How do I set the WWW-Authentication header in an IAuthenticationFilter implementation?

Question

I'm implementing basic authentication using MVC5's IAuthenticationFilter interface. My understanding is that this is now the preferred approach instead of using a DelegatingHandler. I've got it working but the www-authenticate header is not being returned in the response. This is my implementation of ChallengeAsync:

public async Task ChallengeAsync(HttpAuthenticationChallengeContext context, CancellationToken cancellationToken)
    {
        var result = await context.Result.ExecuteAsync(cancellationToken);
        if (result.StatusCode == HttpStatusCode.Unauthorized)
        {
            result.Headers.WwwAuthenticate.Add(new AuthenticationHeaderValue("Basic", "realm=localhost"));
        }
    }

The header is returned if I set it in AuthenticateAsync but I think I'm supposed to set it in ChallengeAsync. Sample implementations have been hard to find.

Answer 1

In ChallengeAsync, set context.Result to an instance of type IHttpActionResult, like so.

public Task ChallengeAsync(HttpAuthenticationChallengeContext context,
                                  CancellationToken cancellationToken)
{
    context.Result = new ResultWithChallenge(context.Result);
    return Task.FromResult(0);
}

Provide an implementation, like so.

public class ResultWithChallenge : IHttpActionResult
{
    private readonly IHttpActionResult next;

    public ResultWithChallenge(IHttpActionResult next)
    {
        this.next = next;
    }

    public async Task<HttpResponseMessage> ExecuteAsync(
                                CancellationToken cancellationToken)
    {
        var response = await next.ExecuteAsync(cancellationToken);
        if (response.StatusCode == HttpStatusCode.Unauthorized)
        {
            response.Headers.WwwAuthenticate.Add(
                   new AuthenticationHeaderValue("Basic", "realm=localhost"));
        }

        return response;
    }
}