A new avenue for spammers to relay mail through an exchange server has been identified.
The concept is outlined here: ZDNet.com.
Essentially, in exchange servers prior to 2003 the guest account, if enabled, will allow anyone to relay mail through the server. The immediate remedy is to ensure that your guest account is disabled. There are exploits that exist to enable an account remotely for local users, but the most likely way the guest account will be enabled is on a server that has been nimda or codered infected and cleaned. The common cleaning tools will not again disable the guest account enabled by these worms.
We’re issuing this alert to remind all exchange admins to ensure that the guest account is disabled, most importantly on servers that have been infected and cleaned in the past, and those exposed to the Internet as an MTA.
We reiterate infotex recommendations, that an exchange server should not be used as a directly Internet exposed MTA, but that another system should be an intermediary to receive mail and forward to an exchange core. This is in large part due to the complexity and number of unknowns in the exchange environment. This being a perfect example of that.
As always, please let us know if you have any questions or require any assistance.