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Network Security Protocols and Defensive Mechanisms
John Mitchell
CS 155 Spring 2017
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Network security
What is the network for?What properties might attackers destroy? Confidentiality : no information revealed to
others Integrity : communication remains intact Availability : messages received in
reasonable time
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Network Attacker
Intercepts and controls network communication
System
• Confidentiality• Integrity• Availability
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Plan for today
Protecting network connections Wireless access– 802.11i/WPA2 IPSEC
Perimeter network defenses Firewall
Packet filter (stateless, stateful), Application layer proxies
Intrusion detection Anomaly and misuse detection
Network infrastructure security BGP instability and S-BGP DNS rebinding and DNSSEC
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Last lecture
Basic network protocols IP, TCP, UDP, BGP, DNS
Problems with them TCP/IP
No SRC authentication: can’t tell where packet is from
Packet sniffing Connection spoofing, sequence numbers
BGP: advertise bad routes or close good ones
DNS: cache poisoning, rebinding Web security mechanisms rely on DNS
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Network Protocol Stack
Application
Transport
Network
Link
Application protocol
TCP protocol
IP protocol
Data
Link
IP
Network Access
IP protocol
Data
Link
Application
Transport
Network
Link
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Protocol and link-layer connectivity
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Authentica-tion Server (RADIUS)No Key
Authenticator UnAuth/UnAssoc802.1X BlockedNo Key
SupplicantUnAuth/UnAssoc802.1X BlockedNo Key
SupplicantAuth/Assoc802.1X BlockedNo Key
Authenticator Auth/Assoc802.1X BlockedNo Key
Authentica-tion Server (RADIUS)No Key
802.11 Association
EAP/802.1X/RADIUS Authentication
SupplicantAuth/Assoc802.1X BlockedMSK
Authenticator Auth/Assoc802.1X BlockedNo Key
Authentica-tion Server (RADIUS)MSK
MSK
SupplicantAuth/Assoc802.1X BlockedPMK
Authenticator Auth/Assoc802.1X BlockedPMK
Authentica-tion Server (RADIUS)No Key
4-Way Handshake
SupplicantAuth/Assoc802.1X UnBlockedPTK/GTK
Authenticator Auth/Assoc802.1X UnBlockedPTK/GTK
Authentica-tion Server (RADIUS)No Key
Group Key Handshake
SupplicantAuth/Assoc802.1X UnBlockedNew GTK
Authenticator Auth/Assoc802.1X UnBlockedNew GTK
Authentica-tion Server (RADIUS)No Key
802.11i Protocol
Data Communication
SupplicantAuth/Assoc802.1X UnBlockedPTK/GTK
Authenticator Auth/Assoc802.1X UnBlockedPTK/GTK
Authentica-tion Server (RADIUS)No Key
Link Layer
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TCP/IP CONNECTIVITY
How can we isolate our conversation from attackers on the Internet?
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Basic Layer 2-3 Security Problems
Network packets pass by untrusted hosts Eavesdropping, packet sniffing Especially easy when attacker controls a
machine close to victim
TCP state can be easy to guess Enables spoofing and session hijacking
Transport layer security (from last lecture)
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Virtual Private Network (VPN)
Three different modes of use: Remote access client connections LAN-to-LAN internetworking Controlled access within an intranet
Several different protocols PPTP – Point-to-point tunneling protocol L2TP – Layer-2 tunneling protocol IPsec (Layer-3: network layer)
Data layer
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12Credit: Checkpoint
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IPSEC
Security extensions for IPv4 and IPv6IP Authentication Header (AH) Authentication and integrity of payload and
header
IP Encapsulating Security Protocol (ESP) Confidentiality of payload
ESP with optional ICV (integrity check value) Confidentiality, authentication and integrity
of payload
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Recall packet formats and layers
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
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IPSec Transport Mode: IPSEC instead of IP header
http://www.tcpipguide.com/free/t_IPSecModesTransportandTunnel.htm
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IPSEC Tunnel Mode
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IPSec Tunnel Mode: IPSEC header + IP header
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Mobile IPv6 Architecture
IPv6
Mobile Node (MN)
Corresponding Node (CN)
Home Agent (HA)
Direct connection via binding update
Authentication is a requirementEarly proposals weakRFC 6618 – use IPSec
Mobility
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Summary
Protecting network connections Wireless access– 802.11i/WPA2
Several subprotocols provide encrypted link between user device and wireless access point
IPSEC Give external Internet connections equivalent
security to local area network connections Mobility
Preserve network connections when a device moves to different physical portions of the network
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Second topic of today’s lecture
Perimeter defenses for local networks Firewall
Packet filter (stateless, stateful) Application layer proxies
Intrusion detection Anomaly and misuse detection
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LOCAL AREA NETWORK
How can we protect our local area network from attackers on the external Internet?
