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RE: SMTP Relaying bug
From: Pauling <pauling () Starwolf biz>
Date: Tue, 14 Jan 2003 17:02:43 -0500 (EST)
That's exactly my point, However I feel that the rule itself is confusing, becuase when looking at the syslog style log messages, you get (Your IP address) -> (adress of attempted relay tester) SMTP Relaying denied. To me it seems that the snort rules language exibits a weakness here, simply by being able to determine which way the "550 5.7.1" goes, rather than showing that the blocked message come to your system. On Tue, 14 Jan 2003, L. Christopher Luther wrote:
If you're talking about the following rule: alert tcp $SMTP 25 -> $EXTERNAL_NET any (msg:"POLICY SMTP relaying denied"; flags:A+; content: "550 5.7.1"; depth:70; reference:url,mail-abuse.org/tsi/ar-fix.html; reference:arachnids,249; classtype:misc-activity; sid:567; rev:8;) then what you have is a rule that traps when *your* mail server responds from its own TCP port 25 to any outside network on any port, and the response contains the text "550 5.7.1". This implies that someone outside your network attempted to use your SMTP server as a relay point and your server denied the relay attempt, not that your server is attempting to send mail through a closed relay. That rule would be something like: alert tcp $EXTERNAL_NET 25 -> $SMTP any (msg:"POLICY SMTP relaying denied"; flags:A+; content: "550 5.7.1"; depth:70; reference:url,mail-abuse.org/tsi/ar-fix.html; reference:arachnids,249; classtype:misc-activity; sid:567; rev:8;) Hope this helps. Christopher -----Original Message----- Date: Tue, 14 Jan 2003 12:22:36 -0500 (EST) From: Pauling <pauling () Starwolf biz> To: snort-users () lists sourceforge net Subject: [Snort-users] SMTP Relaying bug Has anybody noticed this, that the Alert for an SMTP relay attack monitors the 550 RELAING DENIED message, and as such, gives a misleading notification implying that your server is attempting to send mail through a closed relay. I'm not very good at writing snort rules, but is there any way to efectively reverse this, so that the alert reads that a mail message from $EXTERNAL_NET was not relayed through $SMTP_SERVERS
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Current thread:
- SMTP Relaying bug Pauling (Jan 14)
- <Possible follow-ups>
- RE: SMTP Relaying bug L. Christopher Luther (Jan 14)
- RE: SMTP Relaying bug Pauling (Jan 14)
- RE: SMTP Relaying bug L. Christopher Luther (Jan 14)