Network Attack & Defense II

topics

• viruses
• trojans
  • backdoors
  • rootkits
• worms
• how we can protect our systems
  • IDS
  • chkrootkit

viruses

• Viruses self-replicating programs that spread by inserting their own code into other files.
• They are not able to run without their host-programs.
• They can (even without having a real payload) damage binaries, because they fail to insert their code correctly.
• They are rather small in size because changing the filesize of an infected program could be noticed.
• There are several stealty approaches of viruses to hide themselves (metamorphic/polymorphic code etc.)
• In theory they sould be easy to detect, because the have to modify files. comparing checksums of infected files with original-checksums should suffice.
• Viruses consist of three parts:
  • the infector that is able to insert the viral code into other files
  • the trigger
  • the payload
• Not every virus has to have a payload, there are viruses that exist just to spread.

trojans

• Actually trojans are just programs that claim to do something useful but do have unknown side-effects.
• Trojans can be used to install any kind of malware.
• Often 'trojan' is mixed up with backdoor or rootkit

backdoors

• Once a system is compromised, the attacker may want to install some kind of backdoor to have further access

to the system.

• A simple example is: echo "1337 stream tcp nowait root /bin/sh sh">>/etc/inetd.conf
• This, of course, has several disadvantages:
  • Detectable by just viewing /etc/inetd.conf
  • All your activity is visible to the administrator.
  • netstat -an will show port 1337 is listening (and that's suspicous :) )

rootkits

There are more stealthy ways of backdooring a system. The idea is simple: when you've got root on a system you can almost do everything you want to:

• Modify all programs like ps, top, w etc., so that your activity is hidden
• Or even better: modify the kernel to hide your activity
  • This is achieved by an lkm which intercepts some system-calls (or replaces some functions in the vfs-layer)
  • Such a modification can be very hard to detect because you can't rely on what the kernel is telling you.

worms

• Worms spread in networks and don't 'infect' files.
• They exploit a known vulnerability in software and
• can run without any host-program.
• Worms can create a big deal of network-traffic.
• Size and complexity of worms differs (from one udp-packet up to several KB perl)

how we can protect our systems

• When looking from outside the compromised system, it's theoretically possible to identify any malware.
• Of course that's inefficient, but it shows, that there is no 'perfect' malware.
• The running systems behaviour can give us some hints if anything's wrong. From inside the system we can
  • use virus-scanners to find (and sometimes eiliminate) viruses
  • check our logfiles regulary (maybe using a log-analyzer)
  • scan for signs of rootkits
• We can also use an IDS (on a seperate machine!) to monitor network-traffic

ids

• An ids can be really helpful: There are some generic and common patterns in many network-attacks.
  • For example: An http-request containing something like system(foo) can be logged (doesn't mean, that it has to be dangerous, but we can analyze that later).
  • Any packets containing many \x90's followed by some machine-code can be logged for further analysis
• An ids can't help against viruses (that don't spread in networks on their own).

chkrootkit

• chkrootkit is a shellscript that checks for signs of rootkits (and backdoors). For example:
  • suspicious lines in /etc/inetd.conf
  • promiscious-flag of network-devices
  • replaced files (ps, top, w etc.)
  • various sniffers' logs
  • suspicious listening ports
  • rootkit-related kernel-symbols

summary

• Don't rely on tools like chkrootkit or your ids. They can be fooled if the attacker knows what tools you are using.
• Read the relevant logfiles!
• Monitor system-activity
• Use an ids and tools like chkrootkit

references

• Silvio Cesare 'Unix viruses': http://vx.netlux.org/lib/vsc02.html
• Adore rootkit: http://stealth.openwall.net/rootkits/
• Snort ids: http://www.snort.org
• chkrootkit: http://www.chkrootkit.org