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Computer Networks

Computer Networks

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Computer Networks

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  1. Computer Networks Security

  2. Internet no central authority end systems in control no central knowledge of connections no per-packet billing legal issues not well understood anonymity is easy Internet/Telco Comparison • Telephone System • central authority • network in control • billing records per connection • legal issues well understood • provisions for law enforcement (wiretapping)

  3. Internet Security Stinks • Hosts are hard to secure • Bad defaults • Poor software • Fixes rarely applied • Average user/administrator is clueless • An overly secure system is not useful • It’s difficult to coordinate among sites

  4. Authentication are who you say you are Nonrepudiation no denying it Access Control don’t touch that! Reputation Ensure your good name What to Protect • Confidentiality • snooping • encryption • Integrity • deletion, changes • backups • Availability • denial of service attacks

  5. Where to Put the Protection?

  6. Physical Security • Trash bins • Social engineering • It’s much easier to trust a face than a packet • Protect from the whoops • power • spills • the clumsy • software really can kill hardware

  7. Host Based Security • Recall End-to-End Argument • Security is ultimately a host problem • Key idea: protect the DATA • End hosts are in control of data • Users are in control of end hosts • Users can and often will do dumb things • Result: very difficult to protect all hosts

  8. Internal Security • Most often ignored • Most likely the problem • Disgruntled employee • Curious, but dangerous employee • Clueless and dangerous employee

  9. Security by Obscurity • Is no security at all. • However • It’s often best not to advertise unnecessarily • It’s often the only layer used (e.g. passwords) • Probably need more security

  10. Network Based Security • Should augment host based security • Useful for • Protecting groups of users from others • Prohibiting certain types of network usage • Controlling traffic flow • Difficult to inspect traffic • encryption can hide bad things • tunneling can mislead you Good book: Network Security: PRIVATE Communication in a PUBLIC World. Kaufman, Perlman and Speciner.

  11. Layered Defenses • The belt and suspenders approach • Multiple layers make it harder to get through • Multiple layers take longer to get through • Basic statistics and probability apply • If Defense A stops 90% of all attacks and Defense B stops 90% of all attacks, you might be able to stop up to 99% of all attacks • Trade-off in time, money and convenience

  12. Perimeter Security • Boundary between a trusted internal network and a hostile external network

  13. Firewall Solutions • They help, but not a panacea • A network response to a host problem • Packet by packet examination is tough • Don’t forget internal users • Need well defined borders • Can be a false sense of security • Careful not to break standard protocol mechanisms!

  14. Packet Filtering Firewalls • Apply rules to incoming/outgoing packets • Based on • Addresses • Protocols • Ports • Application • Other pattern match

  15. Packet Filtering Firewall Illustrated

  16. Screened Subnet

  17. Example Firewall: ipchains -A input -s 192.168.0.0/255.255.0.0 -d 0.0.0.0/0.0.0.0 -j DENY -A input -s 172.0.0.0/255.240.0.0 -d 0.0.0.0/0.0.0.0 -j DENY -A input -s 10.0.0.0/255.0.0.0 -d 0.0.0.0/0.0.0.0 -j DENY -A input -s 224.0.0.0/224.0.0.0 -d 0.0.0.0/0.0.0.0 -j DENY -A input -s 0.0.0.0/0.0.0.0 -d a.b.c.d/255.255.255.255 22:22 -p 6 -j ACCEPT -A input -s 0.0.0.0/0.0.0.0 -d a.b.c.d/255.255.255.255 1024:65535 -p 6 ! -y -j ACCEPT

  18. Example Firewall: Cisco Router Filters access-list 100 deny ip 192.168.0.0 0.0.255.255 any access-list 100 deny ip 172.0.0.0 0.15.255.255 any access-list 100 deny ip 10.0.0.0 0.255.255.255 any access-list 100 deny ip 0.0.0.0 0.255.255.255 any access-list 100 deny ip 127.0.0.0 0.255.255.255 any access-list 100 deny ip 224.0.0.0 31.255.255.255 any access-list 100 deny ip 1.2.0.0 0.0.255.255 any access-list 100 permit tcp any host 1.2.3.4 eq domain access-list 100 permit udp any host 1.2.3.4 eq domain access-list 100 deny tcp any host 1.2.3.5 eq telnet log access-list 100 deny tcp any host 1.2.3.6 eq syn log access-list 100 deny ip any host 1.2.3.4 access-list 100 permit ip any 1.2.0.0 0.0.255.255 access-list 100 deny ip any any

