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Re: User Enumeration Flaw
From: Valdis.Kletnieks () vt edu
Date: Sat, 18 Feb 2006 18:41:40 -0500

On Sat, 18 Feb 2006 17:03:20 EST, Mar.Shatz () education gov il said:

helo jojo

Lack of a fully qualified domain name.

250 esgeop03.whitehouse.gov Hello [xxx.xxx.xxx.xxx], pleased to meet you
mail from:bob () com com

mail from:<bob () com com>

250 2.1.0 bob () com com    Sender ok
rcpt to:gbush () whitehouse gov

rcpt to:<gbush () whitehouse gov>

Their SMTP server would have been totally within its rights to throw a '501
syntax error' due to the lack of < > characters.  Given that at this point they
have yet to see a command that's correct both syntactically and semantically,
and they're still talking to you, they've obviously decided to try to be as
helpful as possible, rather than being totally strict hard-asses about it.

Of course, they're being self-serving here.  They can either give you a '550
user unknown' reply if the mailbox doesn't exist, or they can give you a '250
user OK' and then try to issue a bounce later - and given that at that point
we're batting 0 for 3 for correct commands, the chances that the MAIL FROM:
will actually work as a bounce destination are slim indeed.

So they get to choose between the small information leak that the 250/550
replies create (said information being extractable via *other* means in most
cases anyhow), or not allowing the leak and getting stuck with the almost
inevitable double bounces that will result.  Obviously, they consider the
security risk of DoS via flooding of double-bounces to be larger than the risk
of leaking information to something that can't get basic SMTP syntax correct...

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