Autor: Marc Sherman Fecha: A: Tony Finch Cc: Exim Users Asunto: Re: [exim] Why is STARTTLS preferred over tls_on_connect_ports?
Tony Finch wrote: >
> It's because of IETF policy. Protocols should only be run on one port
> and should have built-in security, rather than having a separate
> secure version of the protocol on a different port.
That makes sense. But given that I'm interested in "465 or 587 for
submission?" rather than "25 or 25+465 for SMTP?", it seems like a wash
to me.
> Actually the information in the HELO command is completely
> uninteresting to an attacker.
Completely uninteresting? Ok, I'll bite. I'm a junior IT guy at
Acquisicorp. I get minimum wage to sit on a stool in the network
closet, watching the blinking lights, changing the bulbs as they burn
out. I've managed to get a box on the network doing passive
eavesdropping on all our incomming traffic on port 587. No-one was
worried about the possibility, since the only unencrypted traffic on
that port is completely unintersting.
I happen to know that a certain hotshot M&A guy at Acquisicorp has his
laptop set up to send "EHLO hot.grits". It's a bogus EHLO string, but
it is (fairly) unique. My eavesdropping box can record IP/EHLO pairs
for all incomming connections on port 587, before the STARTTLS command.
By searching for hot.grits, and doing reverse lookups on the IP
addresses, I can figure out what possible target companies Mr. Hotshot
M&A guy is scouting for hostile takeover this week, and Buy Low, Sell
High! It's a victimless crime!
> The real reason that TLS-on-connect is more secure than STARTTLS is
> because it is more resistant to downgrade attacks. However this is
> mostly to do with the bad quality of implementation of most SMTP
> clients - they encourage users to make security optional rather than
> required, which makes the attacker's job much easier.
Yeah, I'd thought of that, too, but I've already got my clients secured
against that, so I didn't mention it.
