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Re: IIS SMTP component allows mail relaying via Null Session
From: Todd Sabin <tsabin () razor bindview com>
Date: 04 Mar 2002 23:23:02 -0500

"Toni Lassila" <toni.lassila () mc-europe com> writes:
Overview:
IIS comes with a small SMTP component.  The default settings allow
anyone who can authenticate to it to relay email.  Because the
authentication system supports NTLM, it is possible for anyone to
authenticate using null session credentials, and then relay email.

Workarounds:
Disable the SMTP service.
Disable the ability of authenticated users to relay email.
Firewall off the SMTP service from untrusted networks.

I suspect turning off NTLM authentication and allowing only Basic
Authentication (with or without TLS), 

I tried this, and it appears to be effective.

                                      or alternatively disabling
null session access (details are in many MS KB) from the server
are two possible workarounds as well. Disabling null sessions is
one of those security features one should do when securing a
Windows-based server anyway.

If by "disabling null sessions" you mean setting RestrictAnonymous to
1 or 2, then that is not effective.  RestrictAnonymous doesn't disable
anonymous access, it just places additional restrictions on it.  You
can still authenticate just fine with a null session when RA=2, and
that's all you need for relaying.


Todd

-- 
Todd Sabin                                               <tas () webspan net>
BindView RAZOR Team                            <tsabin () razor bindview com>


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