Forms authentication: disable redirect to the login page
I have an application that uses ASP.NET Forms Authentication. For the most part, it's working great, but I'm trying to add support for a simple API via an .ashx file. I want the ashx file to have optional authentication (i.e. if you don't supply an Authentication header, then it just works anonymously). But, depending on what you do, I want to require authentication under certain conditions.
I thought it would be a simple matter of responding with status code 401 if the required authentication was not supplied, but it seems like the Forms Authentcation module is intercepting that and responding with a redirect to the login page instead. What I mean is, if my ProcessRequest
method looks like this:
public void ProcessRequest(HttpContext context)
{
Response.StatusCode = 401;
Response.StatusDescription = "Authentication required";
}
Then instead of getting a 401 error code on the client, like I expect, I'm actually getting a 302 redirect to the login page.
For nornal HTTP traffic, I can see how that would be useful, but for my API page, I want the 401 to go through unmodified so that the client-side caller can respond to it programmatically instead.
Is there any way to do that?
Solution 1:
ASP.NET 4.5 added the Boolean HttpResponse.SuppressFormsAuthenticationRedirect
property.
public void ProcessRequest(HttpContext context)
{
Response.StatusCode = 401;
Response.StatusDescription = "Authentication required";
Response.SuppressFormsAuthenticationRedirect = true;
}
Solution 2:
After a bit of investigation, it looks like the FormsAuthenticationModule adds a handler for the HttpApplicationContext.EndRequest
event. In it's handler, it checks for a 401 status code and basically does a Response.Redirect(loginUrl)
instead. As far as I can tell, there's no way to override this behaviour if want to use FormsAuthenticationModule
.
The way I ended up getting around it was by disabling the FormsAuthenticationModule
in the web.config like so:
<authentication mode="None" />
And then implementing the Application_AuthenticateEvent
myself:
void Application_AuthenticateRequest(object sender, EventArgs e)
{
if (Context.User == null)
{
var oldTicket = ExtractTicketFromCookie(Context, FormsAuthentication.FormsCookieName);
if (oldTicket != null && !oldTicket.Expired)
{
var ticket = oldTicket;
if (FormsAuthentication.SlidingExpiration)
{
ticket = FormsAuthentication.RenewTicketIfOld(oldTicket);
if (ticket == null)
return;
}
Context.User = new GenericPrincipal(new FormsIdentity(ticket), new string[0]);
if (ticket != oldTicket)
{
// update the cookie since we've refreshed the ticket
string cookieValue = FormsAuthentication.Encrypt(ticket);
var cookie = Context.Request.Cookies[FormsAuthentication.FormsCookieName] ??
new HttpCookie(FormsAuthentication.FormsCookieName, cookieValue) { Path = ticket.CookiePath };
if (ticket.IsPersistent)
cookie.Expires = ticket.Expiration;
cookie.Value = cookieValue;
cookie.Secure = FormsAuthentication.RequireSSL;
cookie.HttpOnly = true;
if (FormsAuthentication.CookieDomain != null)
cookie.Domain = FormsAuthentication.CookieDomain;
Context.Response.Cookies.Remove(cookie.Name);
Context.Response.Cookies.Add(cookie);
}
}
}
}
private static FormsAuthenticationTicket ExtractTicketFromCookie(HttpContext context, string name)
{
FormsAuthenticationTicket ticket = null;
string encryptedTicket = null;
var cookie = context.Request.Cookies[name];
if (cookie != null)
{
encryptedTicket = cookie.Value;
}
if (!string.IsNullOrEmpty(encryptedTicket))
{
try
{
ticket = FormsAuthentication.Decrypt(encryptedTicket);
}
catch
{
context.Request.Cookies.Remove(name);
}
if (ticket != null && !ticket.Expired)
{
return ticket;
}
// if the ticket is expired then remove it
context.Request.Cookies.Remove(name);
return null;
}
}
It's actually slightly more complicated than that, but I basically got the code by looking at the implementation of FormsAuthenticationModule
in reflector. My implementation is different to the built-in FormsAuthenticationModule
in that it doesn't do anything if you respond with a 401 - no redirecting to the login page at all. I guess if that ever becomes a requirement, I can put an item in the context to disable the auto-redirect or something.
Solution 3:
I'm not sure if this will work for everyone, but in IIS7 you can call Response.End() after you've set the status code and description. This way, that #&$^#@*! FormsAuthenticationModule won't do a redirect.
public void ProcessRequest(HttpContext context) {
Response.StatusCode = 401;
Response.StatusDescription = "Authentication required";
Response.End();
}
Solution 4:
To build on zacharydl's answer slightly, I used this to solve my woes. On every request, at the beginning, if it's AJAX, immediately suppress the behavior.
protected void Application_BeginRequest()
{
HttpRequestBase request = new HttpRequestWrapper(Context.Request);
if (request.IsAjaxRequest())
{
Context.Response.SuppressFormsAuthenticationRedirect = true;
}
}
Solution 5:
I don't know how that Response.End() worked for you. I tried it with no joy, then looked at MSDN for Response.End(): 'stops execution of the page, and raises the EndRequest event'.
For what it's worth my hack was:
_response.StatusCode = 401;
_context.Items["401Override"] = true;
_response.End();
Then in Global.cs add an EndRequest handler (which will get called after Authentication HTTPModule):
protected void Application_EndRequest(object sender, EventArgs e)
{
if (HttpContext.Current.Items["401Override"] != null)
{
HttpContext.Current.Response.Clear();
HttpContext.Current.Response.StatusCode = 401;
}
}