Using CAS without the Login Screen

Owned by Konrad Wulf
Last updated: Jan 18, 2013 by Anthony O.Version comment
13 min read

Motivation

We wanted to be more flexible in the use of the login UI, so e.g. wanted to embed it in several places as a small panel. Moreover, we wanted to understand CAS as a pure service, not having to maintain layout information twice. Also, we wanted to support both, login postings from another site and direct login at the CAS server as a fallback.

Other proposed solutions

Proposed Solutions that do not work 

There have been two proposals, which we haven't found to be suitable:

  • AJAX: Ajax requests are sandboxed and that sandbox is even more strict than the two-dot rule applied to cookies: It really has to be exactly the same server name in order to work
  • IFrames: In this posting to the cas-dev mailing list, it has been suggested to use an iFrame to embed a login panel on a remote site. This has two limitations:
    • How to get rid of the Frame after login? in the default view a successful login would lead to redirect only in that frame. If you use target="parent" in the from tag, how do you display errors then?
    • You would have limited layout possibilities.

 So for our requirements, none of the proposed solutions were adequate.

Our Solution

We decided to use Javascript triggered redirects, which allows us to use this mechanism also on static html pages (e.g. CMS based contents). While loading we request a login ticket, then the login form displays and then it submits to the CAS login URL. If Errors are present at CAS we redirect to the referring page, adding a parameter "error_message" to that request.

Client side 

Here is the static remote login form we use:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Test remote Login using JS</title>
<script type="text/javascript">
function prepareLoginForm() {
	$('myLoginForm').action = casLoginURL;
	$("lt").value = loginTicket;
}

function checkForLoginTicket() {
	var loginTicketProvided = false;
	var query	= '';
    	casLoginURL = 'https://pcdt057:8443/cas/login';
    	thisPageURL = 'https://pcdt057:8443/cas/test-remote-login-js+error.html';
    	casLoginURL += '?login-at=' + encodeURIComponent (thisPageURL);

	query	= window.location.search;
	query	= query.substr (1);


	var param	= new Array();
	//var value	= new Array();
	var temp	= new Array();
	param	= query.split ('&');

	i = 0;
	while (param[i]) {
		temp		= param[i].split ('=');
		if (temp[0] == 'lt') {
			loginTicket = temp[1];
			loginTicketProvided = true;
		}
		if (temp[0] == 'error_message') {
        	    error = temp[1];
        	}
		i++;
	}
	if (!loginTicketProvided) {
		location.href = casLoginURL + '&get-lt=true';
	}
}

function $(id) {
	return document.getElementById(id);
}
var loginTicket;
var error;
var casLoginURL;
var thisPageURL;

checkForLoginTicket();
onload = prepareLoginForm;
</script>
</head>
<body>
<h2>Test remote Login using JS</h2>
<form id="myLoginForm" action="" method="post">
<input type="hidden" value="submit" name="_eventId"/>
<table>
<tr>
    <td id="txt_error" colspan="2">

	<script type="text/javascript" language="javascript">
	<!--
	if ( error ) {
		error = decodeURIComponent (error);
		document.write (error);
	}
	//-->
	</script>

	</td>
</tr>
<tr>
	<td>Benutzer:</td>
	<td><input type="text" value="konrad" name="username" ></td>
</tr>
<tr>
	<td>Password:</td>
	<td><input type="text" value="konrad" name="password" ></td>
</tr>
<tr>
	<td>Login Ticket:</td>
	<td><input type="text" name="lt" id="lt" value=""></td>
</tr>
<tr>
	<td>Service:</td>
	<td><input type="text" name="service" value="http://quiz.jacomac.de"></td>
</tr>
<tr>
	<td align="right" colspan="2"><input type="submit" /></td>
</tr>
</table>
</form>
</body>
</html>

 

CAS side

Adapting only the views and introducing if-else statements there meant having logic that belongs to the controllers in the view. Therefore we decided to inject our add-on into the spring web flow. Here are the changes we have made to login-webflow.xml (changes highlighted in blue):

<?xml version="1.0" encoding="UTF-8"?>
<flow xmlns="http://www.springframework.org/schema/webflow"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
              http://www.springframework.org/schema/webflow
              http://www.springframework.org/schema/webflow/spring-webflow-1.0.xsd">

 

<start-state idref="provideLoginTicketToRemoteRequestor" />

<action-state id="provideLoginTicketToRemoteRequestor">
<action bean="provideLoginTicketToRemoteRequestorAction" />
<transition on="loginTicketRequested" to="viewRedirectToRequestor" />
<transition on="continue" to="automaticCookiePathSetter" />
</action-state>

<view-state id="viewRedirectToRequestor" view="bmRedirectToRequestorView">
<transition on="submit" to="bindAndValidate" />
</view-state>

<action-state id="automaticCookiePathSetter">
        <action bean="automaticCookiePathSetterAction" />
        <transition on="success"
            to="ticketGrantingTicketExistsCheckAction" />
    </action-state>

