There are two main problems with "harvest attack". The first is the consumtion of your systems resources which sometimes can cause a total failure. The second one is that the attack is only the start, I mean, you get a storm of mails which try to discover what addresses really exist. If the attack is successful, they have valid email addresses of your organization and they are able to spam you anytime. But the worse is that they can sell and resell the list to other spammers. If the user does not exist in active directory Exchange shows an "550 5.1.1 User unknown" error. There are some antispam products that prevent of forwarding that error. Perhaps you can solve the storm problem but remember that it is possible that the spammer has already the list of valid addresses. For those who are planning a new email infrastructure it advisable to install antispammers at the beggining and create mailboxes with large alias names in order to make the attack more difficult. -Sierr@- ________________________________ De: John T (Lists) [mailto:johnlist@xxxxxxxxxxxxxxxxxxx] Enviado el: viernes, 06 de enero de 2006 16:55 Para: [ExchangeList] Asunto: [exchangelist] RE: Some Advice Please Carácter: Privado http://www.MSExchange.org/ http://www.vamsoft.com/orf/ John T eServices For You -----Original Message----- From: Maglinger, Paul [mailto:PMAGLINGER@xxxxxxxx] Sent: Friday, January 06, 2006 5:42 AM To: [ExchangeList] Subject: [exchangelist] RE: Some Advice Please Sensitivity: Private http://www.MSExchange.org/ John - ORF? ________________________________ From: John T (Lists) [mailto:johnlist@xxxxxxxxxxxxxxxxxxx] Sent: Thursday, January 05, 2006 17:10 To: [ExchangeList] Subject: [exchangelist] RE: Some Advice Please Sensitivity: Private http://www.MSExchange.org/ If it is a harvesting or dictionary attack, your best bet is an automated way to temporarily block connections from an IP after x amount of invalid recipients or tarpit the IP after x amount of invalid recipients. Some one else has posted a couple of links of how to do this on Exchange, but IMHO you want to do this before your Exchange server unless you a small shop and do not have other resources. My clients Exchange servers sit behind my e-mail server which is acting as a gateway for them which that server sits behind 3 MS SMTP servers with ORF running. ORF is actually a very good product that is growing but does not get mentioned much. It can install on any server running IIS as it works directly with the IIS SMTP service. A harvest attack is where the attacking server(s) will "send" an e-mail to every possible address at your domain from a through zzzzzzzzzzz (you get the idea) to find out which are valid addresses. The proper way to fight this is either block the IP after so many invalid recipients or to tarpit which means waiting 30 to 60 seconds to respond with a 5.1.x indicating an invalid address. John T eServices For You ------------------------------------------------------ List Archives: http://www.webelists.com/cgi/lyris.pl?enter=exchangelist Exchange Newsletters: http://www.msexchange.org/pages/newsletter.asp ------------------------------------------------------ Visit TechGenix.com for more information about our other sites: http://www.techgenix.com ------------------------------------------------------ You are currently subscribed to this MSExchange.org Discussion List as: sieferda@xxxxxxx To unsubscribe visit http://www.webelists.com/cgi/lyris.pl?enter=exchangelist Report abuse to info@xxxxxxxxxxxxxx