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![Example DoS vulnerability [Watson’04]
! Suppose attacker can guess seq. number for an
existing connection:
Attacker can send Reset packet to
close connection. Results in DoS.
Naively, success prob. is 1/232 (32-bit seq. #’s).
Most systems allow for a large window of
acceptable seq. #’s
Much higher success probability.
! Attack is most effective against long lived
connections, e.g. BGP](https://image.slidesharecdn.com/08-tcp-dns-201019115048/75/08-tcp-dns-17-2048.jpg)
![BGP example [D. Wetherall]
! Transit: 2 provides transit for 7
! Algorithm seems to work OK in practice
BGP is does not respond well to frequent node outages
3 4
6 5
7
1
8 2
7
7
2 7
2 7
2 7
3 2 7
6 2 7
2 6 52 6 5
2 6 5
3 2 6 5
7 2 6 5
6 5
5
5](https://image.slidesharecdn.com/08-tcp-dns-201019115048/75/08-tcp-dns-23-2048.jpg)
![DNS Rebinding Attack
Read permitted: it’s the “same origin”
Firewall
www.evil.com
web server
ns.evil.com
DNS server
171.64.7.115
www.evil.com?
corporate
web server
171.64.7.115 TTL = 0
<iframe src="http://www.evil.com">
192.168.0.100
192.168.0.100
[DWF’96, R’01]
DNS-SEC cannot
stop this attack](https://image.slidesharecdn.com/08-tcp-dns-201019115048/75/08-tcp-dns-39-2048.jpg)
08 tcp-dns
- 1. Network Protocols and Vulnerabilities DanBoneh CS 155 Spring 2010
- 2. Outline ! BasicNetworking: How things work now plus some problems ! Some network attacks Attacking host-to-host datagram protocols TCP Spoofing, … Attacking network infrastructure Routing Domain Name System
- 3. Backbone ISP ISP Internet Infrastructure ! Local and interdomain routing TCP/IP for routing, connections BGP for routing announcements ! Domain Name System Find IP address from symbolic name (www.cs.stanford.edu)
- 4. TCP Protocol Stack Application Transport Network Link Applicationprotocol TCP protocol IP protocol Data Link IP Network Access IP protocol Data Link Application Transport Network Link
- 5. Data Formats Application Transport (TCP,UDP) Network (IP) Link Layer Application message - data TCP data TCP data TCP data TCP Header dataTCPIP IP Header dataTCPIPETH ETF Link (Ethernet) Header Link (Ethernet) Trailer segment packet frame message
- 6. Internet Protocol ! Connectionless Unreliable Best effort ! Notes: src and dest ports not parts of IP hdr IP Version Header Length Type of Service Total Length Identification Flags Time to Live Protocol Header Checksum Source Address of Originating Host Destination Address of Target Host Options Padding IP Data Fragment Offset
- 7. IP Routing ! Internetrouting uses numeric IP address ! Typical route uses several hops Meg Tom ISP Office gateway 121.42.33.12 132.14.11.51 Source Destination Packet 121.42.33.12 121.42.33.1 132.14.11.51 132.14.11.1
- 8. IP Protocol Functions(Summary) ! Routing IP host knows location of router (gateway) IP gateway must know route to other networks ! Fragmentation and reassembly If max-packet-size less than the user-data-size ! Error reporting ICMP packet to source if packet is dropped ! TTL field: decremented after every hop Packet dropped f TTL=0. Prevents infinite loops.
- 9. Problem: no srcIP authentication ! Client is trusted to embed correct source IP Easy to override using raw sockets Libnet: a library for formatting raw packets with arbitrary IP headers ! Anyone who owns their machine can send packets with arbitrary source IP … response will be sent back to forged source IP Implications: (solutions in DDoS lecture) Anonymous DoS attacks; Anonymous infection attacks (e.g. slammer worm)
- 10. User Datagram Protocol ! Unreliable transport on top of IP: No acknowledgment No congenstion control No message continuation UDP
- 11. Transmission Control Protocol ! Connection-oriented, preserves order Sender Break data into packets Attach packet numbers Receiver Acknowledge receipt; lost packets are resent Reassemble packets in correct order TCP Book Mail each page Reassemble book 19 5 1 1 1
- 12. TCP Header Source PortDest port SEQ Number ACK Number Other stuff U R G P S R A C K P S H S Y N F I N TCP Header
- 13. Review: TCP Handshake CS SYN: SYN/ACK: ACK: Listening Store SNC , SNS Wait Established SNC←randC ANC←0 SNS←randS ANS←SNC SN←SNC+1 AN←SNS Received packets with SN too far out of window are dropped
- 14. Basic Security Problems 1.Network packets pass by untrusted hosts Eavesdropping, packet sniffing Especially easy when attacker controls a machine close to victim 2. TCP state can be easy to guess Enables spoofing and session hijacking 3. Denial of Service (DoS) vulnerabilities DDoS lecture
- 15. 1. Packet Sniffing ! Promiscuous NIC reads all packets Read all unencrypted data (e.g., “wireshark”) ftp, telnet (and POP, IMAP) send passwords in clear! Alice Bob Eve Network Prevention: Encryption (next lecture: IPSEC) Sweet Hall attack installed sniffer on local machine
- 16. 2. TCP ConnectionSpoofing ! Why random initial sequence numbers? (SNC , SNS ) ! Suppose init. sequence numbers are predictable Attacker can create TCP session on behalf of forged source IP Breaks IP-based authentication (e.g. SPF, /etc/hosts ) Victim Server SYN/ACK dstIP=victim SN=server SNS ACK srcIP=victim AN=predicted SNS command server thinks command is from victim IP addr attacker TCP SYN srcIP=victim
- 17. Example DoS vulnerability[Watson’04] ! Suppose attacker can guess seq. number for an existing connection: Attacker can send Reset packet to close connection. Results in DoS. Naively, success prob. is 1/232 (32-bit seq. #’s). Most systems allow for a large window of acceptable seq. #’s Much higher success probability. ! Attack is most effective against long lived connections, e.g. BGP
- 18. Random initial TCPSNs ! Unpredictable SNs prevent basic packet injection … but attacker can inject packets after eavesdropping to obtain current SN ! Most TCP stacks now generate random SNs Random generator should be unpredictable GPR’06: Linux RNG for generating SNs is predictable Attacker repeatedly connects to server Obtains sequence of SNs Can predict next SN Attacker can now do TCP spoofing (create TCP session with forged source IP)
- 19.
- 20. Routing Vulnerabilities ! Commonattack: advertise false routes Causes traffic to go though compromised hosts ! ARP (addr resolution protocol): IP addr -> eth addr Node A can confuse gateway into sending it traffic for B By proxying traffic, attacker A can easily inject packets into B’s session (e.g. WiFi networks) ! OSPF: used for routing within an AS ! BGP: routing between ASs Attacker can cause entire Internet to send traffic for a victim IP to attacker’s address. Example: Youtube mishap (see DDoS lecture)
- 21. Interdomain Routing connected groupof one or more Internet Protocol prefixes under a single routing policy (aka domain) OSPF BGP Autonomous System earthlink.net Stanford.edu
- 22. BGP overview ! Iterativepath announcement Path announcements grow from destination to source Packets flow in reverse direction ! Protocol specification Announcements can be shortest path Not obligated to use announced path
- 23. BGP example [D.Wetherall] ! Transit: 2 provides transit for 7 ! Algorithm seems to work OK in practice BGP is does not respond well to frequent node outages 3 4 6 5 7 1 8 2 7 7 2 7 2 7 2 7 3 2 7 6 2 7 2 6 52 6 5 2 6 5 3 2 6 5 7 2 6 5 6 5 5 5
- 24. Issues ! Security problems Potential for disruptive attacks BGP packets are un-authenticated Attacker can advertise arbitrary routes Advertisement will propagate everywhere Used for DoS and spam (detailed example in DDoS lecture) ! Incentive for dishonesty ISP pays for some routes, others free
- 25.
- 26. Domain Name System ! Hierarchical Name Space root edunetorg ukcom ca wisc ucb stanford cmu mit cs ee www DNS
- 27. DNS Root NameServers ! Hierarchical service Root name servers for top-level domains Authoritative name servers for subdomains Local name resolvers contact authoritative servers when they do not know a name
- 28. DNS Lookup Example Client LocalDNS resolver root & edu DNS server stanford.edu DNS server www.cs.stanford.edu NS stanford.eduwww.cs.stanford.edu NS cs.stanford.edu cs.stanford.edu DNS server DNS record types (partial list): - NS: name server (points to other server) - A: address record (contains IP address) - MX: address in charge of handling email - TXT: generic text (e.g. used to distribute site public keys (DKIM) )
- 29. Caching ! DNSresponses are cached Quick response for repeated translations Useful for finding servers as well as addresses NS records for domains ! DNS negative queries are cached Save time for nonexistent sites, e.g. misspelling ! Cached data periodically times out Lifetime (TTL) of data controlled by owner of data TTL passed with every record
- 30. DNS Packet ! Query ID: 16 bit random value Links response to query (from Steve Friedl)
- 31. Resolver to NSrequest
- 32. Response to resolver Responsecontains IP addr of next NS server (called “glue”) Response ignored if unrecognized QueryID
- 33. Authoritative response toresolver final answer bailiwick checking: response is cached if it is within the same domain of query (i.e. a.com cannot set NS for b.com)
- 34. Basic DNS Vulnerabilities ! Users/hosts trust the host-address mapping provided by DNS: Used as basis for many security policies: Browser same origin policy, URL address bar ! Obvious problems Interception of requests or compromise of DNS servers can result in incorrect or malicious responses e.g.: hijack BGP route to spoof DNS Solution – authenticated requests/responses Provided by DNSsec … but no one uses DNSsec
- 35. DNS cache poisoning(a la Kaminsky’08) ! Victim machine visits attacker’s web site, downloads Javascript user browser local DNS resolver Query: a.bank.com a.bank.com QID=x1 attackerattacker wins if ∃j: x1 = yj response is cached and attacker owns bank.com ns.bank.com IPaddr 256 responses: Random QID y1, y2, … NS bank.com=ns.bank.com A ns.bank.com=attackerIP
- 36. If at firstyou don’t succeed … ! Victim machine visits attacker’s web site, downloads Javascript user browser local DNS resolver Query: b.bank.com b.bank.com QID=x2 attacker 256 responses: Random QID y1, y2, … NS bank.com=ns.bank.com A ns.bank.com=attackerIP attacker wins if ∃j: x2 = yj response is cached and attacker owns bank.com ns.bank.com IPaddr success after ≈ 256 tries (few minutes)
- 37. Defenses ! IncreaseQuery ID size. How? a. Randomize src port, additional 11 bits Now attack takes several hours b. Ask every DNS query twice: Attacker has to guess QueryID correctly twice (32 bits) Apparently DNS system cannot handle the load
- 38. Pharming ! DNSpoisoning attack (less common than phishing) Change IP addresses to redirect URLs to fraudulent sites Potentially more dangerous than phishing attacks No email solicitation is required ! DNS poisoning attacks have occurred: January 2005, the domain name for a large New York ISP, Panix, was hijacked to a site in Australia. In November 2004, Google and Amazon users were sent to Med Network Inc., an online pharmacy In March 2003, a group dubbed the "Freedom Cyber Force Militia" hijacked visitors to the Al-Jazeera Web site and presented them with the message "God Bless Our Troops"
- 39. DNS Rebinding Attack Readpermitted: it’s the “same origin” Firewall www.evil.com web server ns.evil.com DNS server 171.64.7.115 www.evil.com? corporate web server 171.64.7.115 TTL = 0 <iframe src="http://www.evil.com"> 192.168.0.100 192.168.0.100 [DWF’96, R’01] DNS-SEC cannot stop this attack
- 40. DNS Rebinding Defenses ! Browser mitigation: DNS Pinning Refuse to switch to a new IP Interacts poorly with proxies, VPN, dynamic DNS, … Not consistently implemented in any browser ! Server-side defenses Check Host header for unrecognized domains Authenticate users with something other than IP ! Firewall defenses External names can’t resolve to internal addresses Protects browsers inside the organization
- 41. Summary ! Coreprotocols not designed for security Eavesdropping, Packet injection, Route stealing, DNS poisoning Patched over time to prevent basic attacks (e.g. random TCP SN) ! More secure variants exist (next lecture) : IP -> IPsec DNS -> DNSsec BGP -> SBGP