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Mail relay vulnerability in RedHat 5.0, 5.1, 5.2
From: luyer () UCS UWA EDU AU (David Luyer)
Date: Fri, 16 Jul 1999 18:12:54 +0800

We have recently found ourself used as mail relays and put into the ORBS
mail relay blocking system due to a bug in early anti-relay rulesets as
used in both our local rules and RedHat 5.0, 5.1 and 5.2 (even though we
never touch RedHat on serious servers, somehow our home-brew rulesets ended
up bug-compatible).

It seems that some spammers out there have discovered the power of:

RCPT TO: <"target () destination com"@relay.host.name>

where relay.host.name is obtained by reverse DNS lookup.

Users of sendmail 8.9.x of course have no problem, neither do those who have
updated their mail relay prevention rulesets recently, but I think there are
enough RedHat 5.0, 5.1 and 5.2 users who are unaware of the problem to make it
worth sending this out.

I have put out a quick little script which fixes this.  The script
can be found at:

This problem is checked for by my latest relay scanner at:

(some additional information about open relays and some problems they present
can be found at http://typhaon.ucs.uwa.edu.au/presentations.html under
'E-mail Security', but hopefully everyone is well-informed of the issues by
now; that paper is quite dated even if it is under a year old)


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