Murphy's law and computer security
Wietse Venema
Mathematics and Computing Science
Eindhoven University of Technology
The Netherlands
wietse@wzv.win.tue.nl
Abstract
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from security advisories and from unpublished loopholes in the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
network.
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++) {
d();
alue & 0xff;
re
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
nutshell.
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
inode.
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
cryptography.
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
generator.
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
systems.
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:
>/etc/passwd
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
hole.
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
stronger.
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
consequences.
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
attack.
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
trap.
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
[Call95]
B. Callaghan, B. Pawlowski, P. Staubach: NFS version 3 protocol
specification. RFC 1813, June 1995.
[Dik96]
Casper H.S. Dik. Private communication, May 1996.
[East94]
D. Eastlake, S. Crocker, J. Schiller: Randomness recommendations
for security. RFC 1750, December 1994.
[Ellis95]
James T. Ellis. Private communication, May 1995.
[Farm93]
Dan Farmer, Wietse Venema: Improving the security of your
site by breaking into it.
ftp.win.tue.nl:/pub/security/admin-guide-to-cracking-101.Z,
December 1993.
[Hans92]
Stephen E. Hansen, E. Todd Atkins: Automated system
monitoring and notification with Swatch. Proc. UNIX security
III, Baltimore, September 1992.
[Kohl93]
J. Kohl, C. Neuman: The Kerberos network authentication service
(V5). RFC 1510, September 1993.
[Marc93]
Borja Marcos. Private communication, June 1993.
[Muff92]
Alec D.E. Muffett: Crack version 4.1 - a sensible password
checker for UNIX. Part of the Crack distribution,
ftp://cert.org/pub/tools/crack/
[Net95]
Netscape technical documents: Potential vulnerability in
Netscape products.
http://www.netscape.com/newsref/std/random_seed_security.html, September 1995.
[Salz92]
Rich Salz: InterNetNews: Usenet transport for Internet
sites. Proc. Usenix conference, San Antonio, June 1992.
[Schei86]
Robert W. Scheifler, Jim Gettys: The X window system. ACM
transactions on graphics vol. 5, no. 2, April 1986.
[Schei92]
Robert W. Scheifler, Jim Gettys: X window system (third
ed.). Digital Press, 1992.
[Spar92]
Icarus Sparry. Private communication, August 1992.
[Stei88]
Jennifer G. Steiner, Clifford Neuman, and Jeffrey I.
Schiller: Kerberos: an authentication service for open network
systems. Proc. winter Usenix conference, Dallas,
1988.
[Ven92]
Wietse Z. Venema: TCP WRAPPER, network monitoring, access
control and booby traps. Proc. UNIX security III, Baltimore,
September 1992.
[Ven96]
Wietse Z. Venema. The logdaemon utilities provide a login
program and several network daemons with enhanced logging
and authentication.
ftp.win.tue.nl:/pub/security/logdaemon_XX.tar.gz.
[Ylo96]
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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