How to validate AWS Cognito JWT in .NET Core Web API using .AddJwtBearer()

Question

I was having some trouble figuring out how to go about validating a JWT given to the client by AWS Cognito inside my .NET Core Web API.

Not only could I not figure out what the variables for Microsoft.IdentityModel.Tokens.TokenValidationParameters were supposed to be, but once I finally did, I didn't know how to retrieve the JWT key set from https://cognito-idp.{region}.amazonaws.com/{pool ID}/.well-known/jwks.json

Finally, though a lot of random Googling and trial and error, I found a (seemingly-not-very-efficient solution) solution. However, I spent way too much time doing it. Citing that, plus the fact that AWS documentation on the subject is severely lacking, I decided to post this Q&A to help others find this solution more easily in the future.

If there's a better way to do this, somebody please tell me because I have yet to find a way to do this besides my answer listed below.

Answer 1

The answer lies primarily in correctly defining the TokenValidationParameters.IssuerSigningKeyResolver (parameters, etc. seen here: https://docs.microsoft.com/en-us/dotnet/api/microsoft.identitymodel.tokens.issuersigningkeyresolver?view=azure-dotnet).

This is what tells .NET Core what to verify the JWT sent against. One must also tell it where to find the list of keys. One cannot necessarily hard-code the key set, as it is often rotated by AWS.

One way to do it would be to fetch and serialize the list from the URL inside the IssuerSigningKeyResolver method. The whole .AddJwtBearer() might look something like this:

Startup.cs ConfigureServices() method:

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
                .AddJwtBearer(options =>
                {
                    options.TokenValidationParameters = new TokenValidationParameters
                    {
                        IssuerSigningKeyResolver = (s, securityToken, identifier, parameters) =>
                        {
                            // get JsonWebKeySet from AWS
                            var json = new WebClient().DownloadString(parameters.ValidIssuer + "/.well-known/jwks.json");
                            // serialize the result
                            var keys = JsonConvert.DeserializeObject<JsonWebKeySet>(json).Keys;
                            // cast the result to be the type expected by IssuerSigningKeyResolver
                            return (IEnumerable<SecurityKey>)keys;
                        },

                        ValidIssuer = "https://cognito-idp.{region}.amazonaws.com/{pool ID}",
                        ValidateIssuerSigningKey = true,
                        ValidateIssuer = true,
                        ValidateLifetime = true,
                        ValidAudience = "{Cognito AppClientID}",
                        ValidateAudience = true
                    };
                });

If you use a JS library such as AWS Amplify, you can see parameters such as the ValidIssuer and ValidAudience in your browser's console by observing the result of Auth.currentSession()

A REST fetch request from a JS client to a .NET Core Web API utilizing the JWT Authentication achieved above as well as using the [Authorize] tag on your controller might look something like this:

JS Client using @aws-amplify/auth node package:

// get the current logged in user's info
Auth.currentSession().then((user) => {
fetch('https://localhost:5001/api/values',
  {
    method: 'GET',
    headers: {
      // get the user's JWT token given to it by AWS cognito 
      'Authorization': `Bearer ${user.getIdToken().getJwtToken()}`,
      'Content-Type': 'application/json'
    }
  }
).then(response => response.json())
 .then(data => console.log(data))
 .catch(e => console.error(e))
})