From SecurityFocus Bugtraq:
Some ISP networks do not reset open TCP connections of customers that were either cut-off by the ISP or cut off by self-initiation. While it is responsibility of every person to terminate every open connection before link termination, when the ISP initiates this, it cannot be guaranteed. A customer who happens to resume a recycled dynamic IP can then read the previous persons open sessions. With streaming mp3 radio services that work on a per-pay basis, this can result in substantial monetary losses, not to mention porn streaming. Further unencrypted email can be read and website cookies can be assumed to continue private Web sessions.
After a dynamic customer terminates a broadband connection, network access servers should terminate any TCP traffic with an RST reply, and give at least a minute time for any retransmissions to be caught. If this is not wanted by the network architect, perhaps a stateful firewall keeping states for customers per session. This is not a good solution though because some people do not want a firewall between their end-user connection and the open Internet, their privacy from this should be accepted. Another security programmer that I contacted, suggested that dynamic IP addresses perhaps be more static with end-users, but I personally don’t think this is a good solution either as this puts the anonymity of end-users at stake, and goes against the philosophy of anonymity that the forefathers of the Internet thought out. On the consumers side one can protect themselves by only
using encrypted communications, as this makes reading of personal data difficult.