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Basic Firewall Concept
Separate local area net from internet
Router
Firewall
All packets between LAN and internet routed through firewall
Local network Internet
Perimeter security
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Screened Subnet Using Two Routers
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Alternate 1: Dual-Homed Host
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Alternate 2: Screened Host
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Basic Packet Filtering
Uses transport-layer information only IP Source Address, Destination Address Protocol (TCP, UDP, ICMP, etc) TCP or UDP source & destination ports TCP Flags (SYN, ACK, FIN, RST, PSH, etc) ICMP message type
Examples DNS uses port 53
Block incoming port 53 packets except known trusted servers
Issues Stateful filtering Encapsulation: address translation, other
complications Fragmentation
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Source-Address Forgery
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More about networking: port numbering
TCP connection Server port uses number less than 1024 Client port uses number between 1024 and 16383
Permanent assignment Ports <1024 assigned permanently
20,21 for FTP 23 for Telnet 25 for server SMTP 80 for HTTP
Variable use Ports >1024 must be available for client to make
connection Limitation for stateless packet filtering
If client wants port 2048, firewall must allow incoming traffic
Better: stateful filtering knows outgoing requests Only allow incoming traffic on high port to a machine
that has initiated an outgoing request on low port
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Filtering Example: Inbound SMTP
Can block external request to internal server based on port number
Assume we want to block internal server from external attack
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Filtering Example: Outbound SMTP
Known low port out, arbitrary high port inIf firewall blocks incoming port 1357 traffic then connection fails
Assume we want to allow internal access to external server
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Stateful or Dynamic Packet Filtering
Assume we want to allow external UDP only if requested
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Telnet
“PORT 1234”
“ACK”
Telnet ClientTelnet Server
23 1234
Client opens channel to server; tells server its port number. The ACK bit is not set while establishing the connection but will be set on the remaining packets
Server acknowledges
Stateful filtering can use this pattern to identify legitimate sessions
How can stateful filtering identify legitimate session?
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“PORT 5151”
“OK”
DATA CHANNEL
TCP ACK
FTP ClientFTP Server
20Data
21Command 5150 5151 Client opens
command channel to server; tells server second port number
Server acknowledges
Server opens data channel to client’s second port
Client acknowledges
FTPHow can stateful filtering identify legitimate session?
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Normal IP Fragmentation
Flags and offset inside IP header indicate packet fragmentation
Complication for firewalls
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Abnormal Fragmentation
Low offset allows second packet to overwrite TCP header at receiving host
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Packet Fragmentation Attack
Firewall configuration TCP port 23 is blocked but SMTP port 25 is allowed
First packet Fragmentation Offset = 0. DF bit = 0 : "May Fragment" MF bit = 1 : "More Fragments" Destination Port = 25. TCP port 25 is allowed, so firewall allows
packetSecond packet
Fragmentation Offset = 1: second packet overwrites all but first 8 bits of the first packet
DF bit = 0 : "May Fragment" MF bit = 0 : "Last Fragment." Destination Port = 23. Normally be blocked, but sneaks by!
