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Murphy's Law and Computer Security by Weitse Venema

 Murphy's law and computer security

                          Wietse Venema

                Mathematics and Computing Science
               Eindhoven University of Technology
                         The Netherlands


This paper discusses lessons learned from a selection of computer
security problems that have surfaced in the recent past, and that
are likely to show up again in the  future.  Examples  are  taken
from  security  advisories  and from unpublished loopholes in the
author's own work.

1.  Widely-known passwords

Imagine that you choose a password to protect  your  systems  and
then  advertise  that  password in big neon signs for all to see.
That would not be a very responsible thing to do.   Surprisingly,
this  is  almost exactly what some programs have been doing for a
long time.

Passwords are the traditional method  to  authenticate  users  to
computer  systems.   With todays computer networks, large pseudorandom 
numbers are being used as password tokens  or  as  cryptographic  
keys  for communication between computer programs. Examples 
that will be discussed in this paper are  Kerberos  tickets,
X11 magic cookies, and NFS file handles.

These password tokens or cryptographic keys  are  not  chosen  by
people.  Instead, a pseudo-random value is generated programmatically 
whenever one is needed.  Unfortunately it is easy to end up
with a predictable result.

1.1.  Predictable Kerberos keys

Recent advisories allude to a problem that allows an attacker  to
impersonate  users  of  the  Kerberos  version 4 [Stei88, Kohl93]
authentication system.  In order to understand the problem it  is
not  necessary to go into the details of how Kerberos works.  The
problem is with the generation of encryption keys.

In the Kerberos system, temporary encryption  keys  are  used  to
protect authentication information.  One of these encryption keys
is generated by the authentication server at the  very  beginning
of a login session: it is part of the ticket-granting ticket that
allows an authenticated user to obtain service through  the  network 
without having to provide a password each time.

Unfortunately, the key generation algorithm used by the version 4
authentication server was predictable. Ultimately, all encryption
keys were derived from known information (the time of day),  from
constant  information  (a process ID and a machine identity), and
from predictable information: a counter that was incremented with
each call.  The result: a cryptographer's nightmare.

Armed with this knowledge, an intruder  could  impersonate  other
users without even having to know their password.  The code fragment 
below illustrates the problem:

        p = getpid() ^ gethostid();
        gettimeofday(&time, (struct timezone *) 0);
        /* randomize start */
        srandom(time.tv_usec ^ time.tv_sec ^ p ^ n++);

The fragment shows a fine example of a comment that lies:   feeding  
predictable  input  into  a deterministic routine produces a
predictable result.  Curiously, the  Kerberos  version  4  source
code  already  contains an improved key-generation algorithm that
does use secret data as input, but the improved algorithm was 
not put into actual use until the release of Kerberos
version 5.

1.2.  Only 256 different magic cookies

The X Window system [Schei86, Schei92], comes with the XDM graphical  
login tool. Some vendors provide their own alternative, but
the programs all perform the same basic task:  display a logo  on
the  screen  and  prompt  for a login name and a password. When a
correct name and password are given, an  X  session  is  started.
This  can be a default desktop that is provided by the system, or
it can be a desktop as specified by the commands in  a  .xsession
file in the user's home directory.

When an X application program connects to the  X  server  program
(i.e.,  to  the  user's keyboard, screen and mouse), it typically
authenticates according to one of three methods:

 No authentication: every user on the network has  access  to
     the  user's  keyboard, screen and mouse. This is the default
     on too many systems.

 Client network address: all users  on  specific  hosts  have
     access  to the user's keyboard, screen and mouse. The client
     network address is provided by the client host.

 Magic cookie: all users that know  a  128-bit  secret  value
     have  access  to the user's keyboard, screen and mouse.  The
     secret is typically kept in a file .Xauthority in the user's
     home  directory.  The  secret  is sent in the clear over the

Other authentication methods  exist,  based  on  data  encryption
techniques,  but their use is less common. Authentication methods
differ in strength.  Authentication with magic  cookies  is  more
secure than authentication by client network address. Authentication 
by network address, in turn, is a lot more  secure  than  no
authentication at all.

