic auth" do
+        request.headers["Authorization"] = basic_auth_header_for_client(client)
+
+        post :introspect, params: { token: token_for_introspection.token }
+
+        response_status_should_be 200
+
+        expect(json_response).to include("active" => false)
+      end
     end
 
     context "when custom introspection response configured" do