SMTP User Enumeration Utility - Metasploit


This page contains detailed information about how to use the auxiliary/scanner/smtp/smtp_enum metasploit module. For list of all metasploit modules, visit the Metasploit Module Library.

Table Of Contents
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Module Overview

Name: SMTP User Enumeration Utility
Module: auxiliary/scanner/smtp/smtp_enum
Source code: modules/auxiliary/scanner/smtp/smtp_enum.rb
Disclosure date: -
Last modification time: 2019-06-03 13:42:55 +0000
Supported architecture(s): -
Supported platform(s): -
Target service / protocol: smtp, smtps
Target network port(s): 25, 465, 587, 2525, 25000, 25025
List of CVEs: CVE-1999-0531

The SMTP service has two internal commands that allow the enumeration of users: VRFY (confirming the names of valid users) and EXPN (which reveals the actual address of users aliases and lists of e-mail (mailing lists)). Through the implementation of these SMTP commands can reveal a list of valid users.

Module Ranking and Traits

Module Ranking:

  • normal: The exploit is otherwise reliable, but depends on a specific version and can't (or doesn't) reliably autodetect. More information about ranking can be found here.

Basic Usage

This module is a scanner module, and is capable of testing against multiple hosts.

msf > use auxiliary/scanner/smtp/smtp_enum
msf auxiliary(smtp_enum) > show options
    ... show and set options ...
msf auxiliary(smtp_enum) > set RHOSTS ip-range
msf auxiliary(smtp_enum) > exploit

Other examples of setting the RHOSTS option:

Example 1:

msf auxiliary(smtp_enum) > set RHOSTS 192.168.1.3-192.168.1.200

Example 2:

msf auxiliary(smtp_enum) > set RHOSTS 192.168.1.1/24

Example 3:

msf auxiliary(smtp_enum) > set RHOSTS file:/tmp/ip_list.txt

Required Options

  • RHOSTS: The target host(s), range CIDR identifier, or hosts file with syntax 'file:<path>'

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Msfconsole Usage

Here is how the scanner/smtp/smtp_enum auxiliary module looks in the msfconsole:

msf6 > use auxiliary/scanner/smtp/smtp_enum

msf6 auxiliary(scanner/smtp/smtp_enum) > show info

       Name: SMTP User Enumeration Utility
     Module: auxiliary/scanner/smtp/smtp_enum
    License: Metasploit Framework License (BSD)
       Rank: Normal

Provided by:
  Heyder Andrade <[email protected]>
  nebulus

Check supported:
  No

Basic options:
  Name       Current Setting                                                             Required  Description
  ----       ---------------                                                             --------  -----------
  RHOSTS                                                                                 yes       The target host(s), range CIDR identifier, or hosts file with syntax 'file:<path>'
  RPORT      25                                                                          yes       The target port (TCP)
  THREADS    1                                                                           yes       The number of concurrent threads (max one per host)
  UNIXONLY   true                                                                        yes       Skip Microsoft bannered servers when testing unix users
  USER_FILE  /opt/metasploit-framework/embedded/framework/data/wordlists/unix_users.txt  yes       The file that contains a list of probable users accounts.

Description:
  The SMTP service has two internal commands that allow the 
  enumeration of users: VRFY (confirming the names of valid users) and 
  EXPN (which reveals the actual address of users aliases and lists of 
  e-mail (mailing lists)). Through the implementation of these SMTP 
  commands can reveal a list of valid users.

