On the Security of UNIX SMM:17-1
On the Security of UNIX
Dennis M. Ritchie
AT&T Bell Laboratories
Murray Hill, New Jersey 07974
Recently there has been much interest in the security
aspects of operating systems and software. At issue is the abil-
ity to prevent undesired disclosure of information, destruction
of information, and harm to the functioning of the system. This
paper discusses the degree of security which can be provided
under the UNIX- system and offers a number of hints on how to
improve security.
The first fact to face is that UNIX was not developed with
security, in any realistic sense, in mind; this fact alone
guarantees a vast number of holes. (Actually the same statement
can be made with respect to most systems.) The area of security
in which UNIX is theoretically weakest is in protecting against
crashing or at least crippling the operation of the system. The
problem here is not mainly in uncritical acceptance of bad param-
eters to system calls- there may be bugs in this area, but none
are known- but rather in lack of checks for excessive consumption
of resources. Most notably, there is no limit on the amount of
disk storage used, either in total space allocated or in the
number of files or directories. Here is a particularly ghastly
shell sequence guaranteed to stop the system:
while : ; do
mkdir x
cd x
done
Either a panic will occur because all the i-nodes on the device
are used up, or all the disk blocks will be consumed, thus
preventing anyone from writing files on the device.
In this version of the system, users are prevented from
creating more than a set number of processes simultaneously, so
unless users are in collusion it is unlikely that any one can
stop the system altogether. However, creation of 20 or so CPU or
_________________________
- UNIX is a registered trademark of AT&T Bell Labora-
tories in the USA and other countries.
SMM:17-2 On the Security of UNIX
disk-bound jobs leaves few resources available for others. Also,
if many large jobs are run simultaneously, swap space may run
out, causing a panic.
It should be evident that excessive consumption of disk
space, files, swap space, and processes can easily occur acciden-
tally in malfunctioning programs as well as at command level. In
fact UNIX is essentially defenseless against this kind of abuse,
nor is there any easy fix. The best that can be said is that it
is generally fairly easy to detect what has happened when disas-
ter strikes, to identify the user responsible, and take appropri-
ate action. In practice, we have found that difficulties in this
area are rather rare, but we have not been faced with malicious
users, and enjoy a fairly generous supply of resources which have
served to cushion us against accidental overconsumption.
The picture is considerably brighter in the area of protec-
tion of information from unauthorized perusal and destruction.
Here the degree of security seems (almost) adequate theoreti-
cally, and the problems lie more in the necessity for care in the
actual use of the system.
Each UNIX file has associated with it eleven bits of protec-
tion information together with a user identification number and a
user-group identification number (UID and GID). Nine of the pro-
tection bits are used to specify independently permission to
read, to write, and to execute the file to the user himself, to
members of the user's group, and to all other users. Each process
generated by or for a user has associated with it an effective
UID and a real UID, and an effective and real GID. When an
attempt is made to access the file for reading, writing, or exe-
cution, the user process's effective UID is compared against the
file's UID; if a match is obtained, access is granted provided
the read, write, or execute bit respectively for the user himself
is present. If the UID for the file and for the process fail to
match, but the GID's do match, the group bits are used; if the
GID's do not match, the bits for other users are tested. The last
two bits of each file's protection information, called the set-
UID and set-GID bits, are used only when the file is executed as
a program. If, in this case, the set-UID bit is on for the file,
the effective UID for the process is changed to the UID associ-
ated with the file; the change persists until the process ter-
minates or until the UID changed again by another execution of a
set-UID file. Similarly the effective group ID of a process is
changed to the GID associated with a file when that file is exe-
cuted and has the set-GID bit set. The real UID and GID of a pro-
cess do not change when any file is executed, but only as the
result of a privileged system call.
The basic notion of the set-UID and set-GID bits is that one
may write a program which is executable by others and which main-
tains files accessible to others only by that program. The clas-
sical example is the game-playing program which maintains records
of the scores of its players. The program itself has to read and
On the Security of UNIX SMM:17-3
write the score file, but no one but the game's sponsor can be
allowed unrestricted access to the file lest they manipulate the
game to their own advantage. The solution is to turn on the set-
UID bit of the game program. When, and only when, it is invoked
by players of the game, it may update the score file but ordinary
programs executed by others cannot access the score.
There are a number of special cases involved in determining
access permissions. Since executing a directory as a program is a
meaningless operation, the execute-permission bit, for direc-
tories, is taken instead to mean permission to search the direc-
tory for a given file during the scanning of a path name; thus if
a directory has execute permission but no read permission for a
given user, he may access files with known names in the direc-
tory, but may not read (list) the entire contents of the direc-
tory. Write permission on a directory is interpreted to mean that
the user may create and delete files in that directory; it is
impossible for any user to write directly into any directory.
