________________________________________________________________ THE COMPUTER INCIDENT ADVISORY CAPABILITY CIAC ADVISORY NOTICE ________________________________________________________________ Security Holes in UNIX Systems The DOE Computer Incident Advisory Capability (CIAC) at LLNL has learned that there are two security holes which leave certain systems running the UNIX Operating System vulnerable to intrusion. The holes, when used together in a very specific scenario, permit an intruder to attack UNIX systems over the Internet and other pathways, and to assume superuser privileges. Therefore, these vulnerabilities constitute a highly severe threat. Sun systems running SunOS 3.x and 4.0 are susceptible; the hole may exist in other versions of the UNIX operating system as well. A brief description of the holes follow: 1. There is a long-known hole in tftp, the user interface to the Internet TFTP (Trivial File Transfer Protocol). This hole allows any user, without first logging in, to read any readable file and to write any writable file on a remote system via Internet. This hole exists in SunOS 3.x, but has been fixed in SunOS 4.0. To determine whether this hole affects a system, do the following: Enter tftp local_testing_system at the UNIX prompt and follow with a carriage return. For example, % tftp styx The system should answer with the tftp prompt: tftp> Enter the following at the prompt: tftp> get /etc/passwd stolen_pw If "Error code 1: File not found" appears, the tested system is safe from remote attacks. However, it may still be susceptible to local attacks. 2. There are holes in rwalld (the network rwall server daemon) and wall (write to all users). The command "wall" writes to filenames encoded within /etc/utmp. You are vulnerable to this hole if /etc/utmp is world and/or group writable. In essence, an intruder can use this hole to write on any file on the computer system. Any UNIX machine which runs a windowing system is likely to have the privileges which create the described vulnerabilities. The presence of an intruder who exploits these holes is difficult to detect. It is our understanding that knowledge of these holes has been distributed widely within the hacker community. It is advisable, based on your vulnerability, to take appropriate action for your site. CIAC is currently working with Sun Microsystems and DARPA's Computer Emergency Response Team (CERT) to eliminate these vulnerabilities as quickly as possible. You will be notified when a patch is available. For further information, contact CIAC at (415) 422-8193 or (FTS) 532-8193, or send e-mail to gschultz%nsspa@icdc.llnl.gov.