This article will delve into the details of the Mulesoft JWT Validation policy. It is intended for advanced users with a comprehensive understanding of the platform's internal operations. The goal is to provide a concise overview of the policy's inner workings.


To ensure a comprehensive understanding of the Mulesoft JWT Validation policy, this in-depth article will start from the basics. Advanced users can skip over sections that cover topics they already understand well. The article will be divided into the following sections:

  1. JWT Configuration in Keycloak
  2. Validating JWT with Java
  3. Configuring JWT in Mulesoft
  4. Checking the source code of the Mulesoft JWT policy
  5. Conclusion
1. JWT Configuration in Keycloak

Since there is already an abundance of information available online regarding JWT, I won't be delving into its technical details. Instead, for this demonstration, I will be using the Keycloak Identity Server to generate JWTs. I have found it to be both free and straightforward to use.

Keycloak installation

If you have Docker installed on your machine, it is the best way to try it out.

docker run -p 8080:8080 -e KEYCLOAK_ADMIN=admin -e KEYCLOAK_ADMIN_PASSWORD=admin start-dev

After setting up your Keycloak server, you can navigate to http://localhost:8080. Here, you will be greeted by a user-friendly welcome screen, similar to the one shown below.

Blog image april 12 (Keycloak)

Configure a Client

To get started, we need to configure a client. On the left-hand side of the screen, select the "Clients" option and then click on "Create Client".

Blog image april 12 (Keycloak Clients)

Add the client name as shown in the screenshot below:

Blog image april 12 (Keycloak Create Clients)

Add the capabilities to the client:

Blog image april 12 (Capability)


Save and check the Credentials section of the Mule client to find the Client Secret, as shown below:

Blog image april 12 (Credentials)

Finding the Well-Known URL of Keycloak

To find the well-known URL of Keycloak, follow these steps:

  1. Click on "Configure" → "Realm settings" → "General".

Blog image april 12 (General)

If you go to the OpenID Endpoint Configuration link, you will find the well-known endpoints that are exposed by the Keycloak server. This is a useful feature.

In my case, the endpoint is:


To use it, simply invoke the endpoint using Postman or any other platform of your choice, and you'll be ready to go.

Blog image april 12 (Endpoint)

Generate JWT

To generate a JWT, invoke the token endpoint:

  • curl --location '<http://localhost:8080/realms/master/protocol/openid-connect/token>' \\
    --header 'Content-Type: application/x-www-form-urlencoded' \\
    --data-urlencode 'client_id=mule' \\
    --data-urlencode 'grant_type=client_credentials' \\
    --data-urlencode 'client_secret=SIGlvYOaD8cJ3DggiW2UBw9E3zpkpWrS' \\
    --data-urlencode 'scope=openid'

Check my Postman screen-shot:

Blog image april 12 (Postman)
The id_token contains the information we are interested in. We can decode it to verify its contents.

Blog image april 12 (id_token)In the next section, we will validate the token using Java code.

2. Validating JWT with Java

We will use simple code to validate a JWT token against the JWKS URL. It's important to note that we can easily locate the JWKS URL using the well-known URL. In this case, the JWKS URL is:


The functionality of the JWKS URL is straightforward - it exposes a set of keys (kid) that can be used to validate JWTs. While there is a lot of information available online about this topic, we won't go into further detail here.

To write the Java code, we will use the Nimbus JOSE+JWT libraries. The following code snippet demonstrates how to use these libraries:

When the token is valid you get the below output:

package com.utility;

import com.nimbusds.jose.JWSAlgorithm;
import com.nimbusds.jose.jwk.source.JWKSource;
import com.nimbusds.jose.jwk.source.RemoteJWKSet;
import com.nimbusds.jose.proc.JWSKeySelector;
import com.nimbusds.jose.proc.JWSVerificationKeySelector;
import com.nimbusds.jose.proc.SecurityContext;
import com.nimbusds.jwt.JWTClaimsSet;
import com.nimbusds.jwt.proc.ConfigurableJWTProcessor;
import com.nimbusds.jwt.proc.DefaultJWTProcessor;


public class App {
    public static void main(String[] args) {

    public static void validate(String accessToken) {
        try {
            ConfigurableJWTProcessor<SecurityContext> jwtProcessor = new DefaultJWTProcessor<>();

            JWKSource<SecurityContext> keySource = new RemoteJWKSet<>(new URL("http://localhost:8080/realms/master/protocol/openid-connect/certs"));
            JWSKeySelector<SecurityContext> keySelector = new JWSVerificationKeySelector<>(JWSAlgorithm.RS256, keySource);

            JWTClaimsSet claimsSet = jwtProcessor.process(accessToken, null);
        catch (Exception e){



When the token is valid you get the below output:

{at_hash=3qFSJ-5r4GcNOFhylifyHg, sub=2bdb2533-8cfe-47ea-9f4f-e8be012c7a3a, clientHost=, clientId=mule, email_verified=false, iss=http://localhost:8080/realms/master, typ=ID,
preferred_username=service-account-mule, clientAddress=,
aud=mule, acr=1, azp=mule, auth_time=0, exp=1676814856, iat=1676814796, jti=28f1853c-40db-46db-b763-4e9a192d6380}

When the JWT expires you get the below error:

Blog image april 12 (JSON Web Token)Isn't it amazing? This is precisely the magic that Mulesoft employs in its JWT Policy, albeit in a more sophisticated manner. Now you know the secret behind it all!

3. Configuring JWT in Mulesoft

To create an API, follow these steps:

  1. Create a RAML in the Design Center.
  2. Publish the RAML to the Exchange.
  3. Import the RAML in the API Manager.
  4. Create the API.

After creating the API, apply the JWT Validation policy to it.

Blog image april 12 (JWT Validation)
4. Checking the source code of the Mulesoft JWT policy

The next section is particularly exciting because we will be debugging the Mulesoft JWT Policy. In my previous articles, I covered how to debug the entire Mulesoft Runtime, not just debugging a Mulesoft application (which is quite straightforward).

Here's what we'll be doing: first, we need to obtain the JWT Validation policy code. Follow the steps below to do so:

Configure Anypoint Platform credentials

The most effective way to obtain the JWT Validation policy code is to download a standalone Mulesoft Runtime 4.x onto your machine. After that, edit the wrapper.conf file to include the platform client_id and client_secret. In my case, I retrieved the environment's credentials to which I plan to deploy my API.

Blog image april 12 (Anypoint Platform)

Additionally, configure debugging settings:

Blog image april 12 (Anypoint Platform2)
I won't go into too much detail here, as you can find more information in the article I cited.

API Development

Assuming that you received the API Instance ID when you created the API in API Manager, you can create a mock implementation on your local machine using this information.

  • API Development
  1. Next, export the zip file and publish it to your standalone Mule Runtime (not Anypoint Studio). Place the file in the /apps folder.
  2. Run the standalone Mule Runtime and wait a few moments. Note that you will need to attach a debugger to the runtime. I typically use IntelliJ for this purpose.

Although we won't be debugging anything right now, I've set up everything for future use.

Over time, you'll notice that the JWT policy is automatically downloaded and installed in your standalone Mulesoft Runtime, which is located at:

Blog image april 12 (Folder)
Explore the JWT Policy

To explore the contents of the jwt-validation-1.3.1-mule-policy.jar file, unzip it using a tool of your choice. Inside, you'll find several other JAR files, but you only need to focus on the one that is marked or highlighted.

This specific JAR file likely contains the core functionality of the JWT policy that you'll be using in your Mulesoft project. Take a closer look at its contents to gain a better understanding of how it works.

Blog image april 12 (JAR file)If you're ready to test your API using the standalone MuleSoft Runtime, you can make a request to it and check the logs to troubleshoot any issues that arise.

Blog image april 12 (MuleSoft Runtime)This is the method that is executed for JWT validation. If you take a closer look at the code, you will find that Mulesoft uses the Nimbus JOSE + JWT libraries to validate the JWT in a sophisticated manner. There are no secrets involved.


It's great to see that you've taken the time to explore what's happening under the hood in Mulesoft and the JWT validation process. By understanding how things work at a deeper level, developers can gain more confidence in their projects and make more informed decisions.

Using JWT doesn't have to be complicated! While there are intricacies to the validation process, tools such as Mulesoft and libraries like Nimbus JOSE + JWT can simplify it and make it more accessible for developers at all levels of experience.


Anupam Gogoi

Anupam Gogoi is an Integration Engineer at Avenue Code. He has been working in software development for about 9 years, implementing solutions in Java technologies as well as in SOA domain. He is a hardcore JAVA and MIDDLEWARE evangelist.

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