Updating Page Sentry for APEX 4.0 Upgrade

2010-12-09 19:00:00 -0500

We’ve been using Oracle’s Application Express 3.0.x on a client site for about three years now, possibly more. Last week we did the upgrade to Application Express 4.0.2 on a development host, and for the most part it was seamless and our apps worked great. In addition, the new Application Express engine is snappier, easier to work with, and really quite fantastic.

However, we did run into some small troubles that became quite vexing. We use APEX in a single-sign-on environment (Oracle Access Manager is involved), so instead of using APEX’s built-in authentication schemes, we use a custom one we call oamPageSentry, based off of the many examples out there on doing just that (there’s even a kind of unofficial white-paper from Joel Kallman, one of Oracle’s APEX hackers, on creating a “page sentry” authentication scheme in PL/SQL for OAM over here on his blog).

For the sake of brevity, if you’d like to peak at our original page sentry function at the time of the upgrade, in it’s resplendent, mod_plsql glory, check out this gist.

Anyway, the strange behavior we noticed had to do with links used to forward users already authenticated in our SSO environment into a specification application and page in APEX, setting certain required page items in the reports using values passed in through the APEX URL request format. After the upgrade, we saw was that the user was getting a redirect that squashed the Page Item parameters by sending the user to a new session every time, without the original query string preserved.

This seems due not to anything wrong with the original page sentry code (ours or Oracle’s, which were identical in most aspects), but with some slight changes in behavior in APEX to avoid session manipulation, although it was quite tricky to narrow it down.

The primary problem seems to be that when the user visits a session-id-zero link, APEX 4.0 redirects the user to a new version of the URL with a new session ID every time, even if the user already has a session. We found this quite accidentally; we created a simple test app using built-in authentication to narrow things down, and we decided on a lark to include the display of the &APP_SESSION. variable. In other words, for a URL like this:


We’d expect to end up at something like:


And on every subsequent request, we should keep landing at that same link with the same session ID (the value 3343328572473537 in the example above). However, that’s not what was happening, we were getting a new session ID every time. Even more curiously, the value being displayed from APP_SESSION never changed, it was always left at the value of the first session ID we were given in that browser session.

Another problem that was happening was that page items we passed over the original session-id-zero link were indeed retained in the redirect URL, but were not used to set the values in session, despite session protection being off. We think this is because APEX won’t properly manipulate the session from url parameters unless the session IDs agree, and in this case, they wouldn’t.

Now, this was all without the page sentry authentication turned on, so we finally had a culprit — this particular behavior would cause the page sentry function to behave somewhat incorrectly. In our page sentry, which attempts to emulate what APEX normally does, for users that already have a cookie, it just registers the existent session and attempts to forward you to the page you originally requested. Except the session IDs no longer match (because a new session has been generated by APEX thanks to session ID zero), so the URL the user is sent to is for a new session, but the parameters/page item values have been stored in the original session and ignored in the new one.

At this point we decided to adapt the more recent page sentry function listed in Joel Kallman’s blog post as our starting point for getting this working correctly, since his seemed more comprehensive and careful of edge-case scenarios. It looks like this:

create or replace function PASSPORT.oamPageSentry ( p_apex_user in varchar2 default 'APEX_PUBLIC_USER' )
return boolean
l_cgi_var_name varchar2(100) := 'REMOTE_USER';
l_authenticated_username varchar2(256) := upper(owa_util.get_cgi_env(l_cgi_var_name));
l_current_sid number;
-- check to ensure that we are running as the correct database user
if user != upper(p_apex_user) then
return false;
end if;

if l_authenticated_username is null then
return false;
end if;

l_current_sid := apex_custom_auth.get_session_id_from_cookie;
if apex_custom_auth.is_session_valid then
apex_application.g_instance := l_current_sid;
if l_authenticated_username = apex_custom_auth.get_username then
return true;
else -- username mismatch. unset the session cookie and redirect back here to take other branch
apex_application.g_unrecoverable_error := true; -- tell apex engine to quit
return false;
end if;

else -- application session cookie not valid; we need a new apex session
apex_application.g_unrecoverable_error := true; -- tell apex engine to quit
if owa_util.get_cgi_env('REQUEST_METHOD') = 'GET' then
wwv_flow_custom_auth.remember_deep_link(p_url => 'f?'|| wwv_flow_utilities.url_decode2(owa_util.get_cgi_env('QUERY_STRING')));
end if;
-- -- register session in APEX sessions table,set cookie,redirect back
p_uname => l_authenticated_username,
p_session_id => nv('APP_SESSION'),
p_app_page => apex_application.g_flow_id||':'||nvl(apex_application.g_flow_step_id,0));
return false;
end if;
end oamPageSentry;

The first thing we needed to do with the above listing is put in a check for the session ID mis-match caused by linking the user to a page with session zero. If we have the “current”, or originally created session ID in the user’s cookie, we can check to see whether or not it matches what is in the QUERY_STRING, and if they don’t match, redirect the user to the very same URL but with the “current” session ID:

l_current_sid := apex_custom_auth.get_session_id_from_cookie;
l_url := wwv_flow_utilities.url_decode2(owa_util.get_cgi_env('QUERY_STRING'));

-- split on zero or more non-colon characters
l_url_sid := REGEXP_SUBSTR(l_url, '[^:]*', 1, 5);

-- does the current sid match the sid in the url?
if owa_util.get_cgi_env('REQUEST_METHOD') = 'GET' AND l_current_sid <> TO_NUMBER(l_url_sid) then
-- nope, so let's go to the correct url, with the current sid
wwv_flow.debug('oldurl: ' || l_url);
l_url := REGEXP_REPLACE(l_url, '^(p=.+?:.+?):*\d*(.*)$', '\1:' || l_current_sid || '\2');
wwv_flow.debug('newurl: ' || l_url);
owa_util.redirect_url('f?'|| l_url);
return false;
end if;

Now, if the user comes in on session 0 from a link, and they already have a session, but have been redirected to a new/different session ID, page sentry fixes it and redirects the user to the appropriate URL.

However, there are other problematic edge cases. Most Stephen took care of with the clever regex matching used above to identify the session ID if present in the QUERY_STRING, but the real bugger was post_login. In previous versions of APEX, we believe that the remember_deep_link call would cause any subsequent call to post_login to redirect to the user to the target URL. This doesnt appear to be the case in APEX 4.0.

To account for this, we updated the call to pass the target page in to the post_login call directly. We found that post_login will blindly append the session ID to the end of p_app_page parameter string when it redirects, and we can clean that up with the another cleanup-redirect at the beginning of our page sentry function:

-- the post_login call at the end of this function will blindly append the session ID to the URL, even if it is
-- a deep link. Detect this condition, strip the duplicate session identifier, and redirect.
if REGEXP_SUBSTR(l_url, '^.*:' || l_current_sid || ':.+:' || l_current_sid || '$') IS NOT NULL then
l_url := REGEXP_REPLACE(l_url, ':' || l_current_sid || '$', '');
wwv_flow.debug('oamPageSentry: identified duplicate session id on URL, stripping and redirecting to ' || l_url);
owa_util.redirect_url('f?'|| l_url);
return false;
end if;

This set of fixes has gotten us through, straightening out all the redirecting and link remembering that needs to be done when our users come in to link in our pages with params they need and session ID 0.


You can grab the full PL/SQL for our implementation of this page sentry function at this gist.

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