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TUCoPS :: Hacking Techniques :: rootkits.htm

"Root Kits" and hiding files/directories/processes after a break-in RKT:
Title: "Root Kits" and hiding files/directories/processes after a break-in $Revision: 1.3 $ $Date: 2001/06/06 21:22:24 $ $Author: dittrich $ Question: Someone reported my computer was involved in a security incident, but I can't see anything wrong. How do I know if my system was compromised? Answer: The last thing that a system administrator wants to hear is someone on the other end of the phone say, "I have reason to believe your workstation has been compromised and may be running a network sniffer." My system was compromised?! It's running a sniffer? What does that mean? How do I tell if its true? What do I do? History of Unix Trojan Horses ----------------------------- Back in the 1980s, determining what was happening on your Unix system wasn't too hard. o The "last" command would show you what accounts intruders were using, where they were coming from, and when they were in your system. o The "ls" command would show their files. The "ps" command would show the sniffer, password cracking program, and anything else being run by the intruders. o The "netstat" command would show you the current network connections and ports on which ports were listening for incoming connections. o The "ifconfig" command would tell you if the ethernet interface was in promiscuous mode, making visible to the intruder's sniffer program EVERY packet on the network. In short, the system could be trusted to provide you with accurate information. As time went on, clever hackers had developed methods to conceal their activities, and programs to assist this concealment. These methods and programs were documented in "philes" that populated underground bulletin boards and published in magazines -- electronic and hardcopy -- like 2600 and Phrack. For example, "Hiding Out Under Unix," by Black Tie Affair (Phrack Volume Three, Issue 25, File 6, March 25, 1989) includes source code for a program to edit the /etc/wtmp file to remove all logins records for compromised accounts. Over time, other clever programmers kicked into action and wrote programs to modify the timestamp and size of programs like "ls", "netstat", "ps" which were turned into "trojan horses". Just like the Trojan Horse used by the Greeks to sack Troy, these programs appear to be something you know and trust, but instead hold hidden features that trick the person running them into believing the output is truthful, very effectively allowing the intruder to harvest login passwords, conceal their files, network connections, and processes. Since the files had the same timestamp as other programs in the same directory, and appeared to have the same checksums (via another trojan horse technique), the naive administrator of the system would see nothing out of the ordinary and give up, thinking the system to be "clean". These trojan horse programs were bundled together in the form of "Root Kits", the original written for Sun's Berkeley flavor of Unix (SunOS 4) and later for Linux. (SunOS 4 and 5 root kits will be discussed later.) Linux Root Kit version 3 (lrk3), released in December of 1996, further added tcp wrapper trojans and enhanced the programs in the kit. This was the most common method of concealing activity and stealing passwords by sniffing on the new favorite target of intruders, x86 compatible PCs running Linux. Many intruders would leave the default configuration files (the ones that tell the trojan horse programs what to conceal) in their standard location. An administrator who knew the locations of these programs and their configuration files could fairly easily disable them or use the "strings" program to look for suspect strings in the binary programs. Another method commonly used was to use "find" to locate all files modified within the last 24 hours. While "ls" would lie to you, "find" would dutifully report files and directories, exposing them. Again, the ante is upped by the introduction of the Linux Root Kit version 4 (lrk4), released in November 1998. This version adds new trojan horse programs "pidof" and "killall" (used to terminate running processes by name), "find" (used to locate files by type, name, date, etc.), "top" (shows processes), "crontab" (used to schedule periodic processes), and adds a new program to check for the sniffer. The complete list of programs included in the kit, from the README file, is: bindshell port/shell type daemon! chfn Trojaned! User->r00t chsh Trojaned! User->r00t crontab Trojaned! Hidden Crontab Entries du Trojaned! Hide files find Trojaned! Hide files fix File fixer! ifconfig Trojaned! Hide sniffing inetd Trojaned! Remote access killall Trojaned! Wont kill hidden processes linsniffer Packet sniffer! login Trojaned! Remote access ls Trojaned! Hide files netstat Trojaned! Hide connections passwd Trojaned! User->r00t pidof Trojaned! Hide processes