Sendmail 8.12 and preventing unauthorized distribution via list-outgoing

[Jeff Wasilko <jeffw @ smoe . org>]

I'm having problems with the yahoo/party photos virus going
directly to the -outgoing alias.

I've tried a couple of suggestions I've found both here and on
google, but none of them work with sendmail 8.12 (when using
sendmail's MSP daemon). All of the suggestions I've found end up
bouncing all list mail.

Has anyone found a way to protect the -outgoing aliases under
sendmail 8.12?

-jeff

----- Forwarded message from Daniel Liston <dliston@netscape.com> -----

Date: Thu, 22 Nov 2001 23:35:12 -0600
From: Daniel Liston <dliston@netscape.com>
Organization: Netscape Communications Corp.
X-Mailer: Mozilla 4.76 [en] (X11; U; Linux 2.4.2-2 i686)
To: majordomo-users@GreatCircle.COM
Subject: Re: preventing unauthorized distribution via list-outgoing.

Email virii tend to read address books of email recipients, and
send themself to every address in the list.  Apparently, somebody
on your list was using the listname-outgoing address to talk to 
your list rather than listname, which is how the virus found the
-outgoing address to begin with.  This is also what spammers have
done for a long time.  This is not a majordomo issue, but does
tend to reveal itself when majordomo is used.  Any alias pointing
to an :include: file on the RHS has this same weakness.  You can
stop external users, spammers, and email virii from reaching the
weak alias by enabling and using the virtusertable in sendmail.

In your sendmail.mc add
FEATURE(`virtusertable',`hash -o /etc/mail/virtusertable')dnl
and rebuild your sendmail.cf with
m4 /etc/mail/sendmail.mc > /etc/sendmail.cf
(actual locations in the command above need to match your server)

Secondly, hide the -outgoing address by using something less easy
to guess, like listname-20011122 or listname-delivery.  Then add
this address to your virtusertable to prevent outsiders from 
writing to it, even if it is found.

In /etc/mail/virtusertable add
listname-20011122@mydomain.com		error:nouser User unknown
Any time you make a change to this file, you will also need to 
rebuild the binary database associated with the file.
makemap hash -o /etc/mail/virtusertable < /etc/mail/virtusertable
(solaris uses dbm rather than hash)

Thirdly, prevent sendmail from disclosing the weak alias via the
"Received:" headers of the messages.  Sendmail will not expand
or disclose the LHS of an alias if the RHS is to multiple recipients.
Create (if it does not already exist) a "nobody:" alias that gets
redirected to /dev/null.
nobody: /dev/null
Use this nobody: alias as a second recipient on the RHS of list
aliases along with their normal deliver address.  Following the
above example;

listname: "/pathto/majordomo/wrapper resend -l listname listname-20011122,nobody"
listname-20011122: :include:/pathto/majordomo/lists/listname,nobody

Dan Liston

Ezra Bick wrote:
> 
> We run about 20 majordomo lists reaching 16,000 subscribers.
> 
> Lately there has been a virus on some users computers, which
> automatically sends itself in reply to any mail received.
> 
> Hence, it was being sent to the list by infected subscribers.
> 
> I changed all lists to moderated with approval required, so the virus
> messages were bounced to the moderator (and deleted).
> 
> BUT - the virus has taken to sending itself to <list-outgoing>, which
> then resends to the whole list and effectively avoids all restrictions
> found in the list configuration file.
> 
> This seems to be a rather large whole, which could be used by spammers as
> well as viruses. By simply adding -outgoing to the name of a list, anyone
> can send anything to any list.
> 
> What can be done?
> 
> ebick@etzion.org.il

----- End forwarded message -----