What happens Firewall ignores second packet “TCP header” because it is
fragment of first At host, packet reassembled and received at port 23
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TCP Protocol Stack
Application
Transport
Network
Link
Application protocol
TCP protocol
IP protocol
Data
Link
IP
Network Access
IP protocol
Data
Link
Application
Transport
Network
Link
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Proxying Firewall
Application-level proxies Tailored to http, ftp, smtp, etc. Some protocols easier to proxy than others
Policy embedded in proxy programs Proxies filter incoming, outgoing packets Reconstruct application-layer messages Can filter specific application-layer commands, etc.
Example: only allow specific ftp commands Other examples: ?
Several network locations – see next slides
Beyond packet filtering
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Firewall with application proxies
Daemon spawns proxy when communication detected …
Network Connection
Telnet daemon
SMTP daemon
FTP daemon
Telnet
proxy
FTP proxy SMTP
proxy
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Application-level proxies
Enforce policy for specific protocols E.g., Virus scanning for SMTP
Need to understand MIME, encoding, Zip archives Flexible approach, but may introduce network delays
“Batch” protocols are natural to proxy SMTP (E-Mail) NNTP (Net news) DNS (Domain Name System) NTP (Network Time
Protocol)
Must protect host running protocol stack Disable all non-required services; keep it simple Install/modify services you want Run security audit to establish baseline Be prepared for the system to be compromised
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Web traffic scanning
Intercept and proxy web traffic Can be host-based Usually at enterprise gateway
Block known bad sitesBlock pages with known attacksScan attachments Virus, worm, malware, …
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Firewall references
Elizabeth D. ZwickySimon Cooper
D. Brent Chapman
William R CheswickSteven M Bellovin
Aviel D Rubin
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Intrusion detection
Many intrusion detection systems Network-based, host-based, or combination
Two basic models Misuse detection model
Maintain data on known attacks Look for activity with corresponding signatures
Anomaly detection model Try to figure out what is “normal” Report anomalous behavior
Fundamental problem: too many false alarms
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Example: Snort
From: Rafeeq Ur Rehman, Intrusion Detection Systems with Snort: Advanced IDS Techniques with Snort, Apache, MySQL, PHP, and ACID.
http://www.snort.org/
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Snort components
Packet Decoder input from Ethernet, SLIP, PPP…
Preprocessor: detect anomalies in packet headers packet defragmentation decode HTTP URI reassemble TCP streams
Detection Engine: applies rules to packetsLogging and Alerting SystemOutput Modules: alerts, log, other output
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Snort detection rules
rule header rule options
Alert will be generated if criteria met
Apply to all ip packets
Source ip address
Source port #
destination ip address
Destination port
Rule options
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Additional examples
alert tcp any any -> 192.168.1.0/24 111 (content:"|00 01 86 a5|"; msg: "mountd access";)
alert tcp !192.168.1.0/24 any -> 192.168.1.0/24 111 (content: "|00 01 86 a5|"; msg: "external mountd access";)
! = negation operator in addresscontent - match content in packet192.168.1.0/24 - addr from 192.168.1.1 to 192.168.1.255
https://www.snort.org/documents/snort-users-manual
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Snort challenges
Misuse detection – avoid known intrusions Database size continues to grow
Snort version 2.3.2 had 2,600 rules Snort spends 80% of time doing string
match
Anomaly detection – identify new attacks Probability of detection is low
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Difficulties in anomaly detection
Lack of training data Lots of “normal” network, system call data Little data containing realistic attacks,
anomalies
Data drift Statistical methods detect changes in
behavior Attacker can attack gradually and
incrementally
Main characteristics not well understood By many measures, attack may be within
bounds of “normal” range of activities
False identifications are very costly Sys Admin spend many hours examining
evidence
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Summary of this section
Perimeter defenses for local networks Firewall
Packet filter (stateless, stateful), Application layer proxies
Intrusion detection Anomaly and misuse detection
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Last section of today’s lecture
Network infrastructure protocols BGP vulnerabilities and S-BGP DNS security, cache poisoning and
rebinding attacks
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INFRASTRUCTURE PROTOCOLS: BGP, DNS
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BGP example
Transit: 2 provides transit for 7Algorithm seems to work OK in practice
BGP is does not respond well to frequent node outages
3 4
6 57
1
8 2
77
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
Figure: D. Wetherall
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BGP Security Issues
BGP is used for all inter-ISP routingBenign configuration errors affect about 1% of all routing table entries at any timeHighly vulnerable to human errors, malicious attacks
Actual routing policies can be very complicated
MD5 MAC is rarely used, perhaps due to lack of automated key management, addresses only one class of attacks
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S-BGP Design Overview
IPsec: secure point-to-point router communicationPublic Key Infrastructure: authorization for all S-BGP entitiesAttestations: digitally-signed authorizations
Address: authorization to advertise specified address blocks
Route: Validation of UPDATEs based on a new path attribute, using PKI certificates and attestations
Repositories for distribution of certificates, CRLs, and address attestationsTools for ISPs to manage address attestations, process certificates & CRLs, etc.
