HTTP authentication logout via PHP

Question

What is the correct way to log out of HTTP authentication protected folder?

There are workarounds that can achieve this, but they are potentially dangerous because they can be buggy or don't work in certain situations / browsers. That is why I am looking for correct and clean solution.

Answer 1

Typically, once a browser has asked the user for credentials and supplied them to a particular web site, it will continue to do so without further prompting. Unlike the various ways you can clear cookies on the client side, I don't know of a similar way to ask the browser to forget its supplied authentication credentials.

Answer 2

AFAIK, there's no clean way to implement a "logout" function when using htaccess (i.e. HTTP-based) authentication.

This is because such authentication uses the HTTP error code '401' to tell the browser that credentials are required, at which point the browser prompts the user for the details. From then on, until the browser is closed, it will always send the credentials without further prompting.

Answer 3

Workaround (not a clean and nice solution):

Disable his credentials one time.

You can move your HTTP authentication logic to PHP by sending the appropriate headers (if not logged in):

Header('WWW-Authenticate: Basic realm="protected area"');
Header('HTTP/1.0 401 Unauthorized');

And parsing the input with:

$_SERVER['PHP_AUTH_USER'] // httpauth-user
$_SERVER['PHP_AUTH_PW']   // httpauth-password

So disabling his credentials one time should be trivial.

Answer 4

Mu. No correct way exists, not even one that's consistent across browsers.

This is a problem that comes from the HTTP specification (section 15.6):

Existing HTTP clients and user agents typically retain authentication information indefinitely. HTTP/1.1. does not provide a method for a server to direct clients to discard these cached credentials.

On the other hand, section 10.4.2 says:

If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user SHOULD be presented the entity that was given in the response, since that entity might include relevant diagnostic information.

In other words, you may be able to show the login box again (as @Karsten says), but the browser doesn't have to honor your request - so don't depend on this (mis)feature too much.

Answer 5

Method that works nicely in Safari. Also works in Firefox and Opera, but with a warning.

Location: http://logout:[email protected]/

This tells browser to open URL with new username and password, overriding previous one.

Answer 6

The best solution I found so far is (it is sort of pseudo-code, the $isLoggedIn is pseudo variable for http auth):

At the time of "logout" just store some info to the session saying that user is actually logged out.

function logout()
{
  //$isLoggedIn = false; //This does not work (point of this question)
  $_SESSION['logout'] = true;
}

In the place where I check for authentication I expand the condition:

function isLoggedIn()
{
  return $isLoggedIn && !$_SESSION['logout'];
}

Session is somewhat linked to the state of http authentication so user stays logged out as long as he keeps the browser open and as long as http authentication persists in the browser.

Answer 7

My solution tu the problem is the following. You can find the function http_digest_parse , $realm and $users definitionin the second example of this page: http://php.net/manual/en/features.http-auth.php.

session_start();

function LogOut() {
  session_destroy();
  session_unset($_SESSION['session_id']);
  session_unset($_SESSION['logged']);

  header("Location: /", TRUE, 301);   
}

function Login(){

  global $realm;

  if (empty($_SESSION['session_id'])) {
    session_regenerate_id();
    $_SESSION['session_id'] = session_id();
  }

  if (!IsAuthenticated()) {  
    header('HTTP/1.1 401 Unauthorized');
    header('WWW-Authenticate: Digest realm="'.$realm.
   '",qop="auth",nonce="'.$_SESSION['session_id'].'",opaque="'.md5($realm).'"');
    $_SESSION['logged'] = False;
    die('Accesso Negato.');
  }
  $_SESSION['logged'] = True;  
}

function IsAuthenticated(){
  global $realm;
  global $users;


  if  (empty($_SERVER['PHP_AUTH_DIGEST']))
      return False;

  // check PHP_AUTH_DIGEST
  if (!($data = http_digest_parse($_SERVER['PHP_AUTH_DIGEST'])) ||
     !isset($users[$data['username']]))
     return False;// invalid username


  $A1 = md5($data['username'] . ':' . $realm . ':' . $users[$data['username']]);
  $A2 = md5($_SERVER['REQUEST_METHOD'].':'.$data['uri']);

  // Give session id instead of data['nonce']
  $valid_response =   md5($A1.':'.$_SESSION['session_id'].':'.$data['nc'].':'.$data['cnonce'].':'.$data['qop'].':'.$A2);

  if ($data['response'] != $valid_response)
    return False;

  return True;
}

Answer 8

The simple answer is that you can't reliably log out of http-authentication.

The long answer:
Http-auth (like the rest of the HTTP spec) is meant to be stateless. So being "logged in" or "logged out" isn't really a concept that makes sense. The better way to see it is to ask, for each HTTP request (and remember a page load is usually multiple requests), "are you allowed to do what you're requesting?". The server sees each request as new and unrelated to any previous requests.

Browsers have chosen to remember the credentials you tell them on the first 401, and re-send them without the user's explicit permission on subsequent requests. This is an attempt at giving the user the "logged in/logged out" model they expect, but it's purely a kludge. It's the browser that's simulating this persistence of state. The web server is completely unaware of it.

So "logging out", in the context of http-auth is purely a simulation provided by the browser, and so outside the authority of the server.

Yes, there are kludges. But they break RESTful-ness (if that's of value to you) and they are unreliable.

If you absolutely require a logged-in/logged-out model for your site authentication, the best bet is a tracking cookie, with the persistence of state stored on the server in some manner (mysql, sqlite, flatfile, etc). This will require all requests to be evaluated, for instance, with PHP.