The XDM  program  suffers  from  a  problem  much  like  the  one
described  earlier for Kerberos version 4: magic cookies are generated 
from non-secret data (time of day and  process  ID).  Such
cookies  can  be  guessed  in  a  small amount of time if one has
access to the victim's machine.

Cookie guessing  becomes  harder,  but  not  impossible,  without
access  to the victim's machine: it is still possible to find out
the approximate time of login by fingering  the  host.   However,
until  recently,  some  XDM implementations suffered from an even
worse flaw that made them vulnerable to arbitrary  users  on  the
network.  A fix was announced in November 1995.

The problem was than many  implementations  of  the  UNIX  random
number  generator  are  truly  horrible:   the  low 8 bits of its
result repeat with a cycle of length 256. These low  8  bits  are
exactly  what  some  XDM implementations use when they generate a
magic cookie:

      len; i++) {
     alue & 0xff;


With a cycle of length 256, there can be only 256 different magic
cookies! It takes only a fraction of a second to try them all and
to find out which of the 256 magic cookies an X server is  using.
It is as if every other house has the same key to the front door.

Fortunately, many XDM implementations are not vulnerable to  this
particular  problem:  they  either  use  a  better  random number
generator or they use an algorithm that involves cryptography.

1.3.  Identical NFS file handles

In a discussion of security loopholes, the  Network  File  system
[Call95]  cannot  remain  unmentioned.  The  fsirand  flaw that I
describe here was found and fixed in SunOS 4 years ago. In  order
to  explain  the  problem  I will describe the NFS protocols in a

When an NFS client host wants to access a remote file  or  directory,
it  sends  a  request to the file server's NFS daemon. The
request includes an NFS file handle that  identifies  the  object
being accessed.

How does an NFS client obtain an NFS file handle? Honest  clients
use  the  NFS  mount  protocol.  When an NFS client host wants to
access a remote file system for the very first time,  the  client
host  sends  a  mount  request to the file server's mount daemon.
The mount daemon verifies that the client host has permission  to
access  the file system.  When the mount daemon grants access, it
sends an NFS file handle back to the client.

Once an NFS client has a file handle, it  can  send  file  access
commands  directly  to the file server's NFS daemon. In fact, any
client that is in the possession of a valid NFS file  handle  can
use  it.  Export restrictions are primarily enforced by the mount
daemon.  In the most common cases the file  server's  NFS  daemon
does not care what client is talking to it.

How, then, does an NFS server protect  itself  against  malicious
clients  that make up their own NFS file handles?  SUN's solution
was to make NFS file handles hard to guess.  When a  file  system
is  created, the fsirand program initializes all NFS file handles
with pseudo-random numbers.  The program is seeded with a process
ID  and with the time of day. Unfortunately, early fsirand implementations 
did not initialize the time of day variable.   Because
of  this  missing  initialization,  the time of day variable contained 
fixed garbage values, depending on  the  system  architecture.
Only the process ID was being set [Dik96].

Many sites run the  same  suninstall  procedure  to  install  the
operating  system  onto  their  disks.   This procedure is highly
automated, and by implication, the fsirand  process  ID  is  very
predictable.  Unknowingly, many sites initialized their file systems 
with the same NFS file handles world wide.

Thus, in order to access a file system there was no need  to  use
the  NFS  mount  protocol at all. Every other house in the street
did have the same keys to the front door.

While researching this paper I noticed that many systems  do  not
even  have  an  fsirand  command  or  its moral equivalent.  Some
systems simply use the time of day when allocating  a  filesystem

1.4.  Moral

Why all  this  attention  to  problems  with  the  generation  of
pseudo-random  numbers?  The  answer is cryptography.  Whether we
like it or not, cryptography is becoming more and more  important
for  the  protection  of data and systems.  Pseudo-random numbers
are essential for the generation of  hard-to-guess  cryptographic
keys.  The best encryption in the world is of no use when encryption 
keys are taken from a predictable source, or when  the  keys
are taken from a too small domain.

