Although SMTP probes can be legitimate uses of the network, they can also pose potential risks. They are sometimes used to see whether a bug remains unfixed. Sometimes they are used to try to gather user login names or to feed a program unexpected input in such a way that it breaks and gives away root privilege.
An "unfixed bug" probe can use the SMTP
allows the local sendmail to be placed into debugging mode
(as with the
-d command-line switch; see Section 37.1, "The Syntax of -d") from any other machine
anywhere on the network. The SMTP
showq command allows outsiders
to view the contents of the mail queue.
If SMTPDEBUG (see Section 18.8.42, SMTPDEBUG) is defined when sendmail is compiled, the SMTP debug and showq commands are allowed to work; otherwise, they are disabled. SMTPDEBUG should be defined only when modifying the sendmail code and testing a new version. It should never be defined in an official release of sendmail. To see whether it has been defined at your site, run the following command:
telnet localhost 25Trying 18.104.22.168 ... Connected to localhost. Escape character is '^]'. 220 localhost sendmail 8.8.4 ready at Fri, 13 Dec 1996 06:36:12 -0800
debug500 Command unrecognized
quit221 localhost.us.edu closing connection Connection closed by foreign host. %
When connected, enter the command
debug. If you get the
500 Command unrecognized, you know that
SMTPDEBUG is not enabled. If, on the other hand, you get the answer
200 Debug set, SMTPDEBUG is defined on your
system, and you should immediately take steps to correct the situation.
Either contact your vendor and request a new version of sendmail,
or get the sendmail source and compile it with SMTPDEBUG undefined.
When SMTPDEBUG is undefined and
an outsider connects to the local machine and attempts to execute the
sendmail will syslog(3) a message like the following:
Jul 22 07:09:00 here.domain sendmail: "debug" command from there.domain (22.214.171.124)
This message shows the name of the machine that attempts the probe, or
the IP address of that machine. Note that this message is logged only if the
L) option (see Section 34.8.33, LogLevel (L)) is nonzero.
You may be dismayed to learn that the login names of ordinary users
can be used to break into a system.
It is not, for example, all that unusual for a user to select a password
that is simply a copy of his or her login name, first name, last name, or some combination of initials.
A risk of attack can arise
from outsiders guessing login names. Any that they find can be used to try to
break in, and the SMTP
vrfy gives an attacker the means to discover
vrfy command causes sendmail to
verify that it will accept an address for delivery. If a user's login name is given,
the full name and login name are printed:
vrfy george250 George Washington <[email protected]>
Here, the 250 SMTP reply code (see RFC821) means a successful verification.  If the user is unknown, however, sendmail says so:
 See the
F=qflag (Section 30.8.36, F=q) for a way and reason to change this SMTP reply code to 252.
vrfy foo550 foo... User unknown
The SMTP expn command is similar to the
except that in the case of a mailing list, it will show all the members of that list.
The SMTP expn command causes sendmail to
expand (show all the recipients) of an address.
To illustrate the risk, consider that
many sites have aliases that include all or a large segment of
users. Such aliases often have easily guessed names, such as all, everyone, or staff. A probe of all, for example,
might produce something like the following:
expn all250-George Washington <[email protected]> 250-Thomas Jefferson <[email protected]> 250-Ben Franklin <[email protected]> 250-Betsy Ross <[email protected]> 250 John Q. Public <[email protected]>
With well-designed passwords these full and login names can safely
be given to the world at large. But if one user (say
has a poorly designed password (such as jqpublic), your site's
security can easily be compromised.
Note that not all uses of
vrfy or expn
represent probes. Some MUAs,
vrfy each recipient before sending a message.
 The GNU fingerd(8) daemon also uses
vrfyto provide mailbox information.
expn commands are individually
logged in a form like one of the following:
Sep 22 11:40:43 yourhost sendmail[pid]: other.host: vrfy all Sep 22 11:40:43 yourhost sendmail[pid]: [126.96.36.199]: vrfy all Sep 22 11:40:43 yourhost sendmail[pid]: other.host: expn all Sep 22 11:40:43 yourhost sendmail[pid]: [188.8.131.52]: expn all
This shows that someone from the outside (
other.host in the
first and third examples)
attempted to probe for usernames in the mailing list named
In the second and last examples the probing hostname could not be found, so the IP
address is printed instead (in the square brackets).
Note that this form of logging is enabled only if the
L) option (see Section 34.8.33)
is greater than 5.
Pre-V8 versions of sendmail do not report SMTP
expn attempts at all. Some versions of sendmail (such as
the HP_UX version) appear
to verify but really only echo the address stated.
V8 sendmail allows vrfy and expn services to be
selectively accepted or rejected on the basis of the setting of
p) option (see Section 34.8.47, PrivacyOptions (p)).
For best security we recommend this setting: