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STANFORD UNIVERSITY

INFORMATION TECHNOLOGY SERVICES

Weblogin Page Flow

Introduction

WebLogin is the name for the component of WebAuth that handles user authentication interaction. It is a CGI service that runs on the WebKDC and presents HTML login forms, status pages, and handles translating browser cookies into XML requests to the WebKDC and vice versa. See doc/weblogin-config for full configuration and customization documentation.

We want to support SPNEGO (and other authentication methods implemented by other Apache modules) as a possible authentication mechanism for users. However, attempting SPNEGO unconditionally has deleterious effects in some browsers: some versions of Opera hang or stop loading the page with an error, and IE throws up a basic-auth dialog box unless the WebLogin URL has been added to its intranet sites. We therefore want to permit a configuration where the username/password dialog box is always shown first, but the user may choose to attempt SPNEGO, and then may also choose to set a cookie that always directs them to SPNEGO.

This is generalized into a system that handles any Apache authentication type, but makes it optional. weblogin can be configured in any of three modes: always ask for username and password unless the user has a single sign-on cookie, always attempt Apache authentication first and only ask for username and password as a fallback, or ask for username and password by default but provide the option of Apache authentication and a cookie that can be set to always attempt it first. Apache authentication is referred to as REMOTE_USER through the code and the rest of this document, and the abbreviation "remuser" is used in variables and cookies.

This flow documentation is also available in visual form in the file docs/diagrams/weblogin-flow.svg or weblogin-flow.png. This may be a clearer way to understand the page flow, and is worth consulting in conjunction with this document.

Cookie Check

For all methods of visiting WebLogin, the WebLogin script first checks whether a test cookie is already set. If it isn't, it redirects the user back to the same page with an additional query parameter appended, setting the test cookie in the process.

If the cookie still isn't set when the browser requests the page again, the user is shown an error page explaining that cookies must be enabled.

The following page flows assume that this has happened already and the user's test cookie is already set.

Basic Page Flow

Here is the WebLogin page flow in the basic case where no REMOTE_USER complications are introduced:

  1. User is sent to WebLogin with a request token and a webkdc-service token from the WAS. WebLogin passes that request and any single sign-on cookies from the browser to the WebKDC. If the WebKDC says that authentication is successful (based on the cookies, generally, but it is also allowed to make out-of-band decisions even though it generally won't), it returns an id token or proxy token and says it was successful. In that case, go to step 3. Otherwise, proceed to step 2.

  2. WebLogin presents a username/password form, keeping the request token in a hidden field. This form will also have an "I don't want to log in" link if the application provided a cancel URL. If the user submits a username and password, it is validated with the WebKDC. If that validation fails, they are returned to the same page with an error. If it succeeds, they proceed to step 3.

  3. User is presented with a confirmation page that states their authenticated identity, provides a link back to the original service with the id or proxy token in the URL, and also provides an "I don't want to log in" link going to the cancel URL of the application if provided by the initial request.

Page Flow with REMOTE_USER

If we always try Apache authentication first, using the configuration in install-spengo (for example) that sets up an alias to the login script as an authentication failure handler, we get the following page flow:

  1. User is sent to WebLogin with a request token and a webkdc-service token from the WAS. WebLogin (via Apache) attempts to authenticate. If they succeed, WebLogin is called with REMOTE_USER set. It uses that information to generate an artificial single sign-on token and calls the WebKDC with it to get an appropriate id or proxy token. If that authentication is sufficient, go to step 5. Otherwise, go to step 2.

    If Apache authentication does not succeed, Apache throws an authentication error (401) and then dispatches the request to the 401 error handler. Go to step 3.

  2. Apache authentication succeeded, but that wasn't sufficient to authenticate the user. Present the regular username/password dialog with hidden fields as above, repeat until the username/password works, and then go to step 5.

  3. The login page is invoked again as a 401 error handler and reads the request token and webkdc-service token out of the separate environment variables Apache uses for error handlers. This login page must obviously not be protected by Apache authentication. It passes the request to the WebKDC with any single sign-on cookies as described above. Go to step 5 if the user is authenticated, step 4 otherwise.

  4. WebLogin presents the username/password form with hidden fields as above. User submits username/password, which is then validated. When successful, proceed to step 5. The submit target of the form must be a URL not protected by Apache authentication (generally the same failure handler URL is reused).

  5. User is presented with a confirmation page as described above and can continue, authenticated, to the application.

Page Flow with Optional REMOTE_USER

We want to instead allow the user to select whether to attempt Apache authentication or not, and to set a cookie that says to always use Apache authentication if desired. To do that, we will use the following page flow:

  1. User is sent to WebLogin with tokens. WebLogin passes the request and any single sign-on cookies to the WebKDC. If the user is authenticated, go to step 6. Otherwise, continue to step 2.

  2. WebLogin checks for the "always use REMOTE_USER" cookie. If present and the application doesn't require username/password authentication, user is redirected to the configured REMOTE_USER URL, keeping the tokens at the end of the URL. Go to step 3. Otherwise, go to step 5.