Another, and from the point of view of security, much more
serious special case is that there is a ``super user'' who is
able to read any file and write any non-directory. The super-user
is also able to change the protection mode and the owner UID and
GID of any file and to invoke privileged system calls. It must be
recognized that the mere notion of a super-user is a theoretical,
and usually practical, blemish on any protection scheme.
The first necessity for a secure system is of course arrang-
ing that all files and directories have the proper protection
modes. Traditionally, UNIX software has been exceedingly permis-
sive in this regard; essentially all commands create files read-
able and writable by everyone. In the current version, this pol-
icy may be easily adjusted to suit the needs of the installation
or the individual user. Associated with each process and its des-
cendants is a mask, which is in effect and-ed with the mode of
every file and directory created by that process. In this way,
users can arrange that, by default, all their files are no more
accessible than they wish. The standard mask, set by login,
allows all permissions to the user himself and to his group, but
disallows writing by others.
To maintain both data privacy and data integrity, it is
necessary, and largely sufficient, to make one's files inaccessi-
ble to others. The lack of sufficiency could follow from the
existence of set-UID programs created by the user and the possi-
bility of total breach of system security in one of the ways dis-
cussed below (or one of the ways not discussed below). For
greater protection, an encryption scheme is available. Since the
editor is able to create encrypted documents, and the crypt com-
mand can be used to pipe such documents into the other text-
processing programs, the length of time during which cleartext
versions need be available is strictly limited. The encryption
scheme used is not one of the strongest known, but it is judged
adequate, in the sense that cryptanalysis is likely to require
SMM:17-4 On the Security of UNIX
considerably more effort than more direct methods of reading the
encrypted files. For example, a user who stores data that he
regards as truly secret should be aware that he is implicitly
trusting the system administrator not to install a version of the
crypt command that stores every typed password in a file.
Needless to say, the system administrators must be at least
as careful as their most demanding user to place the correct pro-
tection mode on the files under their control. In particular, it
is necessary that special files be protected from writing, and
probably reading, by ordinary users when they store sensitive
files belonging to other users. It is easy to write programs that
examine and change files by accessing the device on which the
files live.
On the issue of password security, UNIX is probably better
than most systems. Passwords are stored in an encrypted form
which, in the absence of serious attention from specialists in
the field, appears reasonably secure, provided its limitations
are understood. In the current version, it is based on a slightly
defective version of the Federal DES; it is purposely defective
so that easily-available hardware is useless for attempts at
exhaustive key-search. Since both the encryption algorithm and
the encrypted passwords are available, exhaustive enumeration of
potential passwords is still feasible up to a point. We have
observed that users choose passwords that are easy to guess: they
are short, or from a limited alphabet, or in a dictionary. Pass-
words should be at least six characters long and randomly chosen
from an alphabet which includes digits and special characters.
Of course there also exist feasible non-cryptanalytic ways
of finding out passwords. For example: write a program which
types out ``login:'' on the typewriter and copies whatever is
typed to a file of your own. Then invoke the command and go away
until the victim arrives.
The set-UID (set-GID) notion must be used carefully if any
security is to be maintained. The first thing to keep in mind is
that a writable set-UID file can have another program copied onto
it. For example, if the super-user (su) command is writable, any-
one can copy the shell onto it and get a password-free version of
su. A more subtle problem can come from set-UID programs which
are not sufficiently careful of what is fed into them. To take an
obsolete example, the previous version of the mail command was
set-UID and owned by the super-user. This version sent mail to
the recipient's own directory. The notion was that one should be
able to send mail to anyone even if they want to protect their
directories from writing. The trouble was that mail was rather
dumb: anyone could mail someone else's private file to himself.
Much more serious is the following scenario: make a file with a
line like one in the password file which allows one to log in as
the super-user. Then make a link named ``.mail'' to the password
file in some writable directory on the same device as the pass-
word file (say /tmp). Finally mail the bogus login line to
On the Security of UNIX SMM:17-5
/tmp/.mail; You can then login as the super-user, clean up the
incriminating evidence, and have your will.
The fact that users can mount their own disks and tapes as
file systems can be another way of gaining super-user status.
Once a disk pack is mounted, the system believes what is on it.
Thus one can take a blank disk pack, put on it anything desired,
and mount it. There are obvious and unfortunate consequences. For
example: a mounted disk with garbage on it will crash the system;
one of the files on the mounted disk can easily be a password-
free version of su; other files can be unprotected entries for
special files. The only easy fix for this problem is to forbid
the use of mount to unprivileged users. A partial solution, not
so restrictive, would be to have the mount command examine the
special file for bad data, set-UID programs owned by others, and
accessible special files, and balk at unprivileged invokers.
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