    <action-state id="ticketGrantingTicketExistsCheckAction">
        <action bean="ticketGrantingTicketExistsAction" />
        <transition on="ticketGrantingTicketExists"
            to="hasServiceCheck" />
        <transition on="noTicketGrantingTicketExists"
            to="gatewayRequestCheck" />
    </action-state>

    <action-state id="gatewayRequestCheck">
        <action bean="gatewayRequestCheckAction" />
        <transition on="gateway" to="redirect" />
        <transition on="authenticationRequired" to="viewLoginForm" />
    </action-state>

    <action-state id="hasServiceCheck">
        <action bean="hasServiceCheckAction" />
        <transition on="authenticatedButNoService"
            to="viewGenericLoginSuccess" />
        <transition on="hasService" to="renewRequestCheck" />
    </action-state>

    <action-state id="renewRequestCheck">
        <action bean="renewRequestCheckAction" />
        <transition on="authenticationRequired" to="viewLoginForm" />
        <transition on="generateServiceTicket"
            to="generateServiceTicket" />
    </action-state>

    <!--
        <action-state id="startAuthenticate">
        <action bean="x509Check" />
        <transition on="success" to="sendTicketGrantingTicket" />
        <transition on="error" to="viewLoginForm" />
        </action-state>
    -->
    <view-state id="viewLoginForm" view="casLoginView">
        <transition on="submit" to="bindAndValidate" />
    </view-state>

    <action-state id="bindAndValidate">
        <action bean="authenticationViaFormAction" />
        <transition on="success" to="submit" />
        <transition on="error" to="viewLoginForm" />
    </action-state>

    <action-state id="submit">
        <action bean="authenticationViaFormAction" method="submit" />
        <transition on="warn" to="warn" />
        <transition on="success" to="sendTicketGrantingTicket" />
        <transition on="error" to="viewLoginForm" />

 

<transition on="errorForRemoteRequestor" to="viewRedirectToRequestor" />

</action-state>

    <action-state id="sendTicketGrantingTicket">
        <action bean="sendTicketGrantingTicketAction" />
        <transition on="success" to="serviceCheck" />
    </action-state>

    <action-state id="serviceCheck">
        <action bean="hasServiceCheckAction" />
        <transition on="authenticatedButNoService"
            to="viewGenericLoginSuccess" />
        <transition on="hasService" to="generateServiceTicket" />
    </action-state>

    <action-state id="generateServiceTicket">
        <action bean="generateServiceTicketAction" />
        <transition on="success" to="warn" />
        <transition on="error" to="viewLoginForm" />
        <transition on="gateway" to="redirect" />
    </action-state>

    <!--
        The "warn" action makes the determination of whether to redirect directly to the requested
        service or display the "confirmation" page to go back to the server.
    -->
    <action-state id="warn">
        <action bean="warnAction" />
        <transition on="redirect" to="redirect" />
        <transition on="warn" to="showWarningView" />
    </action-state>

    <!--
        the "viewGenericLogin" is the end state for when a user attempts to login without coming directly from a service.
        They have only initialized their single-sign on session.
    -->
    <end-state id="viewGenericLoginSuccess"
        view="casLoginGenericSuccessView" />


    <!--
        The "showWarningView" end state is the end state for when the user has requested privacy settings (to be "warned") to be turned on.  It delegates to a
        view defines in default_views.properties that display the "Please click here to go to the service." message.
    -->
    <end-state id="showWarningView" view="casLoginConfirmView" />

    <!--
        The "redirect" end state allows CAS to properly end the workflow while still redirecting
        the user back to the service required.
    -->
    <end-state id="redirect"
        view="externalRedirect:${externalContext.requestParameterMap['service']}${requestScope.ticket == null ? '' : (externalContext.requestParameterMap['service'].indexOf('?') != -1 ? '&amp;' : '?') + 'ticket=' + requestScope.ticket}" />

    <global-transitions>
        <transition to="viewServiceErrorView"
            on-exception="org.jasig.cas.services.UnauthorizedServiceException" />
    </global-transitions>
</flow>

 

 Problems encountered and suggested solutions

The solution works well, but it has a very dirty edge: introducing a new event in the submit method that allows us to distinguish between a login posting from the CAS Site and another site. Here we had to redeclare the submit method in org.jasig.cas.web.flow.AuthenticationViaFormAction (changes marked in blue):

[...]         try {
            final String ticketGrantingTicketId = this.centralAuthenticationService
                .createTicketGrantingTicket(credentials);
            ContextUtils.addAttribute(context,
                AbstractLoginAction.REQUEST_ATTRIBUTE_TICKET_GRANTING_TICKET,
                ticketGrantingTicketId);
            setWarningCookie(response, warn);
            return success();
        } catch (final TicketException e) {
            populateErrorsInstance(context, e);

            // START: ChangesBusinessMart: check, whether the posting has been sent from a remote server
            String myServerName = request.getLocalName();
            String referrer = request.getParameter("login-at");
            if (referrer != null && referrer.indexOf(myServerName) == -1) {
                return result("errorForRemoteRequestor");
            }

return error();
        }
[...]

 This could have been avoided by

  1. making this submit method overridable (non-final) to add new events in a derived class. Or
  2. to make such a distinction of events in the standard CAS distribution.

