Backdoors By Puerto Since the early days of intruders breaking into computers, they have tried to develop techniques or backdoors that allow them to get back into the system. In this paper, it will be focused on many of the common backdoors and possible ways to check for them. Most of focus will be on Unix backdoors with some discussion on future Windows NT backdoors. This will describe the complexity of the issues in trying to determine the methods that intruders use and the basis for administrators understanding on how they might be able to stop the intruders from getting back in. When an administrator understands how difficult it would be to stop intruder once they are in, the appreciation of being proactive to block the intruder from ever getting in becomes better understood. This is intended to cover many of the popular commonly used backdoors by beginner and advanced intruders. This is not intended to cover every possible way to create a backdoor as the possibilities are limitless. The backdoor for most intruders provide two or three main functions: Be able to get back into a machine even if the administrator tries to secure it, e.g., changing all the passwords. Be able to get back into the machine with the least amount of visibility. Most backdoors provide a way to avoid being logged and many times the machine can appear to have no one online even while an intruder is using it. Be able to get back into the machine with the least amount of time. Most intruders want to easily get back into the machine without having to do all the work of exploiting a hole to gain access. In some cases, if the intruder may think the administrator may detect any installed backdoor, they will resort to using the vulnerability repeatedly to get on a machine as the only backdoor. Thus not touching anything that may tip off the administrator. Therefore in some cases, the vulnerabilities on a machine remain the only unnoticed backdoor. Password Cracking Backdoor One of the first and oldest methods of intruders used to gain not only access to a Unix machine but backdoors was to run a password cracker. This uncovers weak passworded accounts. All these new accounts are now possible backdoors into a machine even if the system administrator locks out the intruder's current account. Many times, the intruder will look for unused accounts with easy passwords and change the password to something difficult. When the administrator looked for all the weak passworded accounts, the accounts with modified passwords will not appear. Thus the administrator will not be able to easily determine which accounts to lock out. Rhosts + + Backdoor On networked Unix machines, services like Rsh and Rlogin used a simple authentication method based on hostnames that appear in rhosts. A user could easily configure which machines not to require a password to log into. An intruder that gained access to someone's rhosts file could put a "+ +" in the file and that would allow anyone from anywhere to log into that account without a password. Many intruders use this method especially when NFS is exporting home directories to the world. These accounts become backdoors for intruders to get back into the system. Many intruders prefer using Rsh over Rlogin because it is many times lacking any logging capability. Many administrators check for "+ +" therefore an intruder may actually put in a hostname and username from another compromised account on the network, making it less obvious to spot. Checksum and Timestamp Backdoors Early on, many intruders replaced binaries with their own trojan versions. Many system administrators relied on time-stamping and the system checksum programs, e.g., Unix's sum program, to try to determine when a binary file has been modified. Intruders have developed technology that will recreate the same time-stamp for the trojan file as the original file. This is accomplished by setting the system clock time back to the original file's time and then adjusting the trojan file's time to the system clock. Once the binary trojan file has the exact same time as the original, the system clock is reset to the current time. The sum program relies on a CRC checksum and is easily spoofed. Intruders have developed programs that would modify the trojan binary to have the necessary original checksum, thus fooling the administrators. MD5 checksums is the recommended choice to use today by most vendors. MD5 is based on an algorithm that no one has yet to date proven can be spoofed. Login Backdoor On Unix, the login program is the software that usually does the password authentication when someone telnets to the machine. Intruders grabbed the source code to login.c and modified it that when login compared the user's password with the stored password, it would first check for a backdoor password. If the user typed in the backdoor password, it would allow you to log in regardless of what the administrator sets the passwords to. Thus this allowed the intruder to log into any account, even root. The password backdoor would spawn access before the user actually logged in and appeared in utmp and wtmp. Therefore an intruder could be logged in and have shell access without it appearing anyone is on that machine as that account. Administrators started noticing these backdoors especially if they did a "strings" command to find what text was in the login program. Many times the backdoor password would show up. The intruders then encrypted or hid the backdoor password better so it would not appear by just doing strings. Many of the administrators can detect these backdoors with MD5 checksums. Telnetd Backdoor When a user telnets to the machine, inetd service listens on the port and receive the connection and then passes it to in.telnetd, that then runs login. Some intruders knew the administrator was checking the login program for tampering, so they modified in.telnetd. Within in.telnetd, it does several checks from the user for things like what kind of terminal the user was using. Typically, the terminal setting might be Xterm or VT100. An intruder could backdoor it so that when the terminal was set to "letmein", it would spawn a shell without requiring any authentication. Intruders have backdoored some services so that any connection from a specific source port can spawn a shell. Services Backdoor Almost every network service has at one time been backdoored by an intruder. Backdoored versions of finger, rsh, rexec, rlogin, ftp, even inetd, etc., have been floating around forever. There are programs that are nothing more than a shell connected to a TCP port with maybe a backdoor password to gain access. These programs sometimes replace a service like uucp that never gets used or they get added to the inetd.conf file as a new service. Administrators should be very wary of what services are running and analyze the original services by MD5 checksums. Cronjob backdoor Cronjob on Unix schedules when certain programs should be run. An intruder could add a backdoor shell program to run between 1 AM and 2 AM. So for 1 hour every night, the intruder could gain access. Intruders have also looked at legitimate programs that typically run in cronjob and built backdoors into those programs as well. Library backdoors Almost every UNIX system uses shared libraries. The shared libraries are intended to reuse many of the same routines thus cutting down on the size of programs. Some intruders have backdoored some of the routines like crypt.c and _crypt.c. Programs like login.c would use the crypt() routine and if a backdoor password was used it would spawn a shell. Therefore, even if the administrator was checking the MD5 of the login program, it was still spawning a backdoor routine and many administrators were not checking the libraries as a possible source of backdoors. One problem for many intruders was that some administrators started MD5 checksums of almost everything. One method intruders used to get around that is to backdoor the open() and file access routines. The backdoor routines were configured to read the original files, but execute the trojan backdoors. Therefore, when the MD5 checksum program was reading these files, the checksums always looked good. But when the system ran the program, it executed the trojan version. Even the trojan library itself, could be hidden from the MD5 checksums. One way to an administrator could get around this backdoor was to statically link the MD5 checksum checker and run on the system. The statically linked program does not use the trojan shared libraries. Kernel backdoors The kernel on Unix is the core of how Unix works. The same method used for libraries for bypassing MD5 checksum could be used at the kernel level, except even a statically linked program could not tell the difference. A good backdoored kernel is probably one of the hardest to find by administrators, fortunately kernel backdoor scripts have not yet been widely made available and no one knows how wide spread they really are. File system backdoors An intruder may want to store their loot or data on a server somewhere without the administrator finding the files. The intruder's files can typically contain their toolbox of exploit scripts, backdoors, sniffer logs, copied data like email messages, source code, etc. To hide these sometimes large files from an administrator, an intruder may patch the files system commands like "ls", "du", and "fsck" to hide the existence of certain directories or files. At a very low level, one intruder's backdoor created a section on the hard drive to have a proprietary format that was designated as "bad" sectors on the hard drive. Thus an intruder could access those hidden files with only special tools, but to the regular administrator, it is very difficult to determine that the marked "bad" sectors were indeed storage area for ...
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