How can I suppress the browser's authentication dialog?

My web application has a login page that submits authentication credentials via an AJAX call. If the user enters the correct username and password, everything is fine, but if not, the following happens:

  1. The web server determines that although the request included a well-formed Authorization header, the credentials in the header do not successfully authenticate.
  2. The web server returns a 401 status code and includes one or more WWW-Authenticate headers listing the supported authentication types.
  3. The browser detects that the response to my call on the XMLHttpRequest object is a 401 and the response includes WWW-Authenticate headers. It then pops up an authentication dialog asking, again, for the username and password.

This is all fine up until step 3. I don’t want the dialog to pop up, I want want to handle the 401 response in my AJAX callback function. (For example, by displaying an error message on the login page.) I want the user to re-enter their username and password, of course, but I want them to see my friendly, reassuring login form, not the browser’s ugly, default authentication dialog.

Incidentally, I have no control over the server, so having it return a custom status code (i.e., something other than a 401) is not an option.

Is there any way I can suppress the authentication dialog? In particular, can I suppress the Authentication Required dialog in Firefox 2 or later? Is there any way to suppress the Connect to [host] dialog in IE 6 and later?

Additional information from the author (Sept. 18):
I should add that the real problem with the browser’s authentication dialog popping up is that it give insufficient information to the user.

The user has just entered a username and password via the form on the login page, he believes he has typed them both correctly, and he has clicked the submit button or hit enter. His expectation is that he will be taken to the next page or perhaps told that he has entered his information incorrectly and should try again. However, he is instead presented with an unexpected dialog box.

The dialog makes no acknowledgment of the fact he just did enter a username and password. It does not clearly state that there was a problem and that he should try again. Instead, the dialog box presents the user with cryptic information like “The site says: ‘[realm]‘.” Where [realm] is a short realm name that only a programmer could love.

Web broswer designers take note: no one would ask how to suppress the authentication dialog if the dialog itself were simply more user-friendly. The entire reason that I am doing a login form is that our product management team rightly considers the browsers’ authentication dialogs to be awful.

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