ps Trojaned! Hide processes rshd Trojaned! Remote access sniffchk Program to check if sniffer is up and running syslogd Trojaned! Hide logs tcpd Trojaned! Hide connections, avoid denies top Trojaned! Hide processes wted wtmp/utmp editor! z2 Zap2 utmp/wtmp/lastlog eraser! There are several solutions to this problem. You can try to use alternatives to the trojaned programs that you assume have not been altered. While this is certainly the easiest for an administrator -- to do nothing -- the fact that you cannot trust any programs on the system that is now under the control of an intruder argues for a more thorough job. Beyond doing nothing, you are faced with identifing which programs have been modified and eventually replacing them. Identifying the programs which have been modified can be very difficult if done after the fact. Had the adminstrator generated a list of checksums for all programs installed on the system at the time of installation, using "md5" for example, the process would be much easier. Programs like "tripwire" can help with this task. The investment in time to set up tripwire is arguably less than the time that must be spent after an intrusion to work backwards, but is often not even considered when a system is brought online. Red Hat Linux uses a package distribution mechanism (Red Hat Package Manager, or RPM for short), which has a checksumming capability. The command 'rpm -V -a' will verify all the packages against the RPM database on the local hard drive. As this database is kept on the local drive, it can be modified just as the other programs are trojaned, so you should not trust this file unless you have a copy on a non-writeable medium (e.g., a floppy disc that is write protected), or a copy on another system, possibly checksummed itself with "md5". Instead, it is better to verify the files on your system by comparing packages with the originals on CD-ROM or a Red Hat distribution mirror site. To verify the package containing "/bin/ls" from an FTP mirror, you would use a command similar to the following: # rpm -Vvp S.5....T /bin/ls You can find a complete list of Red Hat FTP mirror sites at: To avoid the cost of trying to identify modified programs, system administrators are often tempted to go straight to replacement. The question then becomes, "what must be replaced to regain control of the system?" Replacement can be targetted (re-installing just the suspect programs), or it can be comprehensive (re-installing the entire operating system). Either way, the act of replacing programs will destroy any evidence of the intruder's actions on your system. This can hamper a thorough investigation, and in those cases where you wish to pursue prosecution, it may become impossible due to lack of usable evidence (and you never know when your system is going to be central to an incident that results in damages warranting prosecution.) Because of this, it is critical that a complete backup of the system be made before any programs are replaced or files deleted. Of the two options -- partial or total replacement of operating system files -- the first risks not getting all of the modified programs, and the second takes significantly more time to complete the re-installation. The risk in the first case is that you will miss one of the many backdoors and will not regain full control of the system, causing you to start over and possibly end up doing the longer full re-install you avoided in the first place. Should you chose to do a partial re-installation, you will need to replace all of the programs modified by the lrk4 Makefile. On a Red Hat 5.2 system, at minimum the following RPM packages must be replaced: util-linux-2.7-18 /usr/bin/chfn /usr/bin/chsh /bin/login fileutils-3.16-9 /bin/ls passwd-0.50-11 /usr/bin/passwd procps-1.2.7-5 /bin/ps /usr/bin/top rsh-0.10-4 /usr/sbin/in.rshd net-tools-1.33-6 /bin/netstat /sbin/ifconfig sysklogd-1.3-22 /usr/sbin/syslogd netkit-base-0.10-10 /usr/sbin/inetd tcp_wrappers-7.6-4 /usr/sbin/tcpd psmisc-17-3 /usr/bin/killall SysVinit-2.74-4 /sbin/pidof findutils-4.1-23 /usr/bin/find Note: There is a script in Appendix A (called "findrpms") which will allow you to generate this list of packages as of the current state of upgrades to your system. You can feed the results of this script to a "for" loop (or "foreach" if you use the C shell) to verify the packages, as shown above. The command would look something like this: # for i in `./findrpms` > do > echo Verifying package $i > rpm -Vp$i > done If some of these packages have been updated from the "updates" mirror, you will need to substitute the location of the newer RPM files in the command above. Since you are going to be replacing files that are already installed on the system (and thus already in the RPM database), you must re-install the packages using the "--force" option to ensure that the existing files are over-written. The command to replace the first of the listed packages above from Red Hat's FTP