Spring 2000 John Kristoff 1

Security

Computer Networks

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What to Protect

Confidentiality snooping encryption

Integrity deletion, changes backups

Availability denial of service

attacks

Authentication are who you say you

are

Nonrepudiation no denying it

Access Control don’t touch that!

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Most Importantly

Protect your reputation

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Good Books

Network Security: PRIVATE Communication in a PUBLIC World. Kaufman, Perlman and Speciner.

Cryptography and Network Security: Principles and Practice. Stallings.

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Where to Put the Protection?

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Host Based Security

Recall End-to-End ArgumentSecurity is ultimately a host problemKey idea: protect the DATAEnd hosts are in control of dataUsers are in control of end hostsUsers can and often will do dumb thingsResult: very difficult to protect all hosts

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Network Based Security

Should augment host based securityUseful 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

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Perimeter Security

Boundary between a trusted internal network and a hostile external network

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Internal Security

Most often ignoredMost likely the problemDisgruntled employeeCurious, but dangerous employeeClueless and dangerous employee

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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

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Layered Defenses

The belt and suspenders approachMultiple layers make it harder to get

throughMultiple layers take longer to get throughBasic 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

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Physical Security

Trash binsSocial engineeringIt’s much easier to trust a face than

a packetProtect from the whoops

power spills the clumsy software really can kill hardware

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Packet Filtering Firewalls

Apply rules to incoming/outgoing packets

Based on Addresses Protocols Ports Application Other pattern match

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Packet Filtering Firewall Illustrated

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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

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Example Firewall: Cisco Router Filters

access-list 100 deny ip 192.168.0.0 0.0.255.255 anyaccess-list 100 deny ip 172.0.0.0 0.15.255.255 anyaccess-list 100 deny ip 10.0.0.0 0.255.255.255 anyaccess-list 100 deny ip 0.0.0.0 0.255.255.255 anyaccess-list 100 deny ip 127.0.0.0 0.255.255.255 anyaccess-list 100 deny ip 224.0.0.0 31.255.255.255 anyaccess-list 100 deny ip 1.2.0.0 0.0.255.255 anyaccess-list 100 permit tcp any host 1.2.3.4 eq domainaccess-list 100 permit udp any host 1.2.3.4 eq domainaccess-list 100 deny tcp any host 1.2.3.5 eq telnet logaccess-list 100 deny tcp any host 1.2.3.6 eq syn logaccess-list 100 deny ip any host 1.2.3.4access-list 100 permit ip any 1.2.0.0 0.0.255.255access-list 100 deny ip any any

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Encryption

Make a readable message unreadableMath intensivePlain text versus cipher textAlgorithms and keys

public private key size

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Encryption?

#include<stdlib.h>#include<stdio.h>main(I,O,O0,OO,l)int I,O0,OO,l;char **O;{

return !!I>=I?!I>=I?!!~I>=~I?!~I>=~I?!OO?!I:OO%2?OO=main(I,O,O0,OO>>!!OO,l),OO=main(I-!I-!!I,O,OO,OO,l),OO=main(I-!I-!!I,O,O0,OO,l),!(OO-!I||I)?l-1:OO

:(OO=main(I,O,O0,OO>>!!OO,l),!(OO-l+!I||I)?l-1:main(I-!I-!!I,O,OO,OO,l))

:(O0+OO)%l:main(I-I/I-I/I,O,O0,OO+OO/OO,

main(0,O,O0,OO,I-I-I)+I+1?1:printf("%d ",I-I-I)+fflush(stdout)):main(I-I-I-I-I,O,I+I-I+I,I,0):main(~!!I-!!I,O,atoi(1[O]),1,atoi(0[O]));

}

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Shared Secret Key

Each party knows a secretThe 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

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Shared Secret Key Illustrated

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Public Key Cryptography

Public Key Everyone can use it to encrypt

messages to youPrivate Key

Only you know this key and only it decrypts messages encrypted with your public key

Keyring

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Public Key Illustrated

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Denial of Service (DoS)

Prevents or impairs standard serviceSYN floodingSMURF attacksDistributed Denial of Service (DDoS)Most effective when source address

can be spoofedDifficult problem to solve

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Example Denial of Service Illustrated

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Example Distributed Denial of Service Illustrated

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Buffer Overflows and Weak Validation of Input

Key idea: overwriting the something on the stack

Popular exploits with CGI scriptsRegular users can gain root accessIf exploit on TCP/UDP service, remote

root can be accomplished

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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.

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Session Hijacking Illustrated

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Password Cracking

Very common todayIf 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

A good password selection strategy

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Viruses and Worms

Programs written with the intent to spreadWorms 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

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Example: Securing Routers

!version 12.0service tcp-keepalives-inservice timestamps debug datetime msec localtimeservice timestamps log datetime msec localtimeservice password-encryptionclock timezone cst -6clock summer-time cdst recurringno ip source-routeno ip fingerno ip bootp server!interface FastEthernet1/0 description backbone router ip address a.b.c.d 255.255.255.0 ip access-group 100 in no ip unreachables no ip directed-broadcast no cdp enable

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Example: Securing Routers [continued]

router rip passive-interface Serial1/0 network a.b.0.0 distribute-list 1 in Serial2/0

logging history warningslogging trap debugginglogging facility local7logging source-interface Loopback0logging a.b.c.d

access-list 1 deny anyaccess-list 10 permit a.b.c.0 0.0.0.255access-list 10 deny anyaccess-list 100 permit tcp a.b.0.0 0.0.255.255 any eq telnet logaccess-list 100 deny ip any any log

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Example: Securing Routers [continued]

snmp-server community password RO 10snmp-server location computing centersnmp-server contact Network Administratorbanner motd^CThis host is to be used by authorized personnel only!^C!line vty 0 4 exec-timeout 0 5 access-class 100 in password 7 823442561E01034A12 login transport input telnet ssh!ntp source Loopback0ntp server a.b.c.dend

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Example: Securing UNIX

Remove unnecessary UDP/TCP servers Startup scripts in /etc/rc.d directories /etc/inetd.conf

Use secure versions of servers ssh tcpwrappers

Many useful tools available Tripwire, IP Filter, ipchains, lsof, tcpwrappers, etc...

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Example: Securing Windows

Disable file/printer sharingUse virus protection softwareKeep current on latest service packsDisable unnecessary protocols

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Network Address Translation

NAT is a hack!Provides some level of security, but

with a great deal of costIf security is the only goal, avoid NATNAT has been required for sites with

IP address allocation problemsRSIP may get NAT users back on track

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NAT Illustrated

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Virtual Private Networks

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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.

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Why Is This Worthwhile?

Cost, Cost, Cost! Ability to make use of a public, insecure

network, rather than building your own private, secure network

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Challenges

Increased overheadComplexityPerformanceQualityManagement

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Oh, and One More Thing

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Some Terms/Technology Thrown Around with VPNs

IPsecPPP/PPTP/L2TP/L2FCHAP, PAPEncapsulationTunnelingAAARADIUS/TACACS/TACACS+Firewalls

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Security Tools

sshTripWiretcpwrappersIP Filter, ipchainsnmaptcpdump, windumpsyslogntp

snortlogcheck, swatchcrack, l0pftcrackkerberosPGPkerberosS/MIMESSL

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Final Thoughts

Network Address Translation Think about long term implications

Security as a end-to-end problemJava, Javascript and ActiveXCertificatesIntrusion Detection