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TUCoPS :: Unix :: General :: unix5724.htm

Sendmail smrsh bypass vulnerabilities



2nd Oct 2002 [SBWID-5724]
COMMAND

	Sendmail smrsh bypass vulnerabilities

SYSTEMS AFFECTED

	The latest versions of  SMRSH  are  vulnerable.  Including  the  version
	packaged with Sendmail 8.12.6 and Sendmail  8.11.6-15  (default  install
	of Redhat 7.3). Older versions of SMRSH do not appear to  be  vulnerable
	(8.11 5/19/1998).

PROBLEM

	In David Endler of iDEFENSE [http://www.idefense.com/] advisory,  thanks
	to the exclusives from zen-parse [zen-parse@gmx.net]  and  Pedram  Amini
	[pamini@idefense.com] :
	

	It is possible for an attacker to bypass  the  restrictions  imposed  by
	The Sendmail Consortium’s Restricted Shell (SMRSH) and execute a  binary
	of his choosing by inserting  a  special  character  sequence  into  his
	.forward file. SMRSH is an application intended as a replacement for  sh
	for use in Sendmail. There are two attack  methods  both  of  which  are
	detailed below.
	

	 METHOD ONE

	

	This method takes advantage of the application's implementation  of  the
	'||' command. The process is best explained with an example:
	

	$ echo "echo unauthorized execute" > /tmp/unauth

	$ smrsh -c ". || . /tmp/unauth || ." 

	  /bin/sh: /etc/smrsh/.: is a directory

	  unauthorized execute

	

	/tmp/unauth is executed despite the fact that it is not located  in  the
	SMRSH restricted directory /etc/smrsh. This happens because SMRSH  first
	checks for '.', which exists, and does no further  verification  on  the
	files listed after '||'. The same attack would look like  the  following
	in the attackers .forward file:
	

	"| . \|| . /tmp/unauth \|| ."

	

	

	 METHOD TWO

	

	This method takes advantage of the following routine from smrsh.c:
	

	    /* search backwards for last / (allow for 0200 bit) */

	    while (cmd > q)

	    {

	            if ((*--cmd & 0177) == '/')

	            {

	                    cmd++;

	                    break;

	            }

	    }

	

	It is possible to feed SMRSH a command  line  that  will  be  internally
	converted to a space thereby  bypassing  all  filters,  yet  will  still
	execute. Examples of these include:
	

	    smrsh -c "/ command"

	    smrsh -c "../ command"

	    smrsh -c "./ command"

	    smrsh -c "././ command"

	

	The listed routine will convert any of the above examples  to  a  space.
	However, when the following execle() call is reached:
	

	(void) execle("/bin/sh", "/bin/sh", "-c", newcmdbuf, NULL, newenv);

	

	SMRSH will execute:
	

	/bin/sh -c  command

	

	Notice that despite the double space ‘command’ will still  execute.  The
	.forward variation of this attack works the same way.
	

	The Common Vulnerabilities and  Exposures  project  (cve.mitre.org)  has
	assigned the name CAN-2002-1165 to this issue.
	

	

	 ANALYSIS

	

	The following are required  conditions  for  successful  and  meaningful
	exploitation of this vulnerability:
	

	 - - -	The target system must be utilizing SMRSH.

	 - - -	The attacker must have a valid local account on the system.

	 - - -	In method one the attacker must be able to create files.

	

	While this exploit obviously removes the restrictions imposed  by  SMRSH
	it also allows users to execute programs on systems  that  they  do  not
	have shell access to. Utilizing either of the  above-described  methods,
	an attacker who can modify his own .forward file can  execute  arbitrary
	commands on the target system with the privileges of  his  own  account.
	Systems that forbid shell access generally do not have  tightened  local
	security, the ability to execute arbitrary commands  through  the  SMRSH
	vulnerablity opens the  target  system  to  local  privilege  escalation
	attacks that otherwise would not be possible.

SOLUTION

	Sendmail.org  has  provided  a  patch  addressing  the   above-described
	issues. The patch is available for download at :
	

	http://www.sendmail.org/patches/smrsh-20020924.patch

	


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