Network SecurityNetwork Security

Jasleen Kaur

OutlineOutline

• Information security• Attacks, services and mechanisms• Security attacks• Security services• Methods of Defense• A model for Internetwork Security• Internet standards and RFCs

Information Security

“Computer Security”Has undergone two major changes:• Timesharing systems: multiple users share

the H/W and S/W resources on a computer.• Remote login is allowed over phone lines.

Measures & tools to protect data and thwart hackers is called

Computer Security

Information Security…

“Network Security”Computer networks are widely used to

connect computers at distant locations.Raises additional security problems:• Data in transmission must be protected.• Network connectivity exposes each

computer to more vulnerabilities.

Attacks, Services, MechanismsThree aspects of Information Security:• Security Attack: Any action that

compromises the security of information.• Security Mechanism: A mechanism that is

designed to detect, prevent, or recover from a security attack.

• Security Service: A service that enhances the security of data processing systems and information transfers. A security service makes use of one or more security mechanisms.

Security AttacksSecurity Attacks

Security AttacksSecurity Attacks

Interruption: An asset of the system is destroyed or becomes unavailable or unusable. •This is an attack on availability.Examples:•Destroying some H/W (disk or wire).•Disabling file system.•Swamping a computer with jobs or communication link with packets.

Security AttacksSecurity Attacks

Interception: An unauthorized party gains access to an asset.•This is an attack on confidentiality.Examples:•Wiretapping to capture data in a network.•Illicitly copying data or programs.

Security AttacksSecurity Attacks

Modification: An unauthorized party gains access and tampers an asset. •This is an attack on integrity.Examples:•Changing data files.•Altering a program.•Altering the contents of a message.

Security AttacksSecurity Attacks

Fabrication: An unauthorized party inserts a counterfeit object into the system.•This is an attack on authenticity.Examples:•Insertion of records in data files.•Insertion of spurious messages in a network. (message replay).

Passive Vs Active Attacks

1. Passive Attacks:• Eavesdropping on information without

modifying it. (difficult to detect ).2. Active Attacks:• Involve modification or creation of

information.

Passive Vs Active Attacks

Passive Threats

• Release of message contents: Contents of a message are read.– A message may be carrying sensitive or

confidential data.

• Traffic analysis: An intruder makes inferences by observing message patterns.– Can be done even if messages are encrypted.– Inferences: location and identity of hosts.

Active Threats• Masquerade: An entity pretends to be

some other entity. It may captures an authentication sequence and replays it later to impersonate the original entity.

• Replay: Involves capture of a data unit and its retransmission to produce an unauthorized effect.

• Modification of messages: A portion of a legitimate message has been altered to produce an undesirable effect.

• Denial of service: Inhibits normal use of computer & communications resources.– Flooding or swamping of computer network.

Security GoalsSecurity Goals

Integrity

Confidentiality

Availability

Methods of DefenceMethods of Defence

• Encryption• Software Controls (access limitations in a

data base, in operating system protect each user from other users)

• Hardware Controls (smartcard)• Policies (frequent changes of passwords)• Physical Controls

Common security attacks and their countermeasures

• Finding a way into the network– Firewalls

• Exploiting software bugs, buffer overflows– Intrusion Detection Systems

• Denial of Service– Ingress filtering, IDS

• TCP hijacking– IPSec

Firewalls

• Basic problem – many network applications and protocols have security problems that are fixed over time– Difficult for users to keep up with changes and

keep host secure– Solution

• Administrators limit access to end hosts by using a firewall

• Firewall is kept up-to-date by administrators

Firewalls

• A firewall is like a castle with a drawbridge– Only one point of access into the network– This can be good or bad

• Can be hardware or software– Some routers come with firewall functionality– ipfw, ipchains, pf on Unix systems, Windows XP

and Mac OS X have built in firewalls

Firewalls

Intranet

DMZInternet

Firew

all

Firew

allWeb server, email server, web proxy, etc

Intrusion Detection

• Used to monitor for “suspicious activity” on a network– Can protect against known software exploits, like

buffer overflows

• Open Source IDS: Snort, www.snort.org• Uses “intrusion signatures”

– Well known patterns of behavior• Ping sweeps, port scanning, web server indexing, OS

fingerprinting, DoS attempts, etc.

• However, IDS is only useful if contingency plans are in place to curb attacks as they are occurring

Dictionary Attack

• We can run a dictionary attack on the passwords– The passwords in /etc/passwd are encrypted

with the crypt(3) function (one-way hash)– Can take a dictionary of words, crypt() them all,

and compare with the hashed passwords

• This is why your passwords should be meaningless random junk!– For example, “sdfo839f” is a good password

• That is not my andrew password• Please don’t try it either

Denial of Service

• Purpose: Make a network service unusable, usually by overloading the server or network

• Many different kinds of DoS attacks– SYN flooding– SMURF– Distributed attacks– Mini Case Study: Code-Red

Denial of Service

• SYN flooding attack sends SYN packets with bogus source address

• Server responds with SYN ACK and keeps state about TCP half-open connection– Eventually, server memory is exhausted with this

state

• Solution: use “SYN cookies”– In response to a SYN, create a special “cookie”

for the connection, and forget everything else– Then, can recreate the forgotten information

when the ACK comes in from a legitimate connection

Denial of Service

• SMURF– Source IP address of a broadcast ping is forged– Large number of machines respond back to

victim, overloading it

• Distributed Denial of Service– Same techniques as regular DoS, but on a much

larger scale

Denial of Service

• How can we protect ourselves?– Ingress filtering

• If the source IP of a packet comes in on an interface which does not have a route to that packet, then drop it

• RFC 2267 has more information about this

– Stay on top of CERT advisories and the latest security patches

• A fix for the IIS buffer overflow was released sixteen days before CodeRed had been deployed!

TCP Attacks

• If an attacker learns the associated TCP state for the connection, then the connection can be hijacked!

• Attacker can insert malicious data into the TCP stream, and the recipient will believe it came from the original source– Example: Instead of downloading and running

new program, you download a virus and execute it

TCP Attacks

• How do we prevent this?• IPSec

– Provides source authentication, so Mr. Big Ears cannot pretend to be Alice

– Encrypts data before transport, so Mr. Big Ears cannot talk to Bob without knowing what the session key is

Conclusions

• The Internet works only because we implicitly trust one another

• It is very easy to exploit this trust• The same holds true for software• It is important to stay on top of the latest

CERT security advisories to know how to patch any security holes