Maybe this can be accomplished somehow in future versions of CAS?

Listing of the other files mentioned in this solution 

provideLoginTicketToRemoteRequestorAction -> introduces a new parameter get-lt to signal that a new login ticket shall be issued to the HTTP Referer:

package de.businessmart.sso.cas.flow;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.jasig.cas.web.flow.util.ContextUtils;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.webflow.action.AbstractAction;
import org.springframework.webflow.core.collection.MutableAttributeMap;
import org.springframework.webflow.execution.Event;
import org.springframework.webflow.execution.RequestContext;

import de.businessmart.sso.cas.CasUtility;

/**
 * Opens up the CAS web flow to allow external retrieval of a login ticket.
 * @author konrad.wulf
 *
 */
public class ProvideLoginTicketToRemoteRequestorAction extends AbstractAction
{

	@Override
	protected Event doExecute(RequestContext context) throws Exception
	{
		final HttpServletRequest request = ContextUtils.getHttpServletRequest(context);

		if (request.getParameter("get-lt") != null && request.getParameter("get-lt").equalsIgnoreCase("true")) {
			return result("loginTicketRequested");
		}
		return result("continue");
	}

}

viewRedirectToRequestor actually does the redirects to the referrer:

<%@page import="de.businessmart.sso.cas.CasUtility"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib prefix="spring" uri="http://www.springframework.org/tags" %>
<% String separator = "";
String referrer = request.getHeader("Referer");
referrer = CasUtility.resetUrl(referrer);
if (referrer != null && referrer.length() > 0) {
    separator =(referrer.indexOf("?") > -1)? "&" : "?"; %>
<html>
    <head>
        <script>
               var redirectURL = "<%= referrer + separator %>lt=${flowExecutionKey}";
        <spring:hasBindErrors name="credentials">
            redirectURL += '&error_message=' + encodeURIComponent ('<c:forEach var="error" items="${errors.allErrors}"><spring:message code="${error.code}" text="${error.defaultMessage}" /></c:forEach>');
        </spring:hasBindErrors>
            window.location.href = redirectURL;
       </script>
    </head>
    <body></body>
</html><%
}
else
{
    out.print("You better know what to do here.");
} %>

 

And for completeness, here is the CasUtility that removes obsolete parameters from the query string:

package de.businessmart.sso.cas;

public class CasUtility {
    /**
     * Removes the previously attached GET parameters "lt" and "error_message" to be able to send new ones.
     * @param casUrl
     * @return
     */
    public static String resetUrl ( String casUrl) {
        String cleanedUrl;
        String[] paramsToBeRemoved = new String[]{"lt", "error_message", "get-lt"};
        cleanedUrl = removeHttpGetParameters(casUrl, paramsToBeRemoved);
        return cleanedUrl;
    }


    /**
     * Removes selected HTTP GET parameters from a given URL
     * @param casUrl
     * @param paramsToBeRemoved
     * @return
     */
    public static String removeHttpGetParameters (String casUrl, String[] paramsToBeRemoved) {
        String cleanedUrl = casUrl;
        if (casUrl != null)
        {
            // check if there is any query string at all
            if (casUrl.indexOf("?") == -1)
            {
                return casUrl;
            } else
            {
                // determine the start and end position of the parameters to be removed
                int startPosition, endPosition;
                boolean containsOneOfTheUnwantedParams = false;
                for (String paramToBeErased : paramsToBeRemoved)
                {
                    startPosition = -1;
                    endPosition = -1;
                    if (cleanedUrl.indexOf("?" + paramToBeErased + "=") > -1)
                    {
                        startPosition = cleanedUrl.indexOf("?"
                                + paramToBeErased + "=") + 1;
                    } else if (cleanedUrl.indexOf("&" + paramToBeErased + "=") > -1)
                    {
                        startPosition = cleanedUrl.indexOf("&"
                                + paramToBeErased + "=") + 1;
                    }
                    if (startPosition > -1)
                    {
                        int temp = cleanedUrl.indexOf("&", startPosition);
                        endPosition = (temp > -1) ? temp + 1 : cleanedUrl
                                .length();
                        // remove that parameter, leaving the rest untouched
                        cleanedUrl = cleanedUrl.substring(0, startPosition)
                                + cleanedUrl.substring(endPosition);
                        containsOneOfTheUnwantedParams = true;
                    }
                }

                // wenn nur noch das Fragezeichen vom query string übrig oder am schluss ein "&", dann auch dieses entfernen
                if (cleanedUrl.endsWith("?") || cleanedUrl.endsWith("&"))
                {
                    cleanedUrl = cleanedUrl.substring(0,
                            cleanedUrl.length() - 1);
                }

                // erst zurückgeben, wenn wir auch überprüft haben, ob nicht ein parameter mehrfach angegeben wurde...
                if (!containsOneOfTheUnwantedParams)
                    return casUrl;
                else
                    cleanedUrl = removeHttpGetParameters(cleanedUrl, paramsToBeRemoved);
            }
        }
        return cleanedUrl;

    }

}