In order to generate a secret password you need a secret to begin
with.   The  time of day and the process ID are often not secret.
Kerberos is just one system that has suffered from key-generation
problems.  Another  example is the Netscape navigator. Until September 
1995, this software generated session keys with only about
30 bits of randomness [Net95].  This amount is even less than the
40 bits that the US government presently  allows  for  exportable

In RFC 1750, Eastlake et al recommend the use of external sources
for  randomness  [East94].   Even  inexpensive  computers provide
excellent opportunities:  for example, keystroke  timings,  mouse
event  timings,  or  disk seek times.  The UNIX kernel gives convenient 
access to all this  information  and  more.   By  running
various incantations of the ps command you get a look at information 
that changes rapidly.  When you are attached  to  a  network
(and  who isn't these days?), statistics from the netstat command
can be a good source, too, with the caveat that  network  traffic
can  be  manipulated  and  monitored.  The UNIX kernel is exactly
what I used as source of randomness for the SATAN [Farm93] cookie
generator:  I  never  even  considered the use of a random-number

2.  Burning yourself with malicious data

Only a few years ago, Eindhoven University was a  peaceful  site.
Some  may  remember its name from the days of the great Professor
Dijkstra who did fundamental computing science work on structured
programming,  deadlock  avoidance, and so on. And of course, Eindhoven 
is the place where Philips began making  light  bulbs  and
thus started its electronics imperium.

The peace came to an end when Eindhoven University was  connected
to  the  global  Internet. The university computer systems became
immensely popular with data travelers. The facilities had  become
an  excellent starting point to get onto `the net'. Most visitors
were careful not to break things. One  visitor,  however,  had  a
problem.   Every  month  or  so  he  would  break into one of the
university computer systems, acquire system privileges, and  wipe
the machine completely clean:

        # rm -rf / &

The TCP Wrapper [Ven92] began as a simple tool to maintain a  log
of  the intruder's network access attempts. In the course of time
I extended the program to learn more and more about the opponent.
A  powerful  extension  was  the  booby  trap.  It  allowed us to
automatically execute shell commands whenever a  suspicious  connection  
attempt was made to our systems. The typical application
was to finger the host that connected to our site,  in  order  to
get  information  about the user who was trying to break into our

        ALL: .bad.domain: finger -l @%h | /usr/ucb/mail root

The booby trap feature is very flexible: before  the  command  is
given  to the shell, it is subjected to substitutions.  For example,
the sequence %h is replaced by the name of the remote  host,
or by its network address when the name is unavailable.

The TCP Wrapper development process is a tedious one: because  so
many  systems  depend  on  it, everything needs to be tested very
extensively.  I had been using booby traps  for  almost  a  whole
year  before  I  included the feature into the public TCP Wrapper
release. Only a few months later I received an email message from
a kind Mr. Icarus Sparry [Spar92].  He informed me that the booby
trap feature could introduce a security loophole.

The problem was as follows. The booby  trap  feature  substitutes
host  names into shell commands. Host names are looked up via the
Domain Name System (DNS), which is a  distributed  database.  The
reply  to  a  DNS  query  can literally come from anywhere on the
Internet. With the booby trap, I was substituting untrusted  data
into   shell   commands   that  were  being  executed  with  root
privileges. This was not nice.

I quickly ran a few tests  and,  indeed,  the  DNS  server  would
accept  almost  anything  for a hostname.  With an unmodified DNS
server I was able to generate hostnames containing various  shell
metacharacters.   In  the  DNS server data base files, only a few
characters have a special meaning (dollar, semicolon, white space
and  a few others). Anything else can be put into a hostname, for
example something as destructive as:


The attack is not as easy as it seems, though.  The  TCP  Wrapper
attempts  to  expose malicious DNS servers by asking for a second
opinion.  The program compares the name and address results  from
a  reverse  lookup  (by  host  address) with the name and address
results  from  a  forward  lookup  (by  host  name)  and  detects
discrepancies.  Thus,  an  attacker  would have to manipulate the
results of both forward and reverse lookups.  Nevertheless, it is
a  bad  idea  to  pass uncensored data from the network into, for
example, commands that are given to a shell.