  3. Apache attempts authentication. If successful, the user is passed to the login script with REMOTE_USER set. Go to step 4. Otherwise, the user is sent to the login script with a different URL (not protected by Apache authentication) as a failure handler. Go to step 5.

  4. The WebLogin script takes the REMOTE_USER value, cobbles up a single sign-on token, and passes that to the WebKDC. If that's sufficient for authentication, proceed to step 6. Otherwise, the username/password dialog is presented again. Note that this case should not happen, since we catch applications that require username/password at step 2 and don't send them through the Apache authentication route. But if it does, loop there until successful authentication, and then go to step 6.

  5. No REMOTE_USER cookie was set, REMOTE_USER was tried and failed, or the WAS requested forced login. If it was tried and failed, read the token information out of the failure handler environment variables. Present a username/password dialog box that also has an alternative that says to try REMOTE_USER, but suppress the latter if called as a failure handler or if forced login was requested. Loop until the user successfully authenticates with username/password and then go to step 6.

  6. The user has successfully authenticated. Show the standard page with the link to the application. If we used REMOTE_USER or if a REMOTE_USER cookie is present, Also show a check box indicating whether the "always use REMOTE_USER" cookie is present and a button that lets the user change that setting while returning them to the same confirmation page with the same link to the calling application.

Implementation

In order to implement this page flow, the login.fcgi script uses the following logic:

  1. If we already have return_url set in the query, we're at the confirmation page and the user has changed their REMOTE_USER configuration. Set or clear the REMOTE_USER cookie as appropriate and then go to step 6 (displaying the confirmation page). Otherwise, continue to step 2.

  2. Check to see if the environment variable REDIRECT_QUERY_STRING is set. If so, set a flag saying we're running as an error handler.

  3. Make sure we have a request token RT and a webkdc-service token ST in the request. If not, throw up the error page.

  4. Construct the message we're going to send to the WebKDC. We always talk to the WebKDC unless we're already at the confirmation page, since the WebKDC may decide to authenticate the user at any step. The WebKDC request contains:

    • The request token from RT.

    • The webkdc-service token from ST.

    • The username and password if provided in the request (via a form submission, for instance).

    • Any tokens stored in cookies beginning with "webauth_wpt_". These are the single sign-on cookies.

    • Remote and local IP address and port information for logging.

    • If Apache REMOTE_USER support is enabled and REMOTE_USER is set, an additional single sign-on token manufactured from the identity in REMOTE_USER.

  5. Submit the request to the WebKDC and read the response. Pull out the login cancel URL, if any, from the response no matter what and store it for later use.

  6. If the WebKDC said that the user is now authenticated, display the confirmation page. Display the configuration checkbox for REMOTE_USER unless the REMOTE_USER cookie is not set and REMOTE_USER is not set in the environment (which means that either we attempted Apache authentication and failed or we never tried). The configuration checkbox, if present, must contain all of the data required to rebuild the confirmation page, namely:

    • The return URL including response token and state token.
    • The authenticated username.
    • The login cancel token, if any.

    We do not have to preserve the proxy cookies, since we will have set them the first time through the confirm page.

    Note that the confirmation page can be suppressed with a WebLogin configuration option. If it is suppressed, it is replaced by a 302 redirect (following REMOTE_USER authentication or authentication based on an existing single sign-on cookie) or a 303 redirect (following username/password form submission).

  7. If we get this far, the user isn't authenticated yet. There are six possibilities.

    • If the WebKDC returns a fatal error, display the error page and we're done. They lose.

    • If the WebKDC returns WK_ERR_USER_AND_PASS_REQUIRED, they do have a "use REMOTE_USER" cookie, and we are not running as a failure handler, redirect to the configured REMOTE_USER URL, including the original request token and webkdc-service token in the URL.

    • If the WebKDC returns WK_ERR_USER_AND_PASS_REQUIRED, they do not have a "use REMOTE_USER" cookie, and we are not running as a failure handler, display the login page including the button that lets them try Apache authentication.

    • If the WebKDC returns WK_ERR_USER_AND_PASS_REQUIRED and neither of the above cases are true, we tried Apache authentication and it failed. Display the login page but omit the button that lets them try Apache authentication.

    • If the WebKDC returns WK_ERR_LOGIN_FORCED, well, username and password are required. We don't get to even try Apache authentication in this case. Present the username/password dialog without the REMOTE_USER button.

    • If the WebKDC returns WK_ERR_LOGIN_FAILED, they already tried using a username and password but got the wrong one. In this case, don't show the REMOTE_USER button. They've already made a choice, and more importantly the original login may have been forced. We don't want someone to have forced login, enter the wrong username and password, and then get a REMOTE_USER option. They may choose to take it, and then end up being authenticated but still having to enter their username and password (very confusing).

License

Copyright 2006, 2008, 2009, 2011

The Board of Trustees of the Leland Stanford Junior University

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.

Last modified Friday, 12-Dec-2014 02:31:11 PM

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