Network Security7-1 Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall

Network Security 7-1

Firewalls

isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.

firewall

administerednetwork

publicInternet

firewall


Firewalls: WhyPrevent denial of service attacks:

Denial-of-Service (DoS) attack:• Send many fake requests to congest link or

consume server resource (CPU, memory) SYN flooding:

• attacker sends many SYNs to victim; victim has to allocate connection resource; victim has no resource left for real connection requests any more.

• Usually with spoofed source IP address

Prevent illegal modification/access of internal data. e.g., attacker replaces CIA’s homepage with

something else


Firewalls: Why

Allow only authorized access to inside network Set of authenticated users Set of authorized IP addresses

Two types of firewalls: application-level

• Checking application level data packet-filtering

• Checking TCP or IP packets only


Packet Filtering

internal network connected to Internet via router firewall

router filters packet-by-packet, decision to forward/drop packet based on: source IP address, destination IP address TCP/UDP source and destination port numbers ICMP message type TCP SYN and ACK bits

Should arriving packet be allowed

in? Departing packet let out?


Packet Filtering

Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. All incoming and outgoing UDP flows and

telnet connections are blocked. Example 2: Block inbound TCP segments with

SYN=1. Prevents external clients from making TCP

connections with internal clients, but allows internal clients to connect to outside.

Example of Windows XP service pack 2 firewall


Application gateways

Filters packets on application data as well as on IP/TCP/UDP fields.

Example: allow select internal users to telnet outside.

host-to-gatewaytelnet session

gateway-to-remote host telnet session

applicationgateway

router and filter

1. Require all telnet users to telnet through gateway.2. For authorized users, gateway sets up telnet

connection to dest host. Gateway relays data between 2 connections

3. Router filter blocks all telnet connections not originating from gateway. Example: block user access to know porn websites Check if the Web URL is in a “black-list”


Limitations of firewalls and gateways

IP spoofing: router can’t know if data “really” comes from claimed source SYN flood attack UDP traffic

client software must know how to contact application gateway. e.g., must set IP

address of proxy in Web browser

Speed constraint on high-bandwidth link Application-level firewall

is time consuming

filters often use all or nothing policy for UDP Usually most incoming

UDP ports are blocked The trouble caused to

real-time Internet video


Limitations of firewalls and gateways

tradeoff: degree of communication with outside world, level of security

Trend --- remote office Blurred boundary between inside <->

outside Employee laptop threat

many highly protected sites still suffer from attacks


Internet security threatsMapping:

before attacking: “case the joint” – find out what services are implemented on network

Use ping to determine what hosts have addresses on network

Port-scanning: try to establish TCP connection to each port in sequence (see what happens)

nmap (http://www.insecure.org/nmap/) mapper: “network exploration and security auditing”

Countermeasures?


Internet security threats

Mapping: countermeasures record traffic entering network look for suspicious activity (IP addresses,

pots being scanned sequentially) Firewall to block incoming TCP/SYN to ports

or computers not providing the services Block ping traffic


Internet security threatsPacket sniffing:

broadcast media promiscuous NIC reads all packets passing by can read all unencrypted data (e.g. passwords) e.g.: C sniffs B’s packets

A

B

C

src:B dest:A payload

Countermeasures?


Internet security threatsPacket sniffing: countermeasures

all hosts in orgnization run software that checks periodically if host interface in promiscuous mode.

one host per segment of broadcast media (switched Ethernet at hub)

A

B

C



Internet security threatsIP Spoofing:

can generate “raw” IP packets directly from application, putting any value into IP source address field

receiver can’t tell if source is spoofed e.g.: C pretends to be B

A

B

C


Countermeasures?


Internet security threatsIP Spoofing: egress filtering

routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network)

great, but egress filtering can not be mandated for all networks

A

B

C



Internet security threatsDenial of service (DOS):

flood of maliciously generated packets “swamp” receiver Distributed DOS (DDOS): multiple coordinated sources swamp

receiver e.g., C and remote host SYN-attack A

A

B

C

SYN

SYNSYNSYN

SYN

SYN

SYN

Countermeasures?


Internet security threatsDenial of service (DOS): countermeasures

filter out flooded packets (e.g., SYN) before reaaching host• Cooperation with source routers• Detect spoofed SYN based on TTL values

traceback to source of floods (most likely an innocent, compromised machine)

A

B

C

SYN

SYNSYNSYN

SYN

SYN

SYN


Secure e-mail (suppose K+B

known)

Alice: generates random symmetric private key, KS. encrypts message with KS (for efficiency) also encrypts KS with Bob’s public key. sends both KS(m) and KB(KS) to Bob.

Alice wants to send confidential e-mail, m, to Bob.

KS( ).

KB( ).+

+

KS(m

)

KB(KS )+

m

KS

KB+

Internet

KS


Secure e-mail (suppose K+B

known)

Bob: uses his private key to decrypt and recover KS

uses KS to decrypt KS(m) to recover m

Alice wants to send confidential e-mail, m, to Bob.

KS( ).

KB( ).+

+ -

KS(m

)

KB(KS )+

m

KS

KS

KB+

Internet

KS( ).

