How does who-has-blocked-me-on-msn sites work?

Question

I understand that this isn't actually possible. So... what do the do? Can a program retrive a list of msn contacts using a valid user/password?

Answer 1

I think there is/was a vulnerability in the MSN protocol that would allow the client to check if it was blocked. Its not fetching the user list of the blocked person. Not sure of the details, but its something along the lines of:

for every one of my contacts
    try to send message
        if server return status == blocked
             return true

Answer 2

What they used to do was exploit the fact that you had to define who you had blocked on MSN for you not to be able to see their status. Then, they issued and update so that only users you allow can see your update. Previously, they'd simply try and IM the user and see if they were online.

Using the MSNPX protocol, yes. See www.hypothetic.org/docs/msn/