References:
  http://www.ietf.org/rfc/rfc2821.txt
  OSVDB (12551)
  https://nvd.nist.gov/vuln/detail/CVE-1999-0531

Module Options

This is a complete list of options available in the scanner/smtp/smtp_enum auxiliary module:

msf6 auxiliary(scanner/smtp/smtp_enum) > show options

Module options (auxiliary/scanner/smtp/smtp_enum):

   Name       Current Setting                                                             Required  Description
   ----       ---------------                                                             --------  -----------
   RHOSTS                                                                                 yes       The target host(s), range CIDR identifier, or hosts file with syntax 'file:<path>'
   RPORT      25                                                                          yes       The target port (TCP)
   THREADS    1                                                                           yes       The number of concurrent threads (max one per host)
   UNIXONLY   true                                                                        yes       Skip Microsoft bannered servers when testing unix users
   USER_FILE  /opt/metasploit-framework/embedded/framework/data/wordlists/unix_users.txt  yes       The file that contains a list of probable users accounts.

Advanced Options

Here is a complete list of advanced options supported by the scanner/smtp/smtp_enum auxiliary module:

msf6 auxiliary(scanner/smtp/smtp_enum) > show advanced

Module advanced options (auxiliary/scanner/smtp/smtp_enum):

   Name                 Current Setting  Required  Description
   ----                 ---------------  --------  -----------
   CHOST                                 no        The local client address
   CPORT                                 no        The local client port
   ConnectTimeout       10               yes       Maximum number of seconds to establish a TCP connection
   Proxies                               no        A proxy chain of format type:host:port[,type:host:port][...]
   SSL                  false            no        Negotiate SSL/TLS for outgoing connections
   SSLCipher                             no        String for SSL cipher - "DHE-RSA-AES256-SHA" or "ADH"
   SSLVerifyMode        PEER             no        SSL verification method (Accepted: CLIENT_ONCE, FAIL_IF_NO_PEER_CERT, NONE, PEER)
   SSLVersion           Auto             yes       Specify the version of SSL/TLS to be used (Auto, TLS and SSL23 are auto-negotiate) (Accepted: Auto, TLS, SSL23, SSL3, TLS1, TLS1.1, TLS1.2)
   ShowProgress         true             yes       Display progress messages during a scan
   ShowProgressPercent  10               yes       The interval in percent that progress should be shown
   VERBOSE              false            no        Enable detailed status messages
   WORKSPACE                             no        Specify the workspace for this module

Auxiliary Actions

This is a list of all auxiliary actions that the scanner/smtp/smtp_enum module can do:

msf6 auxiliary(scanner/smtp/smtp_enum) > show actions

Auxiliary actions:

   Name  Description
   ----  -----------

Evasion Options

Here is the full list of possible evasion options supported by the scanner/smtp/smtp_enum auxiliary module in order to evade defenses (e.g. Antivirus, EDR, Firewall, NIDS etc.):

msf6 auxiliary(scanner/smtp/smtp_enum) > show evasion

Module evasion options:

   Name                Current Setting  Required  Description
   ----                ---------------  --------  -----------
   TCP::max_send_size  0                no        Maxiumum tcp segment size.  (0 = disable)
   TCP::send_delay     0                no        Delays inserted before every send.  (0 = disable)

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Error Messages

This module may fail with the following error messages:

Error Messages

Check for the possible causes from the code snippets below found in the module source code. This can often times help in identifying the root cause of the problem.

<RHOST>:<RPORT> Error smtp_send: '<E.CLASS>' '<E>'

Here is a relevant code snippet related to the "<RHOST>:<RPORT> Error smtp_send: '<E.CLASS>' '<E>'" error message:

60:	      code = result[0..2].to_i if result
61:	      return result, code
62:	    rescue Rex::ConnectionError, Errno::ECONNRESET, ::EOFError
63:	      return result, code
64:	    rescue ::Exception => e
65:	      print_error("#{rhost}:#{rport} Error smtp_send: '#{e.class}' '#{e}'")
66:	      return nil, 0
67:	    end
68:	  end
69:	
70:	  def run_host(ip)

<RHOST>:<RPORT> Connection but no data...skipping

Here is a relevant code snippet related to the "<RHOST>:<RPORT> Connection but no data...skipping" error message:

79:	    cmd = 'HELO' + " " + "localhost" + "\r\n"
80:	    connect
81:	    result, code = smtp_send(cmd)
82:	
83:	    if(not result)
84:	      print_error("#{rhost}:#{rport} Connection but no data...skipping")
85:	      return
86:	    end
87:	    banner.chomp! if (banner)
88:	    if(banner =~ /microsoft/i and datastore['UNIXONLY'])
89:	      print_status("#{rhost}:#{rport} Skipping microsoft (#{banner})")

<RHOST>:<RPORT> RCPT TO: Allowed for random user (<USER>)...not reliable? <CODE> '<RESULT>'

Here is a relevant code snippet related to the "<RHOST>:<RPORT> RCPT TO: Allowed for random user (<USER>)...not reliable? <CODE> '<RESULT>'" error message:

114:	      result, code = smtp_send("MAIL FROM: root\@#{domain}\r\n")
115:	      if(code == 250)
116:	        user = Rex::Text.rand_text_alpha(8)
117:	        result, code = smtp_send("RCPT TO: #{user}\@#{domain}\r\n")
118:	        if(code >= 250 and code <= 259)
119:	          vprint_status("#{rhost}:#{rport} RCPT TO: Allowed for random user (#{user})...not reliable? #{code} '#{result}'")
120:	          rcpt = false
121:	        else
122:	          smtp_send("RSET\r\n")
123:	          users_found = do_rcpt_enum(domain, usernames)
124:	        end

<RHOST>:<RPORT> could not be enumerated (no EXPN, no VRFY, invalid RCPT)

Here is a relevant code snippet related to the "<RHOST>:<RPORT> could not be enumerated (no EXPN, no VRFY, invalid RCPT)" error message:

126:	        rcpt = false
127:	      end
128:	    end
129:	
130:	    if(not vrfy and not expn and not rcpt)
131:	      print_status("#{rhost}:#{rport} could not be enumerated (no EXPN, no VRFY, invalid RCPT)")
132:	      return
133:	    end
134:	    finish_host(users_found)
135:	    disconnect
136:

Error: <RHOST>:<RPORT> '<E.CLASS>' '<E>'

Here is a relevant code snippet related to the "Error: <RHOST>:<RPORT> '<E.CLASS>' '<E>'" error message:

134:	    finish_host(users_found)
135:	    disconnect
136:	
137:	    rescue Rex::ConnectionError, Errno::ECONNRESET, Rex::ConnectionTimeout, EOFError, Errno::ENOPROTOOPT
138:	    rescue ::Exception => e
139:	      print_error("Error: #{rhost}:#{rport} '#{e.class}' '#{e}'")
140:	  end
141:	
142:	  def finish_host(users_found)
143:	    if users_found and not users_found.empty?
144:	      print_good("#{rhost}:#{rport} Users found: #{users_found.sort.join(", ")}")

<RHOST>:<RPORT> MAIL FROM: <USER> NOT allowed during brute...aborting ( '<CODE>' '<RESULT>')

Here is a relevant code snippet related to the "<RHOST>:<RPORT> MAIL FROM: <USER> NOT allowed during brute...aborting ( '<CODE>' '<RESULT>')" error message:

197:	        if(code == 250)
198:	          vprint_status("#{rhost}:#{rport} - Found user: #{user}")
199:	          users.push(user)
200:	        end
201:	      else
202:	        vprint_status("#{rhost}:#{rport} MAIL FROM: #{user} NOT allowed during brute...aborting ( '#{code}' '#{result}')")
203:	        break
204:	      end
205:	      smtp_send("RSET\r\n")
206:	    }
207:	    return users

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References

See Also

Check also the following modules related to this module:

Related Nessus plugins:

Authors

  • Heyder Andrade <heyder[at]alligatorteam.org>
  • nebulus

Version

This page has been produced using Metasploit Framework version 6.1.24-dev. For more modules, visit the Metasploit Module Library.

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