Ramblings & ephemera

The inventor of UNIX on its security … or lack thereof

From Dennis M. Ritchie’s “On the Security of UNIX” (: ):

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 parameters 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. …

The picture is considerably brighter in the area of protection of information from unauthorized perusal and destruction. Here the degree of security seems (almost) adequate theoretically, and the problems lie more in the necessity for care in the actual use of the system. …

It must be recognized that the mere notion of a super-user is a theoretical, and usually practical, blemish on any protection scheme. …

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. … We have observed that users choose passwords that are easy to guess: they are short, or from a limited alphabet, or in a dictionary. Passwords 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 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.

Comments are closed.