Rapid7 Vulnerability & Exploit Database

SMTP server EXPN Vulnerability

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SMTP server EXPN Vulnerability

Severity
5
CVSS
(AV:N/AC:L/Au:N/C:P/I:N/A:N)
Published
11/01/2004
Created
07/25/2018
Added
11/01/2004
Modified
12/04/2013

Description

The SMTP "EXPN" command allows you to expand a mailing list or alias, to see where mail addressed to the alias actually goes. For example, many organizations alias postmaster to root, so that mail addressed to postmaster will get delivered to the system administrator. Issuing "EXPN postmaster" via SMTP would reveal that postmaster is aliased to root.

The "EXPN" command can be used by attackers to learn about valid usernames on the target system. On some SMTP servers, EXPN can be used to show the subscribers of a mailing list -- subscription lists are generally considered to be sensitive information.

The "EXPN" command can be useful when debugging mailing lists setup, but it should be disabled on production servers.

Solution(s)

  • smtp-disable-expn-vrfy

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