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 debug and showq commands. The SMTP debug command allows the local sendmail to be placed into debugging mode (as with the -d command-line switch, Section 16.1) from any other machine anywhere on the network. The SMTP showq command allows outsiders to view the contents of the mail queue.
If SMTPDEBUG (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 25 Trying 220.127.116.11 ... Connected to localhost. Escape character is '^]'. 220 localhost sendmail 8.12 ready at Fri, 13 Dec 2002 06:36:12 -0800 debug 500 Command unrecognized quit 221 localhost.us.edu closing connection Connection closed by foreign host. %
When connected, enter the command debug. If you get the answer 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 debug or showq commands, sendmail will syslog(3) a message such as the following:
Jul 22 07:09:00 here.domain sendmail: "debug" command from there.domain (18.104.22.168)
This message shows the name of the machine that attempts the probe, or there.domain, and the IP address of that machine. Note that this message is logged only if the LogLevel option (LogLevel) is nonzero.
You might 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 login names.
Login names are also a way to gather addresses for spam email messages. The SMTP VRFY command, too, can be used to collect names for that illicit use.
The SMTP 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 george 250 George Washington <firstname.lastname@example.org>
Here, the 250 SMTP reply code (see RFC821) means a successful verification. If the user is unknown, however, sendmail says so:
 See the F=q flag (F=q) for a way and reason to change this SMTP reply code to 252.
vrfy foo 550 5.7.1 foo... User unknown
The SMTP EXPN command is similar to the VRFY command, except that in the case of a mailing list, an aliases, or a ~/.forward file entry, it will show all the members. 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 such as the following:
expn all 250-George Washington <email@example.com> 250-Thomas Jefferson <firstname.lastname@example.org> 250-Ben Franklin <email@example.com> 250-Betsy Ross <firstname.lastname@example.org> 250 John Q. Public <email@example.com>
With well-designed passwords these full and login names can safely be given to the world at large. But if one user (say, jqp) 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, for example, routinely VRFY each recipient before sending a message.
 The fingerd(8) daemon can also reveal login IDs.
 The GNU fingerd(8) daemon also uses VRFY to provide mailbox information.
SMTP VRFY and EXPN commands are individually logged in a form such as one of the following:
Sep 22 11:40:43 yourhost sendmail[pid]: other.host: vrfy all Sep 22 11:40:43 yourhost sendmail[pid]: [22.214.171.124]: vrfy all Sep 22 11:40:43 yourhost sendmail[pid]: other.host: expn all Sep 22 11:40:43 yourhost sendmail[pid]: [126.96.36.199]: 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 all. 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 LogLevel option (LogLevel) is greater than 5.
Pre-V8 versions of sendmail do not report SMTP VRFY or 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 accepted or rejected on the basis of the setting of the PrivacyOptions option (PrivacyOptions). For improved security, we recommend this setting for the PrivacyOptions option:
V8.10 and above sendmail allow VRFY and EXPN services to be selectively accepted or rejected on the basis of rules in the check_vrfy (check_vrfy and check_expn) and check_expn (check_vrfy and check_expn) rule sets. If, for example, you wish to allow VRFY from internal hosts, but wish to deny it for all outside hosts, you can do so by omitting a definition of the PrivacyOptions option as explained earlier, and by designing appropriate rules for the check_vrfy rule set.