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Uitsmijter Documentation

A versatile OAuth2 authorization server and Traefik middleware for Kubernetes and Docker. Uses your existing user-database to prevent vendor lock-in.



Uitsmijter is a standalone OAuth2 authorization server with embedded middleware that provides login mechanisms to your project without changing the existing user database.

The goal of this project is to bring trustworthy and easy-to-integrate security to your project, within a few hours from installation, configuration and implementation to go-live.

Main goals of the project:

  • Easy migration
    • Move from a single application login to a distributed OAuth 2 flow for many kinds of applications in just one day
  • Fast implementation
  • Reliability
  • OAuth 2 compatibility
  • Fast response times
  • Low Memory and CPU consumption

Read more about Uitsmijter

Getting familiar with the terminology used in this documentation


Uitsmijter is a popular breakfast, brunch and lunch dish in the Netherlands. The ingredients are put on top of each other, finishing with a fried egg on top that covers the ham and the cheese.

Legend goes that this dish used to be served late at night, just before the guests are kicked out at closing time, which may explain why the Dutch name for this dish, “uitsmijter,” means “bouncer” or “doorman” in english.

We found this is an excellent name for the product, because it is put on top of your existing products (the ham and the cheese) and makes everything more delicious. The english translation bouncer makes perfect sense, because the applications inside no longer have to worry about their security. The bouncer will keep uninvited guests outside.


We have treated it as normal that migration projects take a long time and involve a lot of risk. Uitsmijter hits a pretty crowded market of authorisation servers, but fills the need that migrations from a monolith into a microservice architecture should be nice and comfortable.

With Uitsmijter it is no longer a hurdle to implement secure and modern authentication methods. The product supports the developers in every project phase. It is such a pleasure to work with Uitsmijter that it makes sense to build new projects upon it, because the flexibility that is needed for smooth migrations are the successors of new ideas.

You may want to read the full motivation page to get a deeper understanding of why we are building Uitsmijter from the ground up.

Getting started


This application is meant to run on Kubernetes (K8s) and protects resources that run on Kubernetes clusters. Uitsmijter is tested on Kubernetes version 1.22.0 and above.

The Interceptor Mode is coupled to a preexisting 🔗 Traefik instance running on the cluster. Other ingress controllers are in planning, but still under development.

You can run Uitsmijter in a Docker environment with a 🔗 Traefik Endpoint or standaloone, but this is not covered in this documentation yet.

Read the list of requirements, including links to set up those on your server.

Quick start guide

The description in the Quick Start Guide sets up Uitsmijter in a couple of minutes. There is also a walkthrough guide for the interceptor mode, that explains how to protect static websites.


Uitsmijter works on the basis of Tenants and Clients where Tenants are the top entity (for example internal users, users of a product etc.). They implement the access to an existing authentification backend via an authentication provider implemenation. Each Tenant has Clients (for example a Website, a mobile app and the web shop). The Clients can connected using different methods to Uitsmijter, depending what they need. Uitsmijter provides two main authentification mechanisms:
The Interceptor mode which authenticates every request to a page (for example of a single page application) and redirects the user to a login page when he is not logged in.
An OAuth 2 server which allows an application to be easily integrated via the OAuth 2 standard, normally used for bigger applications and mobile apps.


When a resource is requested, the middleware checks if the current user is logged in. If not, the request is redirected to the login page. If the user making the request is logged in, the middleware directs the request to the requested resource with an authorization header. The authorization header includes a Bearer token with an JWT encoded user profile.

The interceptor mode ist an excellent choose for:

  • Protecting static websites
  • Protecting landing pages
  • Migrating legacy monolith applications

Read the walkthrough guide for securing static webserver resources to see how to protect a static resource server in a couple of minutes. More example deployment may give you a complete picture about the mode and how to configure it for your needs. If you are migrating an existing project with its own user database to Uitsmijter, you may want to read a longer article about the migration a monolith application.

An in depth documentation can be found in the Interceptor Mode section.

OAuth 2 Server

OAuth 2 is a protocol that allows a user to authorize a third-party application or service to access their data or perform actions on their behalf on another web service. It streamlines the process of granting this authorization, making it more secure and convenient for users.

OAuth 2 is well-suited for authenticating a user on a single page application or mobile app.

If you are new to OAuth, this overview of the OAuth flow will help you to understand the basic concept.

Uitsmijter supports various Grant types:

  • Authorization Code
  • Refresh Token
  • Password

To set up the client library you find detailed information about the available endpoints on the dedicated page. Uitsmijter uses the default endpoints described in 🔗RFC 6749 but some libraries does not play well with the standards and needs information about the endpoint name. You can find all the necessary information on the available endpoints page.

You can decode the JWT on your own with a client library, or you may want to use the info-endpoint that uitsmijter supports.

The purpose of Proof Key for Code Exchange is to prevent attacks where an attacker might intercept the authorization code and use it to obtain an access token. It does this by introducing a “code verifier” and a “code challenge” that are sent to the authorization server during the authorization request. The authorization server uses the code challenge to verify that the authorization code was issued to the same client that is making the token request. This helps to prevent an attacker from being able to exchange the authorization code for an access token.


Providers are glue code between Uitsmijter and external resources, like your own user data store. Providers are very easy code written in plain ECMA-Script (Javascript). Even if you are not familiar with JavaScript, writing a provider is easy and straight forward.

Read the general provider information before digging into the User Login Provider.


Tenants and Clients are described in the entities configuration section. For general configuration about Uitsmijter, please take a look at the quick start guide. To give a better overview the helm configuration page lists all available configuration parameters from the installation process, again.

The relation between tenants and clients are described in the Tenant and client configuration along examples and best practises.

The available log levels and formats are explained on the logging page.


Easy customisation is one of the main benefits of Uitsmijter. The authorisation server uses your user database through the ability of defining provider scripts. The Tenant and client configuration give you a tool to describing your needs to handle all logins to multiple projects with multiple devices with different access levels.

The customisation of login templates is possible by using template files or by defining an S3 bucket as a source.

Contribution And Development

Uitsmijter is written in Swift 5.7. Please read the Development information and get familiar with the toolchain.

When you fix a bug, or add a new feature, you have to commit it back to the community. Please read the development Guidelines first To make sure that you meet the requirements and that your change can be transferred to an official release without any problems. The guidelines are there to motivate. We do not want to disappoint any developer.

Our Code Of Conduct is a vital asset for working together. Please do not oversee it!

Finally, here is a list of used 3rd party tools that is used by uitsmijter. Thanks to all of them for the great work without Uitsmijter won’t be alive.


Uitsmijter is released under the Apache Licence 2.0.