Windows identity getting confused between users



We are observing some strange behaviour in our web server logs where where
the Identity of the currently logged in user seems to be getting swapped with
another user. I will describe our set up before explaining further.

We are running an web site (v3.5 of the framework) on 2 Windows 2008
web servers and use forms authentication.They are load balanced using a
separate server running Apache 2.2 on Linux (Cent OS 5). The load balancing
simply attaches a cookie to a user and directs them to a particular server
for each subsequent request.

We notice on occasion patterns in the log like this (details obfuscated)

First Log Entry

UserName - (e-mail address removed)

UserId - 1111

WebPage - page1

IP - ip1

Time - 2010-06-29 12:56:20.750

SessionId - h3uyz2fsdfegugjy452sdz0far

Second Log Entry

UserName - (e-mail address removed)

UserId - 2222

WebPage - page2

IP - ip2

Time - 2010-06-29 12:57:16.133

SessionId - 21ipjsdfsdfieqqwyfdokgqsb55

We are using forms authentication using the standard forms
authentication framework (the standard login control and we implemented a
custom membership provider).

The UserName is the Windows identity retrieved using
"HttpContext.Current.User.Identity.Name" The UserId is the database Id set in
the session. The sessionId is retrieved using

As you can see the same Windows identity is the same for 2 different users,
under different IP addresses and with different session id's, hitting the
site about the same time. We checked and the IP's were from totally different
locations. The wrong windows identity seems to be getting recorded. UserId
2222 should have a different username recorded.

Since it happens very occasionally, the code is standard and has not changed
substantially for some time we don't "think" it is a coding error. We presume
either a problem with the load balancer or some problem in the web server. I
have never heard of such problems in before.

Recently we did change our set up from IIS6 on Windows 2003 and a Cisco
hardware load balancer to the current setup of IIS7 on Windows 2008 and the
Apache load balancing. Any ideas appreciated.

The forms authentication entry in the web.config is

authentication mode="Forms"

forms loginUrl="LoginPage.aspx" name=".ASPXFORMSAUTH"

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