Is the access to your API under control?

Original name

Máte pod kontrolou prístup na vaše API?


Ivan Macalák






Slovak 🇸🇰



  • ✅ Keycloak and the whole 2nd part that was a pure practice.

  • ⛔ Not different information about OAuth 2.0 in the first half.

  • ⛔ Not for dummies but rather copied from other sites.

APIs are everywhere, modern applications are based on APIs (Mobile, Web, IoT, Backend-to-backend), so it is a magnet for attackers and is a possible security risk.


  • Sensitive data can be exposed to unauthorized person.

  • Stolen money from bank account. *Complete airplane booked (then canceled to lower price).

  • Attach can cause serious damage (IoT).

  • DDoS attacks ruins our business which cannot offer services anymore

  • We can have higher costs for cloud (or other paid) services, for example mailing service

Basic concepts

Identity: Each user has a visible identity which is a set of contextual attributes:

  • Person: name, email, phone, picture

    • This can also change by context: Work identity (name, employee number, department), Social network identity (name, email, avatar), Game identity (name, nickname, rank)

  • Device: ID, IP address, model, version

  • Application

  • Component

Authentication defines who you are. For example a citizen of EU based on the ID card or passport matching your biometry.

Authorization defines what rights you have. For example, members of the EU countries can enter the EU with no limit, though most of the countries need visa = claim.

Access Policy Enforcement is a verification policy, for example checking the visa authenticity and expiration.

Authority issues and verifies tokens, and user identity, and guarantees the information in the tokens are valid.

Centralized IAM (Identity Management)

If a system provides multiple application where each requires an access, it is not convenient to require the user to register into each one → This resolves SSO, but the API gateway needs to verify tokens across the n applications → Centralized IAM that all applications integrate to, including the API Gateway.

  • User has one set of login credentials improving the user experience

  • SSO is relative easy to implement via centralized ADM

  • Central point assures consistency of the identity-related data

  • Integration to external providers

  • Single point of failure, so it has to be secured

OAuth 2.0

Standardized IAM solution created for delegated Authorization scenarios, follows the IETF standard and defines authorization flows. It enables an application to obtain authorization to call APIs:

  • Application can obtain User’s consent to call APIs on User’s behalf.

  • Application can obtain authorization to call APIs on its own behalf.

It does not deal with identities themselves and is not an authentication protocol → Open ID Connect solves it.

OAuth 2.0 schema
1. OAuth 2.0 schema
OAuth 2.0 flow
2. OAuth 2.0 flow

Open ID Connect

It provides an identity service layer on top of OAuth 2.0 as a federated Authentication protocol that allows user to reuse their accounts and can integrate multiple identity providers. It allows auth server to authenticate user for applications in a standard way and enables an application to delegate user authentication to OAuth 2.0 auth server. It supports SSO.

Open ID Connect is a successor to SAML.

  • Relying on party = OAuth 2.0 Client (client application which requests claims about user

  • Open ID Provider = OAuth 2.0 Authorization server (authorization as a service: issues identity tokens)


Access Token is a token used by an application to access an API. It represents the application’s authorization to call an API and has an expiration.

Refresh Token is an optional token that can be used bu an application to request a new access token when a prior access token has expired.

Authentication Code is an intermediary, opaque code returned to the application and used to obtain an access token and optionally refresh token. Each authorization code is used once.

Json Web Tokens (JWT) is a JSON structure which encodes claims, where claim is value asserted about subject. The structure is {header}.{payload}.{signature} and the standard registered claims are:

  • jti - token identifier

  • iss - token issuer (issuer)

  • sub - token principal identification (subject)

  • aud - recipients of a token (audience)

  • exp - token expiration time


It is the open source customizable and extendable IAM solution and authorization server. It supports the latest standard protocols with fine-grained authorization capabilities.

  • User authentication (SSO support)

  • OpenID/SAML 2.0 identity provider

  • High availability, scalability, multi-tenancy support

  • Identity brokering and social login

  • User federation support (LDAP, AD, Kerberos, …​)


  • Auth0

  • Curity

  • Amazon IAS

Spring Security

Spring security provides modules to support OAuth 2.0, Open ID Connect, and JWT.

Postman serves as an API client.


Keycloak can define realms that serve as sandboxes.

Client defines a client with parameters, such as URLs and Authentication flow from the OAuth 2.0 protocol point of view:

  • Standard flow is the most secure and most used

  • Implicit flow is not as secured as the standard flow

  • Direct access grants makes the authorization server to grant the access token directly. Each client also has own Client Secret for communication with Keycloak.

In a simple case, it is needed to register Postman (Root and Home URLs are localhost:8080).

API implementation

1. build.gradle
dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-oauth2-resource-server'
    implementation 'org.springframework.boot:spring-boot-starter-web'
public class SecurityConfig {

    public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
        http.authorizeHttpRequests(authorize -> authorize
            // Authorize using matchers
        // Spring by default cannot decode roles from Keycloak
        .oauth2ResourceServer(oauth2 -> oauth2
            .jwt(jwt -> jwt.jwtAuthenticationConverter(new KeycloakJwtAuthenticationConverter()))

Either use matchers above or declarative authorization below:

@GetMapping(path = "/hello")
@PreAuthorize("hasRole('ROLE_USER') or 'hasRole('ROLE_ADMIN')")
public String helloWorld() { .. }

@GetMapping(path = "/admin/hello")
public String adminHelloWorld() { .. }

It is needed to show Spring where is Keycloak server. Spring automatically uses its public key.


On top of that, Spring Security provides the following:

  • Access Token validation

  • Token Claims mapping

  • Declarative Access Control

API call

Postman OAuth 2.0 authorization:

Only the authorization server can ask the password, not the application itself.

Upon requesting "Get New Access Token", Postman gets redirected to Keycloak login page.

Open Policy Agent

It is a policy-based control for cloud native environments. Read more at the website.


Implement proper authentication and authorization using standards and best practices:

  • OAuth 2.0 and Open ID Connect

  • JWT

  • Encryption

  • Opaque Tokens

  • API design

  • Do not implement what is available (products, frameworks, …​)

  • Include security tests and monitor the system