Security issue - sites.google.com

One person with whom I was chatting send me a link to someone's google
personal site. When I clicked the link I was taken to this individual's
personal site, but I was not logged as myself, instead the site showed my
friend's login. I was able to modify his sites, user settings etc. I was
all this while online on gmail, but the other tab in which I was looking at
this google site I was logged in as my chat friend.
Looks like a security issue. 

What steps will reproduce the problem?
1. 2 friends login into gmail. First friend opens a
site.google.com/site1/somevalidgooglesite/home, logs in as himself on his
browser and sends the link to the second friend
2. Second friend clicks the link from his chat window and the sites.google
link opens in a new tab
3. Second friend finds him on this other site but logged in as the first
friend and is able to modify settings and websites and other content of the
first friend, but when he tries to access the left side gmail, orkut and
other links he lands on those according to his login name and not his
friends. Only the site.google pages seem modifiable and vulnerable to this
kind of attack.

What is the expected output? What do you see instead?
Expected output was I should be able to see the site.google link logged in
the person who clicked the link instead of being logged in as the person
who sent the link.

What version of the product are you using? On what operating system?
Using sites.google.com and gmail.com on Mac OS - Firefox browser 3.x.

What is the url of the corresponding thread at Google Groups?

What is your support ticket from Google support?

Please provide any additional information below.
I have few screenshots that I took of the whole thing and also the chat
records which might be useful. If needed please contact me on my email I
can send them across. I don't want to attach here unless you guys are
serious about it as they contain personal information.
Original issue reported on code.google.com by vandana....@gmail.com on 2 Dec 2009 at 6:36
Zolta / googlebugs

Security issue - sites.google.com #221
