OSEC

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Bugtraq archives for 1st quarter (Jan-Mar) 1997: Re: New Sendmail bug

Re: New Sendmail bug

Claude Scarpelli (claudeINFOBIOGEN.FR)
Tue, 25 Mar 1997 09:57:47 +0100 

In a mail dated Mar 24, bygranzRS6000.CMP.ILSTU.EDU (Gonzo Granzeau) wrote:
> Jeffrey Moyer once rambled this:
> > On Sat, 22 Mar 1997 C0WZ1LL4NETSPACE.ORG wrote:
> >
> > > Hello fellow mongoloids
> > > Try this:
> > > Make hard link of /etc/passwd to /var/tmp/dead.letter
> > > Telnet to port 25, send mail from some bad email address to some
> > > unreacheable hoost.
> > > Watch your message get appended to passwd.
> > > ie:
> > > cowzilla::0:0:c0wz1ll4 0wns u:/:/bin/sh
>
> okay, just want to point out some things about this exploit...
> this won't work on big boxes that are partitioned cause you can only do a
> hard link on the same file system.  another point is that any box that has
> a 'MAILER-DAEMON' defined will get any mail that gets sent there instead of it

Sometimes, sendmail can't send mail to MAILER-DAEMON. In these case,
the message is stored in /var/tmp/dead.letter.

I have seen it appear in the following configuration :

1) sendmail on the best MX host is configured to refuse mail bigger
   than x bytes.

2) sendmail on a lower priority MX host is configured as a null client
   (FEATURE(nullclient)), but without the size limit.

3) a big mail (bigger than x bytes) arrives on the host where sendmail
   is configured as a null client (the low priority MX host).

Here is what happens then:

4) the null client tries to pass the mail to the best MX, which refuse
   it (bigger than x bytes)

5) So the null client tries to bounce back the mail to the
   originator. Since  it is a null client, it sends the mail to the
   best MX host.

6) But the best MX host refuses the mail (bigger than x bytes). So the
   null client tries to send a notification to MAILER-DAEMON. Since it
   is a null client, it sends this mail to the best MX host, which
   refuse it (bigger than x bytes). This a case where sendmail will
   write to /var/tmp/dead.letter.

It may exist other ways for sendmail to write in /var/tmp/dead.letter.


--
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Claude Scarpelli                        | Defenestrate: to exit a window
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l'étude des BIOmolécules et des GÉNomes | Vol 146, No. 20, Nov 13, 1995)