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Firewalls firewall isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. public Internet administered network firewall Network Security 7-1 Firewalls: Why Prevent 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 Network Security 7-2 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 Network Security 7-3 Packet Filtering Should arriving packet be allowed in? Departing packet let out? 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 Network Security 7-4 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 Network Security 7-5 Application gateways Filters packets on gateway-to-remote host telnet session host-to-gateway telnet session application data as well as on IP/TCP/UDP fields. Example: allow select internal users to telnet outside. application gateway 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” Network Security 7-6 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 Network Security 7-7 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 Network Security 7-8 Internet security threats Mapping: 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? Network Security 7-9 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 Network Security 7-10 Internet security threats Packet 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 C A src:B dest:A payload B Countermeasures? Network Security 7-11 Internet security threats Packet 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) C A src:B dest:A payload B Network Security 7-12 Internet security threats IP 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 C A src:B dest:A Countermeasures? payload B Network Security 7-13 Internet security threats IP 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 C A src:B dest:A payload B Network Security 7-14 Internet security threats Denial 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 C A SYN SYN SYN SYN SYN B Countermeasures? SYN SYN Network Security 7-15 Internet security threats Denial 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) C A SYN SYN SYN SYN SYN B SYN SYN Network Security 7-16 Secure e-mail (suppose K+B known) Alice wants to send confidential e-mail, m, to Bob. KS m K (.) S KS(m ) + KS + . K B( ) + Internet + KB(KS ) KB 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. Network Security 7-17 Secure e-mail (suppose K+B known) Alice wants to send confidential e-mail, m, to Bob. KS m K (.) S + KS + . K B( ) + KS(m ) KS(m ) + KB(KS ) . KS( ) - Internet + KB(KS ) KB m KS - . K B( ) - KB Bob: uses his private key to decrypt and recover KS uses KS to decrypt KS(m) to recover m Network Security 7-18 Secure e-mail (continued) • Alice wants to provide sender authentication message integrity. + - KA m H(.) - . KA( ) - - KA(H(m)) KA(H(m)) + Internet m KA + . KA( ) m H(m ) compare . H( ) H(m ) • Alice digitally signs message. • sends both message (in the clear) and digital signature. Network Security 7-19 Secure e-mail (continued) • Alice wants to provide secrecy, sender authentication, message integrity. - KA m . H( ) - . KA( ) - KA(H(m)) + KS . KS( ) + m KS + . K B( ) + Internet + KB(KS ) KB Alice uses three keys: her private key, Bob’s public key, newly created symmetric key Network Security 7-20 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. A PGP signed message: ---BEGIN PGP SIGNED MESSAGE--Hash: SHA1 Bob:My husband is out of town tonight.Passionately yours, Alice ---BEGIN PGP SIGNATURE--Version: PGP 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ hFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- Network Security 7-21 Secure sockets layer (SSL) transport layer security to any TCPbased 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. Network Security 7-22 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 Network Security 7-23 How SSL works? Three-way handshake Request server certificate K-CA(K+B) KB+ Server B Client Certificate from CA K+B(KA-B) Symmetric session key KA-B(m) time Network Security 7-24 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 Network Security 7-25 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 Network Security 7-26 802.11 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 Network Security 7-27 Internet Worm propagation Find new targets IP random scanning Compromise targets Exploit vulnerability Newly infected join infection army Network Security 7-28 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 ! Network Security 7-29 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 Network Security 7-30 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! Network Security 7-31 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 Network Security 7-32 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 attacker controller controller bot bot bot Network Security 7-33