Slide: Steve Kent
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BGP example
3 4
6 57
1
8 2
77
2 7
2 7
2 7
Host1Host2…Hostn
AS
Address blocks
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Address Attestation
Indicates that the final AS listed in the UPDATE is authorized by the owner of those address blocks Includes identification of:
owner’s certificate AS to be advertising the address blocks address blocks expiration date
Digitally signed by owner of the address blocksUsed to protect BGP from erroneous UPDATEs (authenticated but misbehaving or misconfigured BGP speakers)
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Route Attestation
Indicates that the speaker or its AS authorizes the listener’s AS to use the route in the UPDATEIncludes identification of:
AS’s or BGP speaker’s certificate issued by owner of the AS
the address blocks and the list of ASes in the UPDATE the neighbor expiration date
Digitally signed by owner of the AS (or BGP speaker) distributing the UPDATE, traceable to the IANA ...Used to protect BGP from erroneous UPDATEs (authenticated but misbehaving or misconfigured BGP speakers)
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Validating a Route
To validate a route from ASn, ASn+1 needs:
address attestation from each organization owning an address block(s) in the NLRI
address allocation certificate from each organization owning address blocks in the NLRI
route attestation from every AS along the path (AS1 to ASn), where the route attestation for ASk specifies the NLRI and the path up to that point (AS1 through ASk+1)
certificate for each AS or router along path (AS1 to ASn) to check signatures on the route attestations
and, of course, all the relevant CRLs must have been checked Slide: Kent et al.
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INFRASTRUCTURE PROTOCOLS: BGP, DNS
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Recall: DNS LookupQuery: "www.example.com A?"
Local recursive resolver caches these for TTL specified by RR
Reply Resource Records in Reply
3
5
7
8
"com. NS a.gtld.net""a.gtld.net A 192.5.6.30"
"example.com. NS a.iana.net""a.iana.net A 192.0.34.43"
"www.example.com A 1.2.3.4"
"www.example.com A 1.2.3.4"
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DNS is Insecure
Packets sent over UDP, < 512 bytes16-bit TXID, UDP Src port are only “security”Resolver accepts packet if above matchPacket from whom? Was it manipulated?
Cache poisoning Attacker forges record at resolver Forged record cached, attacks future
lookups Kaminsky (BH USA08)
Attacks delegations with “birthday problem”
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“The Domain Name System (DNS) security extensions provide origin authentication and integrity assurance services for DNS data, including mechanisms for authenticated denial of existence of DNS data.”
-RFC 4033
DNSSEC Goal
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DNSSEC
Basically no change to packet format Goal is security of DNS data, not channel security
New Resource Records (RRs) RRSIG : signature of RR by private zone key DNSKEY : public zone key DS : crypto digest of child zone key NSEC / NSEC3 authenticated denial of existence
Lookup referral chain (unsigned) Origin attestation chain (PKI) (signed)
Start at pre-configured trust anchors DS/DNSKEY of zone (should include root)
DