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I'm using OpenIddict for auth in a .NET Core 2 API. Client side I'm relying on any API errors to follow a custom scheme. However, when e.g. a refresh token has been outdated, I can't seem to find out how to customize the error sent back.

The /token endpoint is never reached, so the error is not under "my control".

The result of the request is a status code 400, with the following JSON:

"error":"invalid_grant","error_description":"The specified refresh token is no longer valid."

I've tried to use a custom middleware to catch all status codes (which it does), but the result is returned before the execution of my custom middleware has completed.

How can I properly customize the error or intercept to change it? Thanks!

You can use OpenIddict's event model to customize the token response payloads before they are written to the response stream. Here's an example:

MyApplyTokenResponseHandler.cs

public class MyApplyTokenResponseHandler : IOpenIddictServerEventHandler<OpenIddictServerEvents.ApplyTokenResponse>

 public Task<OpenIddictServerEventState> HandleAsync(OpenIddictServerEvents.ApplyTokenResponse notification)
 
 var response = notification.Context.Response;
 if (string.Equals(response.Error, OpenIddictConstants.Errors.InvalidGrant, StringComparison.Ordinal) &&
 !string.IsNullOrEmpty(response.ErrorDescription))
 
 response.ErrorDescription = "Your customized error";
 

 return Task.FromResult(OpenIddictServerEventState.Unhandled);
 

Startup.cs

services.AddOpenIddict()
 .AddCore(options =>
 
 // ...
 )

 .AddServer(options =>
 
 // ...
 options.AddEventHandler<MyApplyTokenResponseHandler>();
 )

 .AddValidation();

The /token endpoint is never reached, so the error is not under "my control".

In fact ,the /token is reached, and the parameter of grant_type equals refresh_token. But the rejection logic when refresh token expired is not processed by us. It is some kind of "hardcoded" in source code :

if (token == null)

 context.Reject(
 error: OpenIddictConstants.Errors.InvalidGrant,
 description: context.Request.IsAuthorizationCodeGrantType() ?
 "The specified authorization code is no longer valid." :
 "The specified refresh token is no longer valid.");

 return;


if (options.UseRollingTokens || context.Request.IsAuthorizationCodeGrantType())

 if (!await TryRedeemTokenAsync(token))
 
 context.Reject(
 error: OpenIddictConstants.Errors.InvalidGrant,
 description: context.Request.IsAuthorizationCodeGrantType() ?
 "The specified authorization code is no longer valid." :
 "The specified refresh token is no longer valid.");

 return;
 

The context.Reject here comes from the assembly AspNet.Security.OpenIdConnect.Server.

For more details, see source code on GitHub .

I've tried to use a custom middleware to catch all status codes (which it does), but the result is returned before the execution of my custom middleware has completed.

I've tried and I'm pretty sure we can use a custom middleware to catch all status codes. The key point is to detect the status code after the next() invocation:

app.Use(async(context , next )=>

 // passby all other end points
 if(! context.Request.Path.StartsWithSegments("/connect/token"))
 await next();
 return;
 

 // since we might want to detect the Response.Body, I add some stream here .
 // if you only want to detect the status code , there's no need to use these streams
 Stream originalStream = context.Response.Body;
 var hijackedStream = new MemoryStream();
 context.Response.Body = hijackedStream;
 hijackedStream.Seek(0,SeekOrigin.Begin);

 await next();

 // if status code not 400 , pass by
 if(context.Response.StatusCode != 400)
 await CopyStreamToResponseBody(context,hijackedStream,originalStream);
 return;
 

 // read and custom the stream 
 hijackedStream.Seek(0,SeekOrigin.Begin);
 using (StreamReader sr = new StreamReader(hijackedStream))
 
 var raw= sr.ReadToEnd();
 if(raw.Contains("The specified refresh token is no longer valid."))
 // custom your own response
 context.Response.StatusCode = 401;
 // ...
 //context.Response.Body = ... /
 else
 await CopyStreamToResponseBody(context,hijackedStream,originalStream);
 
 
);

// helper to make the copy easy
private async Task CopyStreamToResponseBody(HttpContext context,Stream newStream, Stream originalStream)

 newStream.Seek(0,SeekOrigin.Begin);
 await newStream.CopyToAsync(originalStream);
 context.Response.ContentLength =originalStream.Length;
 context.Response.Body = originalStream;