  19. Application Level Gateway

  20. Encryption • Make a readable message unreadable • Math intensive • Plain text versus cipher text • Algorithms and keys • public • private • key size

  21. Shared Secret Key • Each party knows a secret • The secret is used to decrypt the cipher text • Book: Ulysses • Page: 7 • Line: 23 • Word: 4 • Must know the book and keep it a secret

  22. Shared Secret Key Illustrated

  23. Public Key Cryptography • Public Key • Everyone can use it to encrypt messages to you • Private Key • Only you know this key and only it decrypts messages encrypted with your public key • Keyring

  24. Public Key Illustrated

  25. Exploits Overview • Passwords • hacking and sniffing • System specific • NT, UNIX, NetWare, Linux • Application specific • web browser, ftp, email, finger • Protocol specific • spoofing, TCP hijacking, ICMP redirects, DNS • Denial of Service • PING of death, trinoo, tribe flood

  26. The Process • Reconnaissance • Scanning • Exploit Systems • Keep access with backdoors/trojans • Use system • Often as a springboard • Cover any tracks

  27. Buffer Overflows and Weak Validation of Input • Key idea: overwriting the something on the stack • Popular exploits with CGI scripts • Format strings • Regular users can gain root access • If exploit on TCP/UDP service, remote root can be accomplished

  28. Network Mapping • PING • DNS mapping (don’t need zone transfer) • dig +pfset=0x2020 -x 10.x.x.x • rpcinfo -p <hostname> • nmap <http://www.insecure.org/nmap/> • very nice! • Microsoft Windows is NOT immune • nbtstat, net commands • Just look around the ‘net!

  29. Session Hijacking If you can predict sequence numbers and spoof the source address, you might be able to pretend to be one end of the session. It helps if you can keep one end of the session busy while you’re hijacking.

  30. Session Hijacking Illustrated

  31. Password Cracking • Very common today • If attacker can get a hold of the password file, they can go offline and process it • Recall • passwords are a form of obscurity • multiple defenses may be needed • Given enough time, passwords alone are probably not safe

  32. Viruses and Worms • Programs written with the intent to spread • Worms are very common today • Often email based (e.g. ILOVEYOU) • Viruses infect other programs • Code copied to other programs (e.g. macros) • All require the code to be executed • Proves users continue to do dumb things • Sometimes software is at fault too

  33. Denial of Service (DoS) • Prevents or impairs standard service • SYN flooding • SMURF attacks • Distributed Denial of Service (DDoS) • Most effective when source address can be spoofed • Difficult to differeniate between valid traffic

  34. Example Denial of Service Illustrated

  35. Example Distributed Denial of Service Illustrated

  36. DoS Solutions • Ingress/Egress filtering • ICMP Traceback • Packet Marking • Rate-limiting • Difficult to solve completely!

  37. Network Address Translation • Removes end-to-end addressing • Standardized in RFC 1918 • NAT has been bad for the Internet • Provides relatively no security with a great deal of cost - this slide shouldn't be here • NAT has been required for sites with IP address allocation problems • NAT may be used for IPv6 transition

  38. NAT Illustrated

  39. Virtual Private Networks

  40. Key Idea A session between two endpoints that is secured from eavesdroppers and all threats on the network in between, usually through the use of encryption technology.

  41. Why Is This Worthwhile? • Cost, Cost, Cost! • Ability to make use of a public, insecure network, rather than building your own private, secure network

  42. Challenges • Increased overhead • Complexity • Performance • Quality • Management

  43. Oh, and One More Thing

  44. Other Areas of Interest • IP multicast • Routing protocols • Privacy issues • IPsec • Intrusion Detection Systems (IDS)

  45. References bugtraq mailing list (see http://www.securityfocus.com) http://www.cert.org http://www.cerias.purdue.edu http://www.first.org http://packetstorm.securify.com http://www.research.att.com/~smb/ http://cm.bell-labs.com/who/ches/ http://www.denialinfo.com http://www.cs.georgetown.edu/~denning/ http://www.washington.edu/People/dad/ http://www.sans.org http://theory.lcs.mit.edu/~rivest/ http://networks.depaul.edu/security/