network installation


    Slackware 3.6 Network Installations


    Following is based on ISS  Security Advisory.  Slackware Linux  is
    one of the major distributions  of the Linux operating system  and
    supporting  utilities.   CD-ROM  distributions  are available from
    Walnut Creek, InfoMagic, LinuxMall,  and other suppliers.   It can
    also be  downloaded from  a number  of archive  and mirror  sites.
    Some of  these sites  offer NFS  access to  Slackware (and  other)
    distributions for  direct installation  from the  network.  During
    routine installation of Slackware Linux, there may be a period  of
    time  during  which  the  system  being installed is vulnerable to
    remote root login  via telnet or  other services.   This period of
    time depends  on human  interaction and  may vary  from minutes to
    hours or more.   This vulnerability may  be subject to  high-speed
    automated  attacks  against   which  individual  interaction   and
    correction can not compete.  The vulnerability exists if Slackware
    is installed with the "net.i"  boot image or if a  network enabled
    kernel is installed during the initial installation.  The CD  boot
    image, when booting directly from  CD-ROM, is not subject to  this

    During a routine  Slackware installation, Slackware  completes the
    installation without  prompting to  set the  root password.   Upon
    completion of the installation, the system is rebooted.  After the
    initial rebooting,  the system  boots with  a NULL  root password.
    When  Slackware  is  initially  installed  and  rebooted, inetd is
    active by  default and  both telnetd  and rlogind  are enabled  in
    inetd.conf.   The  /etc/securetty  file  on  the default Slackware
    install has  remote root  login enabled  by including  entries for
    ttyp0 through ttyp3.  The combination of these three factors means
    that,   immediately   after   rebooting   following   a  Slackware
    installation, if the system  was installed with a  network enabled
    kernel such as "net.i", the  system may be accessed via  telnet or
    other  services  from  anywhere  on  a  reachable  network as root
    without  a  password.   Because  of  the  order and speed at which
    different  components  are  started,  inetd  is active and able to
    start  a  telnet  session  well  in  advance  of any console login
    prompt.   Securing the  installation from  this highly  vulnerable
    state  requires  operator  action  to  change  the  root password,
    disable the  services, or  disable remote  root access.   The most
    likely  initial  action  is  to  establish  a root password.  This
    procedure  may  take  from  less  than  a minute to several hours,
    depending on the experience  of the operator and  other activities
    taking place at the time of the installation.  Leaving the  system
    unattended at the  wrong time could  be potentially hazardous  and
    extend the period of vulnerability.  The initial lilo prompt prior
    to boot has a two minute  timeout and may give a false  sense that
    it will not boot up automatically on its own.  Leaving the  system
    unattended  at  an  apparently  safe   point  can  result  in   it
    autobooting unattended into a  highly vulnerable state.   There is
    no  warning  about  this  vulnerable   time  or  the  urgency   in
    establishing a root  password.  Even  though this simple  security
    precaution  would  be  obvious  to experienced administrators, the
    urgency  may  not  be  obvious  to  inexperienced  or  first  time

    A test of this vulnerability used ping from another system on  the
    same  network  to  monitor  a  system  during  this reboot period.
    Immediately upon  receiving a  ping response,  telnet was  used to
    manually connect to the system and log in to the root account from
    the  remote  system.   After  logging  in  as root via telnet, the
    target system was checked and was, only then, just getting  around
    to loading  gpm and  was nowhere  near presenting  a console login
    prompt.  Several more seconds passed before it became possible  to
    log in on the console.  If  a full install of all of the  packages
    is performed, a common action  by new or inexperienced users,  the
    login  and  shell  services  are  also  installed and enabled from
    inetd.   An attacker  can break  in using  rsh or  rlogin as root,
    even faster than with  telnet.  If a  system can be identified  as
    it is being  installed, it is  possible to use  automatic tools to
    monitor the  address and  attack it  as soon  as it's available in
    its vulnerable condition.  Connections to rlogin and telnet  could
    transfer intrusion binaries and exploits to the system within  the
    time the installer takes to login as root and begin to change  the
    password.  More  elaborate attacks are  possible since it  is only
    necessary to  log in  prior to  the root  password being  changed.
    This vulnerability is particularly dangerous in the case of remote
    installation via NFS from a central site, repository, or archive.

    In  the  case  of  network  installations, it may possible through
    tracing,  sniffing,  scanning,  traffic  analysis,  or  NFS server
    accesses  to  identify  a  system  being  installed from a network
    Slackware  Distribution.   This  discovery  makes  it  possible to
    prepare tools  to attack  the identified  system as  soon as it is
    rebooted  into  its  vulnerable   state.   This  procedure   could
    potentially be automated.   Once a system  has been identified  by
    an attacker as  a Slackware installation,  it becomes possible  to
    quickly re-attack the system  if the system gets  reinstalled with
    the same package.  The re-attack can be executed and completed  in
    less  time  than  required  for  the  system  to reboot to a login
    prompt following re-installation of the OS.


    This problem  has been  addressed in  post-3.6 packages  available
    from the  Slackware FTP  site.   Do not  perform any network based
    installations of Slackware Linux.   If Slackware is the  preferred
    installation distribution for Linux,  it should be installed  from
    local  media   only.    Installation  should   take  place   while
    disconnected from a  live network.   Archive sites should  disable
    NFS  export  of  Slackware  installation directories until updates
    become available.   Users installing  over NFS  from such  archive
    sites  are  particularly  vulnerable  to  attack.  When installing
    from local media,  do not connect  the installed system  to a live
    network until after the system has been rebooted and the  security
    conditions  have  been  corrected  as  described  below.   Do  not
    install with the "net.i" boot  image or install a network  enabled
    kernel until the  root password has  been changed, the  securettys
    file has been corrected, or external services have been disabled.

    Change the root password  to a non-guessable password  immediately
    upon initial reboot following  installation.  Disable remote  root
    logins by removing the ptty entries from the /etc/securetty  file.
    If not required for  normal operation of the  workstation, disable
    the telnet  and ftp  service in  the inetd.conf  file and  restart
    inetd.   If  enabled  by  the  particular  installation,   disable
    rlogin, rsh, and rexec services as well.