Understaing Backdoors
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 . Simply We can say that The Milicious Programmes/Codes/Scripts That
allows Hacker to get Back into the system is called The Backdoor
Features Of backdoors
- The Attacked Will be able to get back into The Target System
- My Backdoors Are Complete Least Visible.
- Backdoors Allow Attackers to get Access To the System In Least Amount Of time.
Types Of Backdoors
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
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
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
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 the hidden file system. Bootblock backdoors In the PC world, many viruses have hid themselves within the bootblock section and most antivirus software will check to see if the bootblock has been altered. On Unix, most administrators do not have any software that checks the bootblock, therefore some intruders have hidden some backdoors in the bootblock area. Process hiding backdoors An intruder many times wants to hide the programs they are running. The programs they want to hide are commonly a password cracker or a sniffer. There are quite a few methods and here are some of the more common: An intruder may write the program to modify its own argv[] to make it look like another process name. An intruder could rename the sniffer program to a legitimate service like in.syslog and run it. Thus when an administrator does a "ps" or looks at what is running, the standard service names appear. An intruder could modify the library routines so that "ps" does not show all the processes. An intruder could patch a backdoor or program into an interrupt driven routine so it does not appear in the process table. An example backdoor using this technique is amod.tar.gz available on http://star.niimm.spb.su/~maillist/bugtraq.1/0777.html An intruder could modify the kernel to hide certain processes as well. Rootkit One of the most popular packages to install backdoors is rootkit. It can easily be located using Web search engines. From the Rootkit README, here are the typical files that get installed: z2 - removes entries from utmp, wtmp, and lastlog. Es - rokstar's ethernet sniffer for sun4 based kernels. Fix - try to fake checksums, install with same dates/perms/u/g. Sl - become root via a magic password sent to login. Ic - modified ifconfig to remove PROMISC flag from output. ps: - hides the processes. Ns - modified netstat to hide connections to certain machines. Ls - hides certain directories and files from being listed. du5 - hides how much space is being used on your hard drive. ls5 - hides certain files and directories from being listed. Network traffic backdoors Not only do intruders want to hide their tracks on the machine, but also they want to hide their network traffic as much as possible. These network traffic backdoors sometimes allow an intruder to gain access through a firewall. There are many network backdoor programs that allow an intruder to set up on a certain port number on a machine that will allow access without ever going through the normal services. Because the traffic is going to a non-standard network port, the administrator can overlook the intruder's traffic. These network traffic backdoors are typically using TCP, UDP, and ICMP, but it could be many other kinds of packets. TCP Shell Backdoors The intruder can set up these TCP Shell backdoors on some high port number possibly where the firewall is not blocking that TCP port. Many times, they will be protected with a password just so that an administrator that connects to it, will not immediately see shell access. An administrator can look for these connections with netstat to see what ports are listening and where current connections are going to and from. Many times, these backdoors allow an intruder to get past TCP Wrapper technology. These backdoors could be run on the SMTP port, which many firewalls allow traffic to pass for e-mail. UDP Shell Backdoors Administrator many times can spot a TCP connection and notice the odd behavior, while UDP shell backdoors lack any connection so netstat would not show an intruder accessing the Unix machine. Many firewalls have been configured to allow UDP packets for services like DNS through. Many times, intruders will place the UDP Shell backdoor on that port and it will be allowed to by-pass the firewall.
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