How can I log the generated Access Token in Identity Server 4? - identityserver4

Question

I would like to know how we can log the generated Refresh & AccessToken in IdentityServer 4.
Currently, we've got the custom implementation about the JwtAccessToken and we writes it + userId/name to the central logging system whenever it generates a new Access token. For Apis (we've more than 10), it always writes all incoming requests + JwtToken to the same logging system. So, we can easily trace what the user had done and see the logs/values at that particular time.
Now, we are going to replace that custom security implementation with IDSV4 and we couldn't find out a way to log the generated token in IDSV4.
We know that we can get the Access Token in .Net App by using await HttpContext.GetAccessTokenAsync(). But we don't want to manually send a log from all our apps (.Net, Spas, Apis (Client Credentials)) which are going to integrate with IDSV. We want to manage that AccessToken logging in a central place as we did before.
I looked at the IDSV4 sourcecode TokenEndpoint.cs Line120, LogTokens()
if (response.IdentityToken != null)
{
_logger.LogTrace("Identity token issued for {clientId} / {subjectId}: {token}", clientId, subjectId, response.IdentityToken);
}
if (response.RefreshToken != null)
{
_logger.LogTrace("Refresh token issued for {clientId} / {subjectId}: {token}", clientId, subjectId, response.RefreshToken);
}
if (response.AccessToken != null)
{
_logger.LogTrace("Access token issued for {clientId} / {subjectId}: {token}", clientId, subjectId, response.AccessToken);
}
Actually, they write the TraceLogs for the actual tokens. But we don't want to update the log level to Trace because it'll flood our logging system.
So, I would like to know whether it's possible to implement a feature to write a generated tokens to a log whenever IDSV4 issues an AccessToken. Is there anyway to intercept these tokens after the generation?
Or do we have to manually log AccessTokens whenever it's generated or refreshed in all our clients?
Update:
Thanks to sellotape for giving me an idea for DI. The following is the correct class to intercept the generated Token:
public class CustomTokenResponseGenerator : TokenResponseGenerator
{
public CustomTokenResponseGenerator(ISystemClock clock, ITokenService tokenService, IRefreshTokenService refreshTokenService, IResourceStore resources, IClientStore clients, ILogger<TokenResponseGenerator> logger) : base(clock, tokenService, refreshTokenService, resources, clients, logger)
{
}
public override async Task<TokenResponse> ProcessAsync(TokenRequestValidationResult request)
{
var result = await base.ProcessAsync(request);
// write custom loggings here
return result;
}
}
After that you can replace default class from IDSV4 with your custom class
services.Replace(ServiceDescriptor.Transient<ITokenResponseGenerator, CustomTokenResponseGenerator>());

Answer 1

There are many places to hook in for this; one is to create your own implementation of ITokenService by deriving from DefaultTokenService.
Override CreateAccessTokenAsync() and have it do:
Token result = await base.CreateAccessTokenAsync(request);
// Your logging goes here...
return result;
Swap in your version in your DI container at Startup (make sure it's after the default one has already been added):
services.Replace<ITokenService, MyTokenService>();
... and you should be ready.
As an aside, you should really log hashes of your tokens and not the tokens themselves. You can still match requests and actions to users based on the hash, but then at least nobody will be able to use the logging data to impersonate any of your users.