When a program has to defend itself against malicious data, there
are two ways to fix the problem: the right fix and the wrong fix.
The right fix is to permit only data that is  known  to  give  no
problems: letters, digits, dots, and a few other symbols. This is
the approach that I took with the TCP Wrapper: when doing substitutions  
on shell commands it replaces characters outside the set
of trusted characters by underscores.

Unfortunately, many people choose  the  wrong  fix:   they  allow
everything except the values that are known to give trouble. This
approach is an invitation to disaster.  Only months ago, part  of
the  WWW  (world-wide  web) community discovered that CGI (common
gateway interface) scripts and other WWW server  helper  applications 
can be manipulated by sending data containing newline characters,
especially when that data is passed on to shell commands.

2.1.  Moral

Recent advisories point out that programs can be  manipulated  by
malicious data from name servers. Together with the sendmail program,
TCP Wrappers and CGI servers are not the only programs that
must  defend  themselves against malicious data.  For example, if
you do automated logfile analysis with Swatch  [Hans92]  or  with
other  tools, you'd better be careful when passing data from logfiles 
on to other programs: untrusted systems can often send data
directly to the syslog daemon.

3.  Secrets in user-accessible memory

In a previous life I was a nuclear physicist. Working with  delicate  
equipment  was a matter of daily routine.  Of course things
would stop working at the most inconvenient hour of the day.  One
golden rule that I learned very quickly: if you're fixing a problem,
better be sure that you're not breaking  something  else  in
the process.

In the information technology world, working with fragile  pieces
of software is a matter of daily routine. Of course programs stop
working at the most inconvenient  hour  of  the  day  -  hardware
failures  are  becoming  rare.  One  golden  rule  that  a system
administrator learns very quickly: if  you're  fixing  something,
better  be  sure that the solution does not break something else.
Of course we never break something while fixing a problem.

The programmer is faced with the same problems.   All  too  often
something  breaks  while  a  security  or non-security problem
is being fixed.
Shadow passwords are a good  example:   they  were
invented  to  fix  one  problem  and at the same time opened up a

Shadow passwords attempt to cure a real problem.  By the  end  of
the  eighties,  computers had become fast enough that large-scale
password cracking became practical.  Alec Muffett's crack program
[Muff92]  gave everyone the best available tool at the time. Like
many  UNIX  password  cracking  programs,  Alec's  program  takes
encrypted  passwords from a password file and tries to guess them
in a systematical manner. It is amazing  what  it  found  when  I
first ran it on our own password files.

Some people stated that such programs should not be  made  available  
because  they  might  help intruders to break into systems.
History repeats. We had a  similar  discussion  again  about  the
release  of  SATAN,  only  this  time  the  opposition  was  much

Instead of applying censorship to the distribution of software, a
more  practical  solution  is  to get to the root of the problem:
prevent users from choosing weak passwords in  the  first  place,
and  make  encrypted password information less easily accessible.
The usual approach is to move the encrypted passwords  to  a  so-called 
shadow password file that is accessible only to privileged
programs.  With shadow passwords an attacker can no longer run  a
password  cracker  on a stolen copy of the regular password file.
Instead, one must connect to the machine to try a password.  This
makes the risk of detection much larger.

Shadow passwords can give a false sense of security, though.   It
seems  as  if  there  is  less need to be careful when choosing a
password.  After all, the encrypted password  is  protected  from
password  cracking programs.  However, it is easy to see how shadow 
passwords can actually weaken a system's defenses. Here is  a
simplified description of how the login program works:

 read username and password from user

 look up password and shadow file entry

 validate username and password

 drop privileges and execute login shell

In the second step, the login program is still privileged, so  it
can  read  the secret shadow password file. Every practical login
implementation will read a lot more data than just the entry  for
the user logging in: most likely it reads  a  whole  kilobyte  of
data or even more.  This excess data lingers on in memory buffers
somewhere in the process address space.

The fourth step implies a race condition: in-between the time the
login  program  switches to the user and the time it executes the
login shell, the login program can be signaled by the user.  Shadow  
password  file  information  that  was read in step 2 can be
found in the core dump.

It is easy to fall into the trap and  keep  secret  data  in  the
memory  of  unprivileged  programs.  I fixed my logdaemon [Ven96]
utilities years ago when I ported them to Solaris  2,  which  has
shadow passwords. The fix prevents programs from dumping core.

The SSH [Ylo96] utilities suffered from a similar problem. SSH is
a  re-implementation of rlogin, rsh and of a few other utilities.
It aims to improve host and/or user authentication by the use  of
strong  cryptography.   Some  of this protection was lost when it
was discovered that cryptographic keys remain in memory  after  a
process has switched privilege to the user.

3.1.  Moral

The moral of this story is that one should avoid carrying  secret
information  in  the  memory of unprivileged programs. Preventing
core dumps does not give total protection, though; it takes  kernel  
support  to  protect  a  process against 
manipulation with a debugger program.
It is  therefore  essential  that  a  process
wipes secrets from memory before switching user privileges.  Part
of the solution is to use a modified memory manager (malloc) that
wipes memory upon return to the free memory pool.

4.  Depending on other programs

Being the author of a popular security tool is much like  walking
a  wire  thirty  feet  above the ground without a safety net. The
bright side of such an elevated position is that you get  a  nice
view  of the world. The dark side is that an error can have painful 

A case in point is the booby trap example of a few sections  ago:
the  example  where  we finger a host whenever a connection comes
from a suspicious source.

        ALL: .bad.domain: finger -l @%h | /usr/ucb/mail root

The booby trap depends on two programs: finger and /usr/ucb/mail.
The  finger  program  is relatively harmless:  after all, it just
connects to the host and reads  the  reply  across  the  network.
Unfortunately, it is not as harmless as it appears to be at first
sight.  Imagine what happens when the remote  finger  demon  just
keeps  sending  data  forever:  sooner or later the disk fills up
with an enormous temporary file. A judicious link from  /dev/zero
to  a  user's .plan file is sufficient to effect a denial of service 

The bigger problem is with the dependence on /usr/ucb/mail.  This
is  a  complex program with many features. One feature is that it
has so-called shell escapes: the mail program recognizes commands
in  its  input  when they are preceded by a tilde character.  The
tilde-command feature was useful when composing a message at  the
terminal.   Nowadays,  few people still use the raw /usr/ucb/mail
command in interactive mode, but the feature is still  there.  By
inheritance,  tilde-command  is  also  supported  by the System V
mailx program.

Three years ago, Borja Marcos pointed out in private  email  that
these tilde commands are also recognized when /usr/ucb/mail reads
its input from a pipe [Marc93].  This was bad news: anyone  could
put  shell  escape  commands  into their .plan file and have them
executed remotely by triggering a TCP Wrapper  finger+mail  booby

I decided to silently fix the problem  by  flooding  the  market.
Each  year  brought  a  new major TCP Wrapper release with enough
useful features to make people upgrade their old version  to  the
current  one.   With the safe_finger program I attempted to eliminate 
all known problems with the finger+mail booby trap. While I
was  at it, I took the opportunity to solve a few other problems,
too: fingering a host is not a trivial activity.

This time, at least, I was in  good  company:  a  year  later  it
became  known  that  the  widely-used INN news transport software
[Salz92] had fallen into the same trap. It was possible  to  execute  
shell  commands world-wide on news servers running INN,  by
posting just one single news article.

The problem with /usr/ucb/mail shell escapes is going  stay  with
us  for  quite  a while: I have found that many web sites run CGI
helper  scripts  that   send   data   from   the   network   into
/usr/ucb/mail, without censoring of, for example, newline characters 
embedded in the data.

4.1.  Moral

Insecurity by depending on other programs is an old problem.  The
problem becomes even worse when security depends on programs that
were  not  designed  for  security  purposes.  Both  finger   and
/usr/ucb/mail  are  unprivileged programs. They were not designed
to defend themselves against malicious inputs from  the  network.
The  finger+mail  loophole  is just one type of accident that can
happen.  As the author of the TCP Wrapper, it is not possible for
me to protect every user against every possible accident. The TCP
Wrapper booby trap feature is a sharp tool and it should be  used
with care.

5.  Concluding remarks

The problems discussed in this paper are only the tip of a  large
ice  berg  of  recurring  security  problems.  There was a lot of
material to choose from, and the selection is far from  representative.
UNIX  environment  settings  are  just one example of a
major source of recurring trouble that could not be discussed.

All examples in this paper were taken from  the  UNIX  world.  Is
UNIX such a bad system?  Of course not.  UNIX is a platform where
much pioneering work was done and still  is  being  done.  It  is
therefore  not surprising that many errors were made first in the
UNIX environment.  On the positive side, these experiences should
give UNIX users an advantage.  For example, now that PC operating
systems become capable enough to provide network services, we can
expect to see a lot of familiar problems coming back.

As the examples in this paper show, the path of the security programmer  
is riddled with land mines. The author has had the questionable 
privilege to meet Mr. Murphy several  times  in  person.
Murphy is a cruel teacher.

6.  References

     B. Callaghan, B. Pawlowski, P. Staubach: NFS version 3  protocol 
specification. RFC 1813, June 1995.

     Casper H.S. Dik. Private communication, May 1996.

     D. Eastlake, S. Crocker, J. Schiller: Randomness recommendations 
for security. RFC 1750, December 1994.

     James T. Ellis. Private communication, May 1995.

     Dan Farmer, Wietse Venema: Improving the  security  of  your
     site          by          breaking          into         it.
     December 1993.

     Stephen  E.  Hansen,  E.  Todd  Atkins:   Automated   system
     monitoring and notification with Swatch. Proc. UNIX security
     III, Baltimore, September 1992.

     J. Kohl, C. Neuman: The Kerberos network authentication service 
(V5).  RFC 1510, September 1993.

     Borja Marcos. Private communication, June 1993.

     Alec D.E. Muffett: Crack version 4.1 - a  sensible  password
     checker   for   UNIX.    Part  of  the  Crack  distribution,

     Netscape technical  documents:  Potential  vulnerability  in
     Netscape   products.   
http://www.netscape.com/newsref/std/random_seed_security.html, September 1995.

     Rich Salz:  InterNetNews:   Usenet  transport  for  Internet
     sites.  Proc.  Usenix conference, San Antonio, June 1992.

     Robert W. Scheifler, Jim Gettys: The X window  system.   ACM
     transactions on graphics vol. 5, no. 2, April 1986.

     Robert W. Scheifler, Jim  Gettys:  X  window  system  (third
     ed.).  Digital Press, 1992.

     Icarus Sparry. Private communication, August 1992.

     Jennifer  G.  Steiner,  Clifford  Neuman,  and  Jeffrey   I.
     Schiller:  Kerberos: an authentication service for open network 
systems.   Proc.   winter  Usenix  conference,  Dallas,

     Wietse Z. Venema: TCP WRAPPER,  network  monitoring,  access
     control and booby traps. Proc. UNIX security III, Baltimore,
     September 1992.

     Wietse Z. Venema.  The logdaemon utilities provide  a  login
     program  and  several  network daemons with enhanced logging
     and                                          authentication.

     Tatu Ylonen: SSH - secure login connections over the  internet.  
Proc. UNIX security VI, San Jose, July 1996.

1  Several vendors were already aware of the  problem  and
	had taken measures in their own version of the software.

2  Not to mention the things that break as  a  side  effect
	of fixing a non-problem.

3.   In particular, the UNIX kernel should not allow
	unprivileged users  to  debug a process with an effective, real, 
	or saved user or group ID that belongs to another user [Ellis95].

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