Multiple Mail Server EXPN/VRFY Information Disclosure - Nessus

Medium Plugin ID: 10249

This page contains detailed information about the Multiple Mail Server EXPN/VRFY Information Disclosure Nessus plugin including available exploits and PoCs found on GitHub, in Metasploit or Exploit-DB for verifying of this vulnerability.

Table Of Contents

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Plugin Overview
Vulnerability Information

Synopsis
Description
Solution

Public Exploits
Risk Information
Plugin Source
How to Run
References
Version

Plugin Overview

ID: 10249
Name: Multiple Mail Server EXPN/VRFY Information Disclosure
Filename: sendmail_expn.nasl
Vulnerability Published: 1982-08-01
This Plugin Published: 1999-06-22
Last Modification Time: 2018-08-13
Plugin Version: 1.59
Plugin Type: remote
Plugin Family: SMTP problems
Dependencies: find_service1.nasl, smtpserver_detect.nasl

Vulnerability Information

Severity: Medium
Vulnerability Published: 1982-08-01
Patch Published: N/A
CVE [?]: N/A
CPE [?]: N/A

Synopsis

It is possible to enumerate the names of valid users on the remote host.

Description

The remote SMTP server answers to the EXPN and/or VRFY commands.

The EXPN command can be used to find the delivery address of mail aliases, or even the full name of the recipients, and the VRFY command may be used to check the validity of an account.

Your mailer should not allow remote users to use any of these commands, because it gives them too much information.

Solution

If you are using Sendmail, add the option :

O PrivacyOptions=goaway

in /etc/sendmail.cf.

Public Exploits

Target Network Port(s): 25
Target Asset(s): Services/smtp
Exploit Available: True (Metasploit Framework)
Exploit Ease: Exploits are available

Here's the list of publicly known exploits and PoCs for verifying the Multiple Mail Server EXPN/VRFY Information Disclosure vulnerability:

Metasploit: auxiliary/scanner/smtp/smtp_enum
[SMTP User Enumeration Utility]

Before running any exploit against any system, make sure you are authorized by the owner of the target system(s) to perform such activity. In any other case, this would be considered as an illegal activity.

Risk Information

CVSS V2 Vector [?]: AV:N/AC:L/Au:N/C:P/I:N/A:N

CVSS Base Score:	5.0 (Medium)
Impact Subscore:	2.9
Exploitability Subscore:	10.0
CVSS Temporal Score:	NA (None)
CVSS Environmental Score:	NA (None)
Modified Impact Subscore:	NA
Overall CVSS Score:	5.0 (Medium)

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Plugin Source

#
# (C) Tenable Network Security, Inc.
#

include("compat.inc");

if (description)
{
 script_id(10249);
 script_version("1.59");
 script_cvs_date("Date: 2018/08/13 14:32:38");


 script_name(english:"Multiple Mail Server EXPN/VRFY Information Disclosure");
 script_summary(english:"EXPN and VRFY checks");

 script_set_attribute(attribute:"synopsis", value:
"It is possible to enumerate the names of valid users on the remote
host.");
 script_set_attribute(attribute:"description", value:
"The remote SMTP server answers to the EXPN and/or VRFY commands.

The EXPN command can be used to find the delivery address of mail
aliases, or even the full name of the recipients, and the VRFY command
may be used to check the validity of an account.

Your mailer should not allow remote users to use any of these
commands, because it gives them too much information.");
 script_set_attribute(attribute:"solution", value:
"If you are using Sendmail, add the option :

 O PrivacyOptions=goaway

in /etc/sendmail.cf.");
 script_set_cvss_base_vector("CVSS2#AV:N/AC:L/Au:N/C:P/I:N/A:N");
 script_set_attribute(attribute:"exploitability_ease", value:"Exploits are available");
 script_set_attribute(attribute:"exploit_available", value:"true");
 script_set_attribute(attribute:"metasploit_name", value:'SMTP User Enumeration Utility');
 script_set_attribute(attribute:"exploit_framework_metasploit", value:"true");

 script_set_attribute(attribute:"vuln_publication_date", value:"1982/08/01");
 script_set_attribute(attribute:"plugin_publication_date", value:"1999/06/22");

 script_set_attribute(attribute:"plugin_type", value:"remote");
 script_end_attributes();

 script_category(ACT_GATHER_INFO);
 script_copyright(english:"This script is Copyright (C) 1999-2018 Tenable Network Security, Inc.");
 script_family(english:"SMTP problems");

 script_dependencies("find_service1.nasl", "smtpserver_detect.nasl");
 script_require_ports("Services/smtp", 25);
 exit(0);
}

include("audit.inc");
include("global_settings.inc");
include("misc_func.inc");
include("smtp_func.inc");

port = get_service(svc:"smtp", default:25, exit_on_fail:1);
if (get_kb_item('SMTP/'+port+'/broken')) exit(0, "The SMTP server listening on port "+port+" is broken.");

soc = open_sock_tcp(port);
if (!soc) audit(AUDIT_SOCK_FAIL, port);

b = smtp_recv_banner(socket:soc);
if (!b) audit(AUDIT_NO_BANNER, port);
trace = '\n  ' + chomp(b);

s = 'HELO example.com';
send(socket:soc, data:s+'\r\n');
r = smtp_recv_line(socket:soc);
trace += '\n  ' + s +
         '\n  ' + chomp(r);

vuln = 0;

s = 'EXPN root';
send(socket:soc, data:s+'\r\n');
expn_root = r = smtp_recv_line(socket:soc);
trace_expn = '\n  ' + s +
             '\n  ' + chomp(r);

