We have an ASP.NET application that uses Forms Auth. When users log in, a session ID cookie and a Forms Auth ticket (stored as a cookie) are generated. These are session cookies, not permanent cookies. It is intentional and desirable that when the browser closes, the user is effectively logged out.
Once a user logs in, a new window is popped up using window.open('location here');. The page that is opened is effectively the workspace the user works in throughout the rest of their session. From this page, other pop-ups are also used.
Lately, we've had a number of customers (all using latest versions of IE8) complaining that the when they log in, the initial pop-up takes them back to the log in screen rather than their homepage. Alternately, users can sometimes log in, get to the homepage (which again, is in a new pop up window), and it all seems fine, until any additional pop-ups are created, where it starts redirecting them to the log in screen again.
In attempting to troubleshoot the issue, I've used good old Fiddler. When the problem starts manifesting, I've noticed that the browser is not sending up the ASP.NET session ID session cookie OR the Forms Auth ticket session cookie, even though the response to the log in POST clearly pushes down those cookies.
What's more strange is if I CTRL+N to open a new window from the popped-up window that is missing the session cookies, then manually type in the URL to the home page, those cookies magically appear again. However, subsequent window.open(); calls will continue to be broken, not sending the session cookies and taking the user to the log in screen.
It's important to note that sometimes, for seemingly no good reason, those same users can suddenly log in and work normally for a while, then it goes back to broken.
Now, I've ensured that there are no browser add-ons, plug-ins, toolbars, etc. are running. I've added our site as a trusted site and dropped the security settings to Low, I've modified the Cookie Privacy policy to "accept all" and even disabled automatic policy settings, manually forcing it to accept everything and include session cookies. Nothing appears to affect it.
Also note the web application resides on a single server. There is no load balancing, web gardens, server farms, clusters, etc. The server does reside behind an ISA server, but other than that it's pretty straight forward.
I've been searching around for days and haven't found anything actionable. Heck, sometimes I can't even reproduce it reliably. I have found a few references to people having this same problem, but they seem to be referencing an issue that was allegedly fixed in a beta or RC release (example: http://stackoverflow.com/questions/179260/ie8-loses-cookies-when-opening-a-new-window-after-a-redirect). These are release versions of IE, with up-to-date patches.
I'm aware that I can try to set permanent cookies instead of session cookies. However, this has drastic security implications for our application.
Update
It seems that the problem automagically goes away when the user is added as a Local Administrator on the machine. Only time will tell if this change permanently (and positively) affects this problem.
Time to bust out ProcMon and see if there is a resource access problem.
Update #2
It seems there are multiple angles to what appears to be a singular problem. I reported long ago that making the user a local administrator seemed to help. And it did, for a number of users. Of course, that's not really a solution, but it did let us hobble along.
Then more users started reporting the issue, and the admin fix was not helping. The users seemed to be mostly Win7, but Vista was also affected. They also seemed to mostly be 64-bit installations.
Setting the TabProcGrowth to 0 or 1 (either worked) as suggested by some members below seems to have largely addressed the issue. So, I'm going to move my accepted answer to the first person that suggested that, as it has had significantly more impact.
This has been an incredibly frustrating issue to attempt to solve, since it is difficult to reproduce and often occurs with users that I do not have direct communication with, or by the time I get to them it doesn't appear to be working. All I can say is something is not right with the session merging feature, but I don't have much data to feed to Microsoft to find a permanent fix.