Web API and ValidateAntiForgeryToken

You could implement such authorization attribute:

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
public sealed class ValidateAntiForgeryTokenAttribute : FilterAttribute, IAuthorizationFilter
{
    public Task<HttpResponseMessage> ExecuteAuthorizationFilterAsync(HttpActionContext actionContext, CancellationToken cancellationToken, Func<Task<HttpResponseMessage>> continuation)
    {
        try
        {
            AntiForgery.Validate();
        }
        catch
        {
            actionContext.Response = new HttpResponseMessage 
            { 
                StatusCode = HttpStatusCode.Forbidden, 
                RequestMessage = actionContext.ControllerContext.Request 
            };
            return FromResult(actionContext.Response);
        }
        return continuation();
    }

    private Task<HttpResponseMessage> FromResult(HttpResponseMessage result)
    {
        var source = new TaskCompletionSource<HttpResponseMessage>();
        source.SetResult(result);
        return source.Task;
    }
}

and then decorate your API actions with it:

[ValidateAntiForgeryToken]
public HttpResponseMessage Post()
{
    // some work
    return Request.CreateResponse(HttpStatusCode.Accepted);
}