server would be: rpm -Uvh For more on RPM, see Alternatives for trojaned programs ---------------------------------- A good alternative for "ps" and "netstat" is the the public domain program "lsof" (LiSt Open Files). Using "lsof", you can see a list of processes and uids running them (exposing sniffers, IRC bots, login shells), network connections, and all currently open files (showing the sniffer log file and current working directory in which it resides). You can get "lsof" in RPM package form from your local Red Hat ftp archive, or in source code form via anonymous ftp from: There are a few ways to get around trojaned versions of "ls". You can use the "echo" command (a built-in command on most C shell variants) to list files, and if you are using the tcsh shell, you can use its built-in variant of "ls -F", by typing "ls-F" (note there is no space between "ls" and "-F" in the built-in version.) An alternative to "find" is to use "tar" instead, creating an archive on standard output, then listing the contents of this archive and using "egrep" to select what you want to see. Of course this depends on how "tar" lists archive contents, but if you use GNU tar you will have a rough equivalent to an "ls -lR" style listing and can identify directories by the leading "d" character on the line, e.g.: % gtar -cf - . | gtar -tvf - | egrep "^d|\/\." drwx------ dittrich/users 0 Sep 25 16:30 1999 ./ drwxr-x--- dittrich/users 0 Aug 18 16:26 1999 ptyf/ drwxr-x--- dittrich/users 0 Sep 7 16:06 1999 f/ drwxrwx--- dittrich/users 0 Sep 25 16:36 1999 .hidden/ drwxrwx--- dittrich/users 0 Sep 25 16:30 1999 .hidden/.../ -rw-rw---- dittrich/users 0 Sep 25 16:36 1999 .hidden/.file Another alternative is to keep copies of any/all of the trojaned programs commonly found in rootkits on a floppy disc, which can be mounted and run when needed. Another source of information on modifying system programs on Linux as part of concealing intrusions can be found in Phrack 52-18, "Weakening the Linux Kernel". You can find Phrack at locations like: Various root kits (some of which are now being included in worms) can be found at sites such as: Various tools for detecting and cleaning up rootkits can be found on these pages: [Note that I have not tried all of these tools, so let me know which ones don't work and I'll remove the links, or others that should be included.] You should also be familiar with the incident response cycle and computer forensic techniques. Using tool kits like "The Coroner's Toolkit", you can do a very thorough job of analyzing the file system and finding a lot of footprints. Read more about these subjects at: The Honeynet Project Forensic Challenge involved detailed forensic analysis of a system with a custom rootkit (comprised of parts of several rootkits, some of which actually broke the host system.) You can see examples of the analyses (and learn the tools and try out the techniques yourself) at: While you are at it, take a look at the "Know your enemy" series of papers by the Honeynet Project. The third paper focuses on root kits and how intruders cover their tracks once they are in a system: David Brumley of Stanford has an article in the September issue of ;login: magazine on root kits: Mixter has a paper on rootkits on his web site: The future is here - Loadable Kernel Modules (LKMs) --------------------------------------------------- LKMs have been around for a while. SunOS 4.x had a loadable module interface, as do most new *nix operating systems. An attack on SunOS systems using this method was tty snooping using "tap" (references anyone?) Phrack 50 (April 9, 1997) featured a brief, but instructive, article by halflife on hacking the Linux kernel using LKMs: More recently, other implementations of LKMs for hiding processes, files, and directories have come about that can get around the above described methods of defeating standard root kits, as well as cryptographic checksumming programs like "tripwire" that must trust the operating system to present them with valid bits from disc and memory. The Hacker's Choice (THC) from Germany has write-ups on loadable kernel modules for Linux, FreeBSD, and Solaris, which describe this methods of hiding out on a rooted box: TESO has another Linux LKM ("adore") along these same lines: Using methods such as these, integrity checking programs like "tripwire" and NIPC's "find_ddos" programs can be subverted, as the kernel could not even be trusted to give correct results when searching process tables, network structures, or file systems. You might think that simply disabling LKM support in the kernel -- which is still a good idea to improve security on a server whose configuration will be stable -- is the final answer. Not exactly. Another method of inserting code into running kernels -- even if LKM support is not present -- is described by Silvio Cesare: Various ideas for defending against kernel level attacks are being considered by many. One such solution is proposed