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Network Security

MET CS-625 Unit 6

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Objectives

• Discuss security tradeoffs• Examine specific threats in an

enterprise network• Discuss development of a site security

policy

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Changes in security requirements

• It used to be that computers were kept in locked rooms

• Often users did not even have an account on the computer

• Now everyone has a computer that attaches to a network of secure and insecure devices

• When attached to the Internet the machine is potentially open to anyone in the world

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The Orange Book

• Government publication describing security of computing systems

• Uses a gradation of security from D (insecure ie DOS) to A1 (super secure)

• Security is not black and white...it is a continuum

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Tradeoffs in security design

• Services offered versus level of security• Ease of use versus security• Cost of security versus cost of loss

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Extending the definition of security

• Security doesn’t only mean theft or compromise of data

• Can also mean complete loss of data or assets – Example: A fire destroys your disk farm

• For our discussion we’ll assume that the earthquake won’t hit

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Why have a security policy?

• Inform users of the requirements for protecting information and assets

• Spell out procedures mechanisms to meet the requirements

• Provides a baseline to audit the site for compliance

• May also include an appropriate use policy

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Physical threats

• Orange Book A1 security requires a machine in a vault with no external connections...including power

• It points out that if someone can touch your machine, it can be compromised

• This extends to all components of the network...switches, phone lines, etc

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Social engineering

• Kevin Mitnick testified before Congress that most of his hacker work was accomplished without the use of technology

• Users are surprisingly naive when confronted by technology

• Humans tend to fall into easily discernible patterns

• Part of the security policy must acknowledge and plan for this

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Network threats

• Any time a computer is connected to a network it is open to a variety of threats

• We’ll look at three broad categories– Unauthorized access– Impersonation– Denial of service

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Unauthorized access

• Intruder gains access to information• Hardest to protect against• Many avenues

– Social engineering– Packet snooping– Eavesdropping

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Impersonation

• Related to unauthorized access• Means the ability to present credentials

to make it seem you are someone or something you are not

• Spoofing• Replay

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Sequence number attack

• TCP packets use a sequence number that increments in a known, non-random way for identification of a connection

• By making valid connections to a machine it may be possible to deduce the next sequence

• Attacker then starts sending packets to server with valid sequence numbers (possibly using DoS attack on some other machine)

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Session hijacking

• Intruder monitors traffic between two machines

• Captures packets• Starts to send packets with authorized

machine’s credentials• Somewhat easier than sequence

number attack

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Denial of service (DoS)

• Purpose: prevent use of a network resource

• Many scenarios• Most rely on incapacitating a server

with an overload of traffic• Often very difficult to trace

– SYN– Ping of Death– SMURF

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SYN attack (LAND.C)

• Not used much anymore due to updates in server code

• Send forged packets to server with the server’s address in the destination field

• Standard SYN flood generates multiple SYN requests to a server using bogus destination addresses– Server must allocate buffers for each

connection– Eventually memory is exhausted

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Ping of death

• Max size of an IP packet is 64k• However packets may be fragmented• Fragments rely on sequence numbers

and offsets• Using an offset with multiple large IP

packets can result in buffer overflows and server crashes

• This one is extremely difficult to trace

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SMURF

• Smurfing also uses ICMP Echo packets (pings)

• In this attack the destination and source addresses are forged to be broadcast addresses

• In a poorly protected network this may generate a cascade of thousands of echo responses for each individual smurf packet

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teardrop.c

• Teardrop attacks use IP fragment vulnerabilities

• Ping of Death simply sends an extremely large IP packet

• Teardrop sends fragments that overlap• Result: Server crash

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SMTP/Email attacks

• Bombing– Multiple identical messages to a single

recipient

• Spamming– Multiple messages to multiple recipients

• Filters are useful in preventing an attack, however spam continues to be used as a marketing tool

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Session replay

• Record an entire TCP/IP stream• Modify the stream• Replay it

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Cookie Poisoning

• Analyze the format of data stored in a cookie– Not all sites encrypt data

• Modify cookie• Log back on to site• Ex: LastPageVisited%2Fkidpub%2Fschools

%2Fkidpub-schools-directory.phpwww.kidpub.com/1024249897292829604757327755579229604673*AuthorIDP3c2236b9bc029www.kidpub.com/102462152012829604682327735579229604673*

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Parameter tampering

• Change parameters in URL request strings– Ex:

http://www.com/order?price=9.99&item=1234

• Can also examine hidden fields in forms• Simple to avoid by using POST instead

of GET in http sources

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Buffer overflows

• Attacker crafts code that overwrites a portion of stack

• Code replaces return address on stack with one attacker chooses

• Return address point either to Attacking code or somewhere else malicious

• Results can be crash or control

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Cross-site scripts

• Insert script code (such as JavaScript) into form fields

• Script is executed on the browser• Social engineering attack• To avoid, use server-side parsing of

inputs (data validation)

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Code injection

• Pass extra SQL commands on http request string

• Ex: http://my .com/getCart?SQL= ‘select creditCard from master where ID=12345’

• Mod: http://my .com/getCart?SQL= ‘select creditCard from master where ID=12345’+ ’OR ID=*’

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File enumeration

• Examine source code and site to find file names, directories, etc

• Use files to determine if site is vulnerable to other attack modes

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Forceful browsing

• Access site pages out of order• May be able to bypass security checks• Data validation may also be weak on

pages deep in site• Can be used with other attacks such as

parameter tampering

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Other vulnerabilities

• Weak encryption• Open access to admin pages• Information leakage• Access to logs

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Way to avoid problems

• Practice least privilege– Users get no more access than what they

need to do job

• Secure defaults• Validate all data from external sources

– Data are called ‘tainted’ if from outside

• Prevent information leakage• Defense in depth

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Application layer options

• SHTML• PGP• Block encryption

– 3DES– Blowfish– IDEA– RC5

• Message digests (MD5 etc)

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Hashing

• Used to provide evidence that message has not been tampered with

• No key involved• Algorithms are collision resistant• Hash algorithm is one-way

– SHA1 and MD5 are in common use

• Typically will hash an encrypted message twice– Original + encrypted

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SHTML

• Uses encryption of data stream between client and server

• Only recently has strong encryption become available in the US

• Still vulnerable to certain attacks because key exchange must happen in the clear

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Symmetric vs asymmetric cryptography

• Symmetric– Algorithm uses same key on both sides of

transaction– Keys must be exchanged in trusted manner– Rotation keys often used

• Asymmetric – Only one key is available to public– No need to exchange keys– PGP/PKI is example

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PGP

• Pretty Good Privacy• Uses private/public key encryption• Extremely strong encryption• Used both for encryption and digital

signatures• Until recent;y PGP was a controlled

technology

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Public-Key Encryption Components

• Plaintext• Encryption algorithm• Public and private key• Ciphertext• Decryption algorithm

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Public-Key Encryption Operation

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Public-Key Signature Operation

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Characteristics of Public-Key

• Computationally infeasible to determine the decryption key given knowledge of the cryptographic algorithm and the encryption key

• Either of the two related keys can be used for encryption, with the other used for decryption

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Steps in Public Key Encryption

• Each user generates a pair of keys to be used for the encryption and decryption of messages.

• Each user places one of the two keys in a public register or other accessible file. This is the public key. The companion key is kept private.

• If Bob wishes to send a private message to Alice, Bob encrypts the message using Alice's public key.

• When Alice receives the message, she decrypts it using her private key. No other recipient can decrypt the message because only Alice knows Alice's private key.

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Digital Signature Process

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RSA Encryption Algorithm

• Developed in 1977, first published in 1978• Widely accepted and implemented approach to public-key

encryption • For plaintext block M and ciphertext block C

– C = Me mod n– M = Cd mod n = (Me)d mod n = Med mod n

• Both sender and receiver must know values of n and e; only receiver knows value of d

• Public key of KU = {e, n} • Private key of KR = {d, n}.