# Some Postfix reply as follow:
#
# 220-localhost.localhost ESMTP Postfix
# HELO there
# 220 localhost.localhost ESMTP Postfix
# 250 localhost.localhost
# EXPN root
# 502 5.5.2 Error: command not recognized
#
if ( r =~ "^250 " )
{
  r2 = smtp_recv_line(socket:soc);
  if (r2)
  {
    expn_root = r = r2;
    trace_expn += '\n  ' + chomp(r2);
  }
}

if (
  ereg(string:r, pattern:"^(250|550)(-| ).*$") &&
  # exim hack
  !ereg(string:r, pattern:"^550 EXPN not available.*$") &&
  !ereg(string:r, pattern:"^550.*Administrative prohibition.*$") &&
  !ereg(string:r, pattern:"^550.*Access denied.*$") &&
  # QHMail 4.6.3.1
  !ereg(string:r, pattern:"^550 lists are confidential.*$") &&
  # PMDF MTA
  !ereg(string:r, pattern:"^550.* EXPN command has been disabled\.$") &&
  !ereg(string:r, pattern:"^550.*EXPN")
)
{
  # nb: check for a bogus account to avoid false positives; eg,
  #     Postfix with EXPN in 'smtpd_noop_commands'.
  if (report_paranoia < 2)
  {
    s2 = 'EXPN random' + rand();
    send(socket:soc, data:s2+'\r\n');
    r2 = smtp_recv_line(socket:soc);

    trace_expn += '\n  ' + s2 +
                  '\n  ' + chomp(r2);
  }
  else r2 = s2 = NULL;

  if (
    isnull(s2) ||
    (
      ereg(string:r2, pattern:"^(250|550)(-| ).*$") &&
      substr(expn_root, 0, 2) != substr(r2, 0, 2)
    )
  )
  {
    trace += trace_expn;

    set_kb_item(name:"SMTP/expn", value:TRUE);
    vuln++;
  }
}

s = 'VRFY root';
send(socket:soc, data:s+'\r\n');
vrfy_root = r = smtp_recv_line(socket:soc);
trace_vrfy = '\n  ' + s +
             '\n  ' + chomp(r);

if (ereg(string:r, pattern:"^(250|550)(-| ).*$"))
{
  if (report_paranoia < 2)
  {
    s2 = 'VRFY random' + rand();
    send(socket:soc, data:s2+'\r\n');
    r2 = smtp_recv_line(socket:soc);

    trace_vrfy += '\n  ' + s2 +
                  '\n  ' + chomp(r2);
  }
  else r2 = s2 = NULL;

  if (
    isnull(s2) ||
    (
      ereg(string:r2, pattern:"^(250|550)(-| ).*$") &&
      substr(vrfy_root, 0, 2) != substr(r2, 0, 2)
    )
  )
  {
    trace += trace_vrfy;

    set_kb_item(name:"SMTP/vrfy", value:TRUE);
    vuln++;
  }
}
smtp_close(socket:soc);


if (vuln > 0)
{
  if (report_verbosity > 0)
  {
    report = '\n' + 'Here is a trace of the SMTP traffic that demonstrates the issue';
    if (vuln > 1) report += 's';
    report += ' : ' +
              '\n' + trace +
              '\n';
    security_warning(port:port, extra:report);
  }
  else security_warning(port);
}
else exit(0, "The SMTP server listening on port "+port+" is not affected.");

The latest version of this script can be found in these locations depending on your platform:

Linux / Unix:
/opt/nessus/lib/nessus/plugins/sendmail_expn.nasl
Windows:
C:\ProgramData\Tenable\Nessus\nessus\plugins\sendmail_expn.nasl
Mac OS X:
/Library/Nessus/run/lib/nessus/plugins/sendmail_expn.nasl

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How to Run

Here is how to run the Multiple Mail Server EXPN/VRFY Information Disclosure as a standalone plugin via the Nessus web user interface (https://localhost:8834/):

Click to start a New Scan.
Select Advanced Scan.
Navigate to the Plugins tab.
On the top right corner click to Disable All plugins.
On the left side table select SMTP problems plugin family.
On the right side table select Multiple Mail Server EXPN/VRFY Information Disclosure plugin ID 10249.
Specify the target on the Settings tab and click to Save the scan.
Run the scan.

Here are a few examples of how to run the plugin in the command line. Note that the examples below demonstrate the usage on the Linux / Unix platform.

Basic usage:

/opt/nessus/bin/nasl sendmail_expn.nasl -t <IP/HOST>

Run the plugin with audit trail message on the console:

/opt/nessus/bin/nasl -a sendmail_expn.nasl -t <IP/HOST>

Run the plugin with trace script execution written to the console (useful for debugging):

/opt/nessus/bin/nasl -T - sendmail_expn.nasl -t <IP/HOST>

Run the plugin with using a state file for the target and updating it (useful for running multiple plugins on the target):

/opt/nessus/bin/nasl -K /tmp/state sendmail_expn.nasl -t <IP/HOST>

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References

See also:

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Version

This page has been produced using Nessus Professional 10.1.2 (#68) LINUX, Plugin set 202205072148.
Plugin file sendmail_expn.nasl version 1.59. For more plugins, visit the Nessus Plugin Library.

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Multiple Mail Server EXPN/VRFY Information Disclosure - Nessus

Plugin Overview

Vulnerability Information

Synopsis

Description

Solution

Public Exploits

Risk Information

Plugin Source

How to Run

References

Version

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