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TUCoPS :: HP/UX :: man9.htm

HP-UX 10.20, 11.xx 'man' symlink bugs



Vulnerability

    man

Affected

    HPUX 10.20 and 11.00 and probably other revs

Description

    Jason  Axley  found  following.   The  'man'  command  potentially
    allows attackers  to overwrite  any arbitrary  file on  the system
    via symlink bugs.

    The programmers of the 'man' command on various HPUX releases have
    made several fatal  mistakes that allow  an attacker to  trivially
    set  a  trap  that  could  result  in  any  arbitrary  file  being
    overwritten on the system when root runs the 'man' command.

    Mistakes:
    0) HP  *still*  insists  on   NOT  setting  the  sticky  bit    on
       world-writeable temporary  directories (/tmp  and /var/tmp)  on
       default installs  of HPUX.   This can  be exploited  to  delete
       existing  catXXXX  and  manXXXX  files  and  replace  them with
       symlinks since  anyone is  free to  delete any  file from these
       directories, regardless of  file ownership.   This can also  be
       used to play fun race conditions in other exploits where a race
       exists between stat()ing or creation of a file and the  opening
       of that  file.   This could  potentially be  used here to watch
       /tmp for catXXXX and manXXXX file creation, delete one of those
       files, and symlink it to  the file you want overwritten  before
       the file is  opened for writing  and truncation.   Fortunately,
       the man  command is  not setuid  on HPUX  or else  normal users
       would be able to  get root without having  to wait for root  to
       run man.

    1) man  creates  temporary  files  with  predictable filenames  in
       world-writeable directories.  The  two files are named  catXXXX
       and manXXXX where  XXXX is the  PID of the  man process (highly
       predictable).

    2) man blindly follows symlinks.

    3) man explicitly opens the  temp files with mode 666  and ignores
       the existing umask.   Jason verified that  this doesn't  change
       the mode of existing files to 666, but it allows for  attackers
       to  edit  the  tempfiles  and  potentially  insert  harmful man
       commands that will get  (like recent Bugtraq discussions  about
       malicious manpages).

    4) man opens the tempfiles with  O_TRUNC.  This means that when  a
       file is  symlinked to,  that file  is blindly  truncated.  This
       could lead to easy denial-of-service  if you want to trash  the
       password file or  a hard disk  device file.    This could  also
       have bad effects on  sane man program operation,  regardless of
       security, if a user runs  man and leaves it running,  then PIDs
       are wrapped around and someone of higher privilege runs man and
       overwrites your tempfiles!

    To exploit,  create ~65535  catXXXX or  manXXXX symlinks  in /tmp,
    pointing to  the file  you want  to overwrite  (e.g. /etc/passwd).
    Then  wait.   When  root  runs  man,  the  file  will  be  blindly
    overwritten  with  the  formatted  manpage  contents  (cat????) or
    unformatted (man????) are written to the symlinked file.

Solution

    Patch is forthcoming soon.

    For HP Admins:   You could create  root-owned catXXXX and  manXXXX
    files in /tmp AFTER chmod'ing /tmp to 1777 to keep attackers  from
    making the symlinks.

    For  HP  and  other  programmers  who  have  or  will make similar
    mistakes:
    0) Ensure  that  people  verify  that  /tmp and /var/tmp have  the
       sticky bit  set.   Also, ensure  the sticky  bit is  set on  by
       default in future releases of HPUX!

    1) Do not  create tempfiles in  world-writeable directories.   Or,
       use  mkstemp()  or  a  similar  function  to  generate  unique,
       difficult  to  guess  tempfilenames.   HPUX  does  not  have an
       entropy source to draw from  so this may not be  a bullet-proof
       solution.

    2) Rewrite man to  not follow symlinks by  doing a secure stat  of
       the file to  check if it  is a symlink  and error out  if it is
       (security message to  syslog too would  be nice).   Use lstat()
       (NOT stat()) to stat the file  and be sure it isn't a  symlink.
       Follow the example in the URL below to verify the file and open
       it securely.

    3) Rewrite  man to  honor the  existing umask!   Don't  explicitly
       create world-writeable files.

    4) Do  not blindly  use O_TRUNC  when opening  the tempfile.   The
       program should error or try a different filename if the  target
       exists.  Again, be aware  of race conditions when checking  for
       file existence and then the subsequent open of that file.   You
       should use O_CREAT  and O_EXCL together  to cause open  to fail
       if  the  target  file  already  exists.   You  should  be using
       ftruncate() (available  on HPUX)  to truncate  the opened  file
       (that you've securely  opened, based on  the guidelines at  the
       URL below)  if you  are sure  that you  aren't going  to hose a
       file in use  or a system  file (e.g.   fix the symlink  problem
       first!).


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