by Sebastian Krahmer at the University of Potsdam, Germany: Sebastian's method is to monitor and log any program execution when execve() calls are made. Combine this with off-system logging, and you can preserve a record of program execution on a system (e.g., a web server, or file server). Add a Perl script to monitor these log entries, and you can trigger actions like alarms or even kill the resulting shell to try to stop the intruder's actions. These are interesting ideas, but may have various weaknesses that could allow a very sophisticated hacker to subvert them (although they would catch the much more common "script kiddie" attacks). More research in this area will surely continue. ----------------------------------------------------------------------------- Appendix A - findrpms script. ----------------------------------------------------------------------------- #!/bin/sh # # findrpms - locates the packages containing verious system binaries # # The following simple script will return the list of Red Hat packages # which are currently installed that contain any of the files listed. # Note that someone hacking your system may alter configuration files, # install other "backdoors," run programs that listen on ports, etc., # so this is by no means a definitive list of packages that must be # backed up and replaced. TMP="$HOME/..list.$$" FILES=`cat <<EOD /usr/bin/chfn /usr/bin/chsh /bin/login /bin/ls /usr/bin/passwd /bin/ps /usr/bin/top /usr/sbin/in.rshd /bin/netstat /sbin/ifconfig /usr/sbin/syslogd /usr/sbin/inetd /usr/sbin/tcpd /usr/bin/killall /sbin/pidof /usr/bin/find EOD ` umask 077 rm -f $TMP cp /dev/null $TMP for i in $FILES do rpm -qf $i >> $TMP done sort $TMP | uniq rm -f $TMP exit 0 ----------------------------------------------------------------------------- Addendum for Windows NT ======================= Windows NT is not immune to root kits, in fact, one was discussed in Phrack Magazine, Issue 55, Article 5: Addendum for SunOS 4.x/5.x Root Kits ==================================== Let me start with a little diatribe on Sun's operating systems and naming conventions. Sun originally shipped their systems with a Berkeley Software Distribution (BSD) base. The final release of this was SunOS 4.x. In the late 1980's, Sun wished to switch to a System V Release 4 flavor of Unix (an entirely different kernel, shell commands, file system layout, device file structure, etc.) They were to name this SunOS 5.x. Sun's marketing department, in the infinite wisdom of marketing types, retroactively renamed SunOS 4.x to Solaris 1.x, and SunOS 5.x to Solaris 2.x, thereby making ther customers think this is just an upgrade, not an entirely new operating system. They then call the former "SunOS" and the latter "Solaris" (even though both names are equally correct, in an extremely vague and confusing way.) That said, names that include "sun", "sun4", or "sunos" *may* mean SunOS 4.x programs, while names that include "sol" or "sun5" *may* mean SunOS 5.x. Since the original root kits for Sun systems addressed SunOS 4.x (Berkeley style Unix) commands, and SunOS 5.x is System V Release 4 based Unix, the contents of various publically available root kits are likewise confused as to which programs are to be replaced. E.g., there are as many as three versions of a program like "ls" on a Solaris 2.x system: ----------------------------------------------------------------------------- # ls -l /usr/bin/ls /usr/ucb/ls /usr/xpg4/bin/ls -r-xr-xr-x 1 bin bin 16644 May 2 1996 /usr/bin/ls -rwxr-xr-x 1 bin bin 13612 May 2 1996 /usr/ucb/ls -r-xr-xr-x 1 bin bin 16676 May 2 1996 /usr/xpg4/bin/ls ----------------------------------------------------------------------------- One Sun root kit replaces: /usr/ucb/netstat /usr/kvm/ps /bin/ls /bin/du /usr/bin/login /usr/etc/ifconfig /usr/5bin/du /usr/5bin/ls It puts backup copies of the original programs replaced by root kit in a directory created by the intruder (default name "rk-bak") with the extension ".bak". The default name of configuration files are "/dev/pty[pqr]". Another kit replaces: /bin/ls /bin/du /usr/bin/du /bin/login /bin/ps /bin/netstat It does not create backups, and uses "/dev/ttyp" (for "ps") and "/dev/ptyr" (for "ls") for its configuration files. Until someone adds a complete set of hacked shell that alter the behaviour of the internal "echo" command, you can locate files using shell wildcards like this: # echo /dev/pty? /dev/tty? ...but remember that the directory location, configuration file names, and backup extensions can be changed, so don't rely on them being the same in all cases. One kit provides a flag ("-/") that instructs "ls", "ps", "netstat" etc. to show things they otherwise would hide. Try this flag to see if it is accepted. Provided that "truss" and/or the kernel have not been modified, a method you can use to determine the names of the root kit configuration files (which are read each time "ls", "ps", "netstat", etc. are run) is to use the "truss" program to trace calls to open(). If you see "ps" open a file named "/dev/hdaa", that's a strong hint that this is not your average "ps". Once you've identified the configuration files this way, and renamed them, these programs should work normally again. ----------------------------------------------------------------------------- Example of normal "/bin/ls": # truss -t open /bin/ls open("/dev/zero", O_RDONLY) = 3 open("/usr/lib/", O_RDONLY) = 4 open("/usr/lib/", O_RDONLY) = 4 open("/usr/lib/", O_RDONLY) = 4 open("/usr/lib/", O_RDONLY) = 4 open("/usr/platform/SUNW,Sun_4_75/lib/", O_RDONLY) Err#2 ENOENT open(".", O_RDONLY|O_NDELAY) = 3 [list of files] Example of normal "/usr/ucb/ls": # truss -t open /usr/ucb/ls open("/dev/zero", O_RDONLY) = 3 open("/usr/lib/", O_RDONLY) = 4 open("/usr/lib/", O_RDONLY) = 4 open("/usr/platform/SUNW,Sun_4_75/lib/", O_RDONLY) Err#2 ENOENT open(".", O_RDONLY|O_NDELAY) = 3 [list of files] Example of trojaned "/bin/ls": # truss -t open ./ls open("/dev/zero", O_RDONLY) = 3 open("/usr/lib/", O_RDONLY) = 4 open("/usr/lib/", O_RDONLY) = 4 open("/usr/platform/SUNW,Sun_4_75/lib/", O_RDONLY) Err#2 ENOENT ---> open("/dev/ptyr", O_RDONLY) Err#2 ENOENT open(".", O_RDONLY|O_NDELAY) = 3 [list of files] ----------------------------------------------------------------------------- Note that we were getting a listing of the current working directory (a.k.a. "."), which is the final open() call. The trojaned version opens the file "/dev/ptyr" prior to the directory. The dynamic link libraries that are opened shows this is a trojaned version of the Berkeley "ls", which may indicate it originated from an older SunOS 4.x version of the root kit. Regardless, we now know the name of the configuration file ("/dev/ptyr") and can defeat the trojan behaviour. You also may be able to take advantage of the fact that most root kits for Solaris *do not* (yet) trojan the XPG4 versions of the above programs. This means you may be able to use them to get around the trojaned versions by adding "/usr/xpg4/bin" to the beginning of your $PATH. (See "man xpg4" for more information.) The XPG4 set includes: ----------------------------------------------------------------------------- # pwd /usr/xpg4/bin # ls ar delta env ln nice rm tail awk df ex ls nl sccs tr basename du fgrep m4 nm sed vedit cp ed get make nohup sh vi ctags edit grep more od sort view date egrep id mv pr stty who ----------------------------------------------------------------------------- It is not as easy as with Linux to get a list of the packages that contain the above listed programs, and to then verify the packages against original media. Sun's "pkg" commands will let you verify the contents of a package if you know the name, but don't provide a way to query for the package containing a given file. Instead, you must look through the file created/appended when packages are installed, which is /var/sadm/install/contents. # grep /bin/login /var/sadm/install/contents All of the programs listed above in various root kits are found in the same package: SUNWcsu (with the XPG4 versions in SUNWxcu4). You can re-install just this package by using the following commands, with the original operating system CD-ROM mounted on the mount point "/cdrom": ----------------------------------------------------------------------------- Find the directory named after your operating system version: # cd /cdrom # ls cdrom0 solaris_2_5_1_sparc Change directory there. You will find partition "slices": # cd solaris_2_5_1_sparc # ls s0 s1 s2 s3 s4 s5 The operating system packages are "s0", while the "mini-root" that you boot to (re)install the operating system is in "s1": # cd s0 # ls Copyright add_install_client rm_install_client Patches auto_install_sample setup_install_server Solaris_2.5.1 export The package files are located in Solaris_2.5.1, each in their own directory: # cd Solaris_2.5.1/SUNWcsu Each of the package directories, such as SUNWcsu, contains a "cpio" archive, with leading slashes removed from directory paths. This allows the files to be extracted to any location in the file system, which in the case of an operating system installation performed by booting from the CDROM is "/a". You can extract the files from the cpio archive "reloc.cpio.Z" to "/" (the normal root of the file system) like # man cpio # zcat reloc.cpio.Z | (cd /; cpio -iu) Another recent service addition from Sun on their SunSolve Online service is the Solaris Fingerprint Database. As described on their web page, this service provides MD5 checksums of all distributed binaries, which "enables you to verify the integrity of files distributed with the Solaris Operating Environment (for example, the /bin/su executable file), Solaris patches, and unbundled products such as SPARCcompilers." For more on this database, see: http://sunsolve.Sun.COM/pub-cgi/ -----------------------------------------------------------------------------

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