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RSA Requirements

• It is possible to find values of e, d, n such that Med = M mod n for all M < n.

• It is relatively easy to calculate Me and Cd for all values of M < n.

• It is infeasible to determine d given e and n.– This requirement can be met with large values of e and

n

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Approaches to Defeating RSA

• Brute force approach: try all possible private keys. – The larger the number of bits in e and d, the more secure the

algorithm. – However, the larger the size of the key, the slower the system will

run.

• Cryptanalysis: factoring n into its two prime factors– A hard problem, but not as hard as it used to be– Currently, a 1024-bit key size is considered strong enough for

virtually all applications

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Key Management

• Symmetric encryption requires both parties to share a secret key

• Secure distribution of keys is the most difficult problem for symmetric encryption

• Public key encryption solves this problem, but adds the issue of authenticity

• Public key certiciates address this issue

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Public Key Certificates

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Public Key Certificate Process

1. A public key is generated by the user and submitted to Agency X for certification.

2. X determines by some procedure, such as a face-to-face meeting, that this is authentically the user’s public key.

3. X appends a timestamp to the public key, generates the hash code of the result, and encrypts that result with X’s private key forming the signature.

4. The signature is attached to the public key.

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Virtual Private Networks (VPNs)

• Internet connectivity provides easier access for telecommuters and off-site employees

• Use of a public network exposes corporate traffic to eavesdropping and provides an entry point for unauthorized users

• A variety of encryption and authentication packages and products are available to secure and authenticate remote access

• Need for a standard that allows a variety of platforms to interconnect securely

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Applications of IPSec

• Secures communications across a LAN, WANs, and/or the Internet

• Can encrypt and/or authenticate all traffic at the IP level• Examples of use:

– Secure branch office connectivity over the Internet– Secure remote access over the Internet– Establishing extranet and intranet connectivity with

partners– Enhancing electronic commerce security

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Benefits of IPSec

• When implemented in a firewall or router, provides strong security for all traffic crossing the perimeter

• IPSec in a firewall is resistant to bypass• Runs below the transport layer (TCP, UDP) and

so is transparent to applications• Can be transparent to end users• Can provide security for individual users if

needed

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IPSec Functions

• IPSec provides three main facilities– authentication-only function referred to as

Authentication Header (AH)– combined authentication/encryption

function called Encapsulating Security Payload (ESP)

– a key exchange function

• For VPNs, both authentication and encryption are generally desired

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ESP Transport and Tunneling

• Transport Mode– provides protection primarily

for upper-layer protocols. – Typically used for end-to-end

communication between two hosts

– encrypts and optionally authenticates the IP payload but not the IP header

– useful for relatively small networks; for a full-blown VPN, tunnel mode is far more efficient

• Tunnel Mode– Provides protection to

the entire packet– Original packet is

encapsulated in ESP fields, protecting contents from examination

– Used when one or both ends is a security gateway

– Multiple hosts on networks behind firewalls may engage in secure communications without implementing IPSec

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IPSec Key Management

• Manual– System administrator manually configures each

system with its own keys and with the keys of other communicating systems

– Practical for small, relatively static environments

• Automated– Enables the on-demand creation of keys for SAs and

facilitates the use of keys in a large distributed system

– Most flexible but requires more effort to configure and requires more software

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IPSec and VPNs

• Organizations need to isolate their networks and at the same time send and receive traffic over the Internet

• Authentication and privacy mechanisms of secure IP allow for security strategy

• IPSec can be implemented in routers or firewalls owned and operated by the organization, allowing the network manager complete control over security aspects of the VPN

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Transport layer

• Secure socket layer (SSL)• Secure shell (SSH)• Socket security (SOCKS)

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Network layer / Link layer

• IPSec (IP Security suite)• Cisco layer 2 forwarding protocol for

VPN• Point to point tunneling

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Creating security policies

• What are you trying to protect?• What are you protecting it from?• How likely are the threats?• Implement measures to protect your

assets• Continuously review and revise your

policy

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Resources

• www.rootshell.com• www.cert.org