KB( ).-

KB-

KS

mKS(m

)

KB(KS )+


Secure e-mail (continued)

• Alice wants to provide sender authentication message integrity.

• Alice digitally signs message.• sends both message (in the clear) and digital signature.

H( ). KA( ).-

+ -

H(m )KA(H(m))-

m

KA-

Internet

m

KA( ).+

KA+

KA(H(m))-

mH( ). H(m )

compare


Secure e-mail (continued)

• Alice wants to provide secrecy, sender authentication, message integrity.

Alice uses three keys: her private key, Bob’s public key, newly created symmetric key

H( ). KA( ).-

+

KA(H(m))-

m

KA-

m

KS( ).

KB( ).+

+

KB(KS )+

KS

KB+

Internet

KS


Pretty good privacy (PGP)

Internet e-mail encryption scheme, de-facto standard.

uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.

provides secrecy, sender authentication, integrity.

inventor, Phil Zimmerman, was target of 3-year federal investigation.

---BEGIN PGP SIGNED MESSAGE---Hash: SHA1

Bob:My husband is out of town tonight.Passionately yours, Alice

---BEGIN PGP SIGNATURE---Version: PGP 5.0Charset: noconvyhHJRHhGJGhgg/

12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2

---END PGP SIGNATURE---

A PGP signed message:


Secure sockets layer (SSL)

transport layer security to any TCP-based app using SSL services.

used between Web browsers, servers for e-commerce (https).

security services: server authentication data encryption client authentication

(optional)

server authentication: SSL-enabled browser

includes public keys for trusted CAs.

Browser requests server certificate, issued by trusted CA.

Browser uses CA’s public key to extract server’s public key from certificate.

check your browser’s security menu to see its trusted CAs.


SSL (continued)

Encrypted SSL session: Browser generates

symmetric session key, encrypts it with server’s public key, sends encrypted key to server.

Using private key, server decrypts session key.

Browser, server know session key All data sent into TCP

socket (by client or server) encrypted with session key.

SSL: basis of IETF Transport Layer Security (TLS).

SSL can be used for non-Web applications, e.g., IMAP.

Client authentication can be done with client certificates. Not widely used since

too many clients


How SSL works?

K B+

ClientServer B

time

Three-way handshake

Request server certificate

K-CA(K+

B)

K+B(KA-B)

KA-B(m)

Symmetric session key

Certificate from CA


IPsec: Network Layer Security Network-layer secrecy:

sending host encrypts the data in IP datagram Applicable toTCP and UDP segments; ICMP and SNMP

messages. IP header in clear text, the other is in encrypted text

Network-layer authentication destination host can authenticate source IP address Also use a similar public key authority for public key

distribution


IEEE 802.11 security

Packet sniffing is unavoidable War-driving: drive around Bay area, see what 802.11

networks available? More than 9000 accessible from public roadways 85% use no encryption/authentication packet-sniffing and various attacks easy!

Wired Equivalent Privacy (WEP): authentication as in protocol ap4.0 (require shared symmetric key) host requests authentication from access point access point sends 128 bit nonce host encrypts nonce using shared symmetric key access point decrypts nonce, authenticates host


802.11 WEP Security Concern

40 bits in encryption is too short RC4 is not properly used in 802.11 A more sure protocol is just

standardized, 802.11i


Internet Worm propagation

Find new targets IP random scanning

Compromise targets Exploit

vulnerability

Newly infected join infection army


Worm Infection Incidents

Code Red (Jul. 2001) : 360,000 infected in 14 hours Slammer (Jan. 2003) : 75,000 infected in 10 minutes

Congested parts of Internet (ATMs down…) Blaster (Aug. 2003) : 150,000 ~ 8 million infected

DDOS attack (shut down domain windowsupdate.com) Witty (Mar. 2004) : 12,000 infected in half an hour

Attack vulnerability in ISS security products Sasser (May 2004) : 500,000 infected within two days

Infection faster than human response !


Email Virus and Attacks

Email Viruses: Executable code in email attachment Social engineering trick to fool users to click

attachment• Sender is your friend (faked email header information)• Email appears to come from security, failed report, etc

Infection procedure: Set up SMTP engine Find all email addresses to send email to

• Avoid some email domains


Email Spam

You know how easy to fake sender info Money is the driving force

Sending millions of spam email costs pennies Using many compromised machines sending spam

Defense is not perfect Most email servers have spam filters

• Check your cs.ucf email header! Email users do not tolerate false alarms 10% going through will be good enough for

spammers!


Honeypot and HoneyNet

Honeypot: A honeypot is a fictitious vulnerable IT system used for the purpose of being attacked, probed, exploited and compromised Attract attack Analyze attacking code, attacking behavior Find out how to defend

HoneyNet: a network (physical/virtual) of honeypots Covering a large number of IP addresses Monitor more attacking incidents


New Attack Trend --- Botnet

Botnet: a network of infected hosts controlled by an attacker Each host is installed with “bot” Hosts can be compromised by any mechansims

• Email, worm scan, network share, malicious web… Attacks: DDoS (extortion), spam, phishing, ads abuse, new attack

bot bot

controller controller

attacker

bot

Documents

Network Security7-1 Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall