Ramblings & ephemera

A technical look at the Morris Worm of 1988

From Donn Seeley’s “The Internet Worm of 1988: A Tour of the Worm“:

November 3, 1988 is already coming to be known as Black Thursday. System administrators around the country came to work on that day and discovered that their networks of computers were laboring under a huge load. If they were able to log in and generate a system status listing, they saw what appeared to be dozens or hundreds of “shell” (command interpreter) processes. If they tried to kill the processes, they found that new processes appeared faster than they could kill them. Rebooting the computer seemed to have no effect within minutes after starting up again, the machine was overloaded by these mysterious processes.

… The worm had taken advantage of lapses in security on systems that were running 4.2 or 4.3 BSD UNIX or derivatives like SunOS. These lapses allowed it to connect to machines across a network, bypass their login authentication, copy itself and then proceed to attack still more machines. The massive system load was generated by multitudes of worms trying to propagate the epidemic. …

The worm consists of a 99-line bootstrap program written in the C language, plus a large relocatable object file that comes in VAX and Sun-3 flavors. …

The activities of the worm break down into the categories of attack and defense. Attack consists of locating hosts (and accounts) to penetrate, then exploiting security holes on remote systems to pass across a copy of the worm and run it. The worm obtains host addresses by examining the system tables /etc/hosts.equiv and /.rhosts, user files like .forward and. rhosts, dynamic routing information produced by the netstat program, and finally randomly generated host addresses on local networks. It ranks these by order of preference, trying a file like /etc/hosts.equiv first because it contains names of local machines that are likely to permit unauthenticated connections. Penetration of a remote system can be accomplished in any of three ways. The worm can take advantage of a bug in the finger server that allows it to download code in place of a finger request and trick the server into executing it. The worm can use a “trap door” in the sendmail SMTP mail service, exercising a bug in the debugging code that allows it to execute a command interpreter and download code across a mail connection. If the worm can penetrate a local account by guessing its password, it can use the rexec and rsh remote command interpreter services to attack hosts that share that account. In each case the worm arranges to get a remote command interpreter which it can use to copy over, compile and execute the 99-line bootstrap. The bootstrap sets up its own network connection with the local worm and copies over the other files it needs, and using these pieces a remote worm is built and the infection procedure starts over again. …

When studying a tricky program like this, it’s just as important to establish what the program does not do as what it does do. The worm does not delete a system’s files: it only removes files that it created in the process of bootstrapping. The program does not attempt to incapacitate a system by deleting important files, or indeed any files. It does not remove log files or otherwise interfere with normal operation other than by consuming system resources. The worm does not modify existing files: it is not a virus. The worm propagates by copying itself and compiling itself on each system; it does not modify other programs to do its work for it. Due to its method of infection, it can’t count on sufficient privileges to be able to modify programs. The worm does not install trojan horses: its method of attack is strictly active, it never waits for a user to trip over a trap. Part of the reason for this is that the worm can’t afford to waste time waiting for trojan horses-it must reproduce before it is discovered. Finally, the worm does not record or transmit decrypted passwords: except for its own static list of favorite passwords, the worm does not propagate cracked passwords on to new worms nor does it transmit them back to some home base. This is not to say that the accounts that the worm penetrated are secure merely because the worm did not tell anyone what their passwords were, of course-if the worm can guess an account’s password, certainly others can too. The worm does not try to capture superuser privileges: while it does try to break into accounts, it doesn’t depend on having particular privileges to propagate, and never makes special use of such privileges if it somehow gets them. The worm does not propagate over uucp or X.25 or DECNET or BITNET: it specifically requires TCP/IP. The worm does not infect System V systems unless they have been modified to use Berkeley network programs like sendmail, fingerd and rexec.

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