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.
ICMP Shell Backdoors
Ping is one of the most common ways to find out if a machine is alive by sending and receiving ICMP packets. Many firewalls allow outsiders to ping internal machines. An intruder can put data in the Ping ICMP packets and tunnel a shell between the pinging machines. An administrator may notice a flurry of Ping packets, but unless the administrator looks at the data in the packets, an intruder can be unnoticed.
