SMTP EXPN command
| smtp-expn (128) |  Medium Risk |
Description:
Simple Mail Transfer Protocol (SMTP)-compliant applications, such as the Sendmail program EXPN, could allow an attacker to determine if an account exists on a system. Such information could provide an attacker significant assistance in executing a brute force attack on user accounts. EXPN provides additional information concerning users on the system, such as if particular users exist and users' full names. This information could also assist an attacker in further attacks.
Platforms Affected:
- Compaq, Tru64
- Data General, DG/UX
- HP, HP-UX 10.20
- HP, HP-UX 11
- HP, HP-UX
- IBM, AIX 4
- IBM, AIX
- Linux, Kernel
- RedHat, Linux 6.0
- RedHat, Linux 7
- RedHat, Linux 7.1
- RedHat, Linux 7.2
- RedHat, Linux 7.3
- SCO, SCO Unix
- SGI, IRIX
- Sun, Solaris 2.5.1
- Sun, Solaris 2.6
- Sun, Solaris 7.0
- Sun, Solaris 8
- Sun, Solaris
- Various vendors, SMTP servers
- WindRiver, BSDOS
Remedy:
If you are running Sendmail, add the line 'Opnoexpn' to your Sendmail configuration file, usually located in /etc/sendmail.cf. For other mail servers, contact your vendor for information on how to disable the expand command.
—AND—Upgrade to the latest version of Sendmail (8.11.4 or later), available from the Sendmail Consortium Web site. See References.
—OR—Apply the appropriate patch for your system, available from the Sendmail Consortium FTP site. See References.
Consequences:
Obtain Information
References:
- Sendmail Consortium FTP site, FTP site at ftp://ftp.cs.berkeley.edu/ucb/sendmail/.
- Sendmail Consortium Web site, Welcome to sendmail.org at http://www.sendmail.org/.
- CVE-1999-0531: An SMTP service supports EXPN, VRFY, HELP, ESMTP, and/or EHLO.
Reported:
Not available
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