Re: Blocking Outlook External POP/SMTP

[Ansgar Wiechers <bugtraq () planetcobalt net>]

On 2010-03-11 Hart, Lee Anne wrote:
> I'm looking for information on preventing/prohibiting users from
> utilizing Outlook (at work) to retrieve their personal email from
> Gmail, AOL, Yahoo, Comcast, Verizon, etc.
>
> Looking for the following:
>
> * Reasons why users should not be allowed to use Outlook on their work
> computers to retrieve their personal email.

It's their employer's infrastructure, and he's also the one paying the
salary, so it's entirley his decision whether or not he wants to allow
private e-mail in the workplace.

That said, I don't see much harm in allowing private e-mail as long as
people get their job done.

Another issue might be data leakage. However, there's so many routes for
data to leak, that blocking access to e-mail providers doesn't really
make a difference if that's your only measure.

In any case, make sure you have an Acceptable Use Policy that specifies
whether or not private e-mail is allowed and have the employees sign it.

> * Tools/techniques to block this type of traffic. It seems the most
> common ports are 110, 465, 587, 993, and 995. Are there others? 

Deny access to ports 110 and 995 if you want to block POP3(s).
Deny access to ports 143, 220 and 993 if you want to block IMAP(s).

Ports 465 (ssmtp) and 587 (submission) are used for sending mail, not
for fetching mail.

Be aware of the fact that the above are only the default ports. The
administrator of a server can have any service listen on any port they
want.

Also be aware of the fact that blacklisting particular ports (or IP
addresses for that matter) can be easily bypassed by tunneling
connections.

If you want more reliable enforcement of the restriction, you need to
apply more draconian settings (whitelisting instead of blacklisting,
traffic inspection at protocol level, etc.). However, doing that will
require a lot of maintenance, which would hardly be justifiable unless
you have rather high security requirements.

> Any reference are much appreciated. 

http://www.iana.org/assignments/port-numbers

Regards
Ansgar Wiechers
-- 
"All vulnerabilities deserve a public fear period prior to patches
becoming available."
--Jason Coombs on Bugtraq

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