Network Attack & Defense

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  • introduction
  • network attacks
    • overview
    • port-scans
    • nessus
  • summary
  • references


  • IBM: increases of network attacks

number of the attacks on networks of state institutions between july and august last yearly around 55 % risen

  • 80% of all network attacks are committed within the firewall

of protected range ComputerWorld, Januar 2002

historical outline

  • 1971 John Draper find out that a toy whistle from a Muesli box

reproduces exactly the clay/tone that a free voice grade channel opens

  • 1984 in the USA are discharged the Comprehensive Crime

control act, a law that more possibilities to the secret service gives to put to credit card cheats and hackers the handicraft

  • 1986 in the USA two further laws, which concern themselves

with attacks on computer systems, are adopted: The computer Fraud and electronics Communications Privacy act

  • 1988 Robert Morris bring 6.000 computers in the internet with

a virus to the crash and to a punishment of $10.000 are condemned

  • 1994, summer Vladimir Levin, graduate of the pc. Petersburg

Universit, steal with a Russian group of hackers 10 millions $ of the Citibank. He is arrested 1995 in London.

  • 1998, 19 May members of the group of hackers of L0pht warn

of serious safety gaps. They maintain the internet in a half hour to paralyze to be able.

network attacks


  • term clarifying
    • the term network attack is legally problematic
    • The legal definition of an attack assumes this took place only if someone arrived into the network!

  • possible aggressors - hackers (private or professional)
    • classically
      • overcoming of entrance barriers
      • no destruction of data
      • no change of data
    • criminal (Cracker)
      • spying data
      • manipulation of data
      • destruction of data and systems

  • possible attack targets
    • everyone is endangers
    • everyone is a goal
    • nearly everyone was already a goal

  • goals of the aggressor
    • feigning a wrong identity
    • seeing confidential enterprise data
    • changing and falsifying data/messages
    • transfer of dangerous programs into the system
    • enterprises in discredit bring

  • motivation
    • tests of the own abilities and borders
    • monetary goals
    • revenge of quit coworkers

  • points of attack and weak points
    • a main cause for the multiplicity at safety problems in the InterNet represents the architecture in principle of the communication protocols TCP/IP and UDP
    • to using the ignorance of users
    • using safety gaps in programs, those on the attacked computer runs (e.g. Web Browser)
    • weak passwords

  • what can we do?
    • own systems and of them (normal) behavior (very much) good know!
    • on remarkablenesses concentrate, e.g.:
      • computer is unexpectedly slow
      • non removable disk is the being obvious for unexplainable reasons fully
      • first examines
    • calm remains; Panic causes errors
    • be prepared!
    • do allways
      • uninstall
      • switching off
      • configuring
      • patching
      • virus protection
      • user behaviors:
        • optimal: users do not install software
        • for minimum requirement: training of the users: software from the internet saves risks

with attention of these measures is avoidable over 99% all "Hacks"!

network analysis

  • data security consider!
  • only within the own range permits!
  • difficult with purposeful masking
  • motivation:
    • weak points recognize:
  • missing Patches
  • unsatisfactory configuration
  • foreigner software (Filesharing Tools, Serv u)
    • abuse/intruders recognize
  • possibilities:
    • port-scan
      • search for services, which are attainable over the net
    • computer scan
      • attempt, over the net additional information about the configuration computers to
    • network monitorings receive
      • monitoring and analysis of connections


  • in principle: each program on a computer, which is to accept connections from the outside, must open a haven before (status: Listening)
  • open ones of haven can be recognized
  • which be attainable should, are also visibly
  • problem: Some haven under certain conditions are only activated
  • which haven? (purposefully at particulars of haven, "waving Known" haven, haven rank (theoretical 1-65535))
  • with what?
    • freeware haven scanner: NMAP
    • recognizing the haven status
    • the behavior of the computer on detailed packages depends on the operating system, IP stack and the applications

  • OS finger print (result can supply reference)
  • Suspekte computer locally to examine
    • opened port lists
    • which process keeps which haven open?
    • interesting are doubtful haven with status "LISTENING"
  • local analysis potentially compromised computer

computer analysis - nessus

  • reaction of a computer when responding a port
  • OS finger prints
  • analysis of weak points (missing Patches/unsatisfactory configuration)
  • with what?

  • freeware scanner NESSUS (
    • two-piece:
      • server ("Daemon") on Unix
      • Client on Unix or Windows 32
  • NESSUS be based on Nmap, has however additional functions
    • scanning on safety gaps
    • contains many prefabricated "Plugins"
    • script language for supplementing own Plugins
    • NESSUS scans not only the system to open ports, it tries also, dependent on the selected scanmethod, to recognize existing weak points of the individual server services
    • with the option "intensively" attacks on the target computer are implemented, which can lead also to system crashes

  • evaluation of the Nessus results:
    • doubtful
    • knowledge about scanned computers is necessary
    • often false alarm, results
    • results can be stored in data base
    • comparison to earlier results or same computer configuration verifies classification of importance possible and meaningfully

  • Negative result does not mean that the computer is safe!


  • there is no chance to be save
  • but you can be close to