Internet Security: How the Internet works and some basic vulnerabilities
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1 CS 155 Internet Security: How the Internet works and some basic vulnerabilities Dan Boneh Acknowledgments: Lecture slides are from the Computer Security course taught by Dan Boneh and John Mitchell at Stanford University. When slides are obtained from other sources, a a reference will be noted on the bottom of that slide. A full list of references is provided on the last slide.
2 Internet Infrastructure ISP Backbone ISP Local and interdomain routing TCP/IP for routing and messaging BGP for routing announcements Domain Name System Find IP address from symbolic name (
3 TCP Protocol Stack Application Transport Application protocol TCP protocol Application Transport Network IP protocol IP IP protocol Network Link Data Link Network Access Data Link Link
4 Data Formats Application message TCP Header Application message - data Transport (TCP, UDP) segment TCP data TCP data TCP data Network (IP) packet IP TCP data Link Layer frame ETH IP TCP data ETF IP Header Link (Ethernet) Header Link (Ethernet) Trailer
5 IP Internet Protocol Connectionless Unreliable Best effort Version Flags Header Length Type of Service Total Length Identification Fragment Offset Notes: src and dest ports not parts of IP hdr Time to Live Protocol Header Checksum Source Address of Originating Host Destination Address of Target Host Options Padding IP Data
6 IP Routing Meg Packet Source Destination Office gateway Tom ISP Typical route uses several hops IP: no ordering or delivery guarantees
7 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 if TTL=0. Prevents infinite loops.
8 Problem: no src IP 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: Anonymous DoS attacks; (solutions in DDoS lecture) Anonymous infection attacks (e.g. slammer worm)
9 TCP 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 Book Mail each page Reassemble book
10 TCP Header (protocol=6) Source Port U R G A C K SEQ Number ACK Number P S H P S R S Y N Dest port F I N TCP Header Other stuff
11 Review: TCP Handshake C S SYN: SN C rand C AN C 0 Listening SYN/ACK: SN S rand S AN S SN C Store SN C, SN S ACK: SN SN C +1 AN SN S Wait Established Received packets with SN too far out of window are dropped
12 Basic Security Problems 1. Network packets pass by untrusted hosts Eavesdropping, packet sniffing Especially easy when attacker controls a machine close to victim (e.g. WiFi routers) 2. TCP state easily obtained by eavesdropping Enables spoofing and session hijacking 3. Denial of Service (DoS) vulnerabilities DDoS lecture
13 Why random initial sequence numbers? Suppose initial seq. numbers (SN C, SN S ) are predictable: Attacker can create TCP session on behalf of forged source IP Breaks IP-based authentication (e.g. SPF, /etc/hosts ) Random seq. num. do not prevent attack, but make it harder attacker TCP SYN srcip=victim ACK srcip=victim AN=predicted SN S Server SYN/ACK dstip=victim SN=server SN S Victim command server thinks command is from victim IP addr
14 Example DoS vulnerability: Reset Attacker sends a Reset packet to an open socket If correct SN S then connection will close DoS Naively, success prob. is 1/2 32 (32-bit seq. # s). but, many systems allow for a large window of acceptable seq. # s. Much higher success probability. Attacker can flood with RST packets until one works Most effective against long lived connections, e.g. BGP
15 Routing Security ARP, OSPF, BGP
16 Interdomain Routing (AS#32) Stanford.edu earthlink.net (AS#4355) BGP Autonomous System OSPF connected group of one or more Internet Protocol prefixes under a single routing policy (aka domain)
17 Routing Protocols ARP (addr resolution protocol): Security issues: (local network attacks) IP addr eth addr Node A can confuse gateway into sending it traffic for Node B By proxying traffic, node A can read/inject packets into B s session (e.g. WiFi networks) OSPF: used for routing within an AS BGP: routing between Autonomous Systems Security issues: unauthenticated route updates Anyone can cause entire Internet to send traffic for a victim IP to attacker s address Example: Youtube-Pakistan mishap (see DDoS lecture) Anyone can hijack route to victim (next slides)
18 BGP example [D. Wetherall]
19 Security Issues BGP path attestations are un-authenticated Anyone can inject advertisements for arbitrary routes Advertisement will propagate everywhere Used for DoS, spam, and eavesdropping (details in DDoS lecture) Often a result of human error Solutions: RPKI: AS obtains a certificate (ROA) from regional authority (RIR) and attaches ROA to path advertisement. Advertisements without a valid ROA are ignored. Defends against a malicious AS (but not a network attacker) SBGP: sign every hop of a path advertisement
20 Example path hijack (source: Renesys 2013) Feb 2013: Guadalajara Washington DC via Belarus route in effect for several hours Normally: Alestra (Mexico) PCCW (Texas) Qwest (DC) Reverse route (DC Guadalajara) is unaffected: Person browsing the Web in DC cannot tell by traceroute that HTTP responses are routed through Moscow
21 Domain Name System
22 DNS Domain Name System Hierarchical Name Space root org net edu com uk ca wisc ucb stanford cmu mit cs ee www
23 DNS Root Name Servers 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
24 DNS Lookup Example Client Local DNS resolver NS stanford.edu NS cs.stanford.edu A www=ipaddr root & edu DNS server stanford.edu DNS server 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 - TXT: generic text (e.g. used to distribute site public keys (DKIM) )
25 Caching DNS responses are cached Quick response for repeated translations Note: NS records for domains also cached 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
26 DNS Packet Query ID: 16 bit random value Links response to query (from Steve Friedl)
27 Resolver to NS request
28 Response to resolver Response contains IP addr of next NS server (called glue ) Response ignored if unrecognized QueryID
29 Authoritative response to resolver bailiwick checking: response is cached if it is within the same domain of query (i.e. a.com cannot set NS for b.com) final answer
30 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.: malicious access point in a Cafe Solution authenticated requests/responses Provided by DNSsec but few use DNSsec
31 DNS cache poisoning (a la Kaminsky 08) Victim machine visits attacker s web site, downloads Javascript user browser Query: a.bank.com local DNS resolver a.bank.com QID=x 1 response.com attacker wins if j: x 1 = y j response is cached and attacker owns bank.com 256 responses: Random QID y 1, y 2, NS bank.com=ns.bank.com A ns.bank.com=attackerip attacker
32 If at first you don t succeed Victim machine visits attacker s web site, downloads Javascript user browser Query: b.bank.com local DNS resolver b.bank.com QID=x 2 response.com attacker wins if j: x 2 = y j 256 responses: Random QID y 1, y 2, NS bank.com=ns.bank.com A ns.bank.com=attackerip attacker response is cached and attacker owns bank.com success after 256 tries (few minutes)
33 Defenses Increase Query ID size. How? Randomize src port, additional 11 bits Now attack takes several hours Ask every DNS query twice: Attacker has to guess QueryID correctly twice (32 bits) but Apparently DNS system cannot handle the load
34 [DWF 96, R 01] DNS Rebinding Attack <iframe src=" TTL = DNS-SEC cannot stop this attack ns.evil.com DNS server Firewall corporate web server Read permitted: it s the same origin web server
35 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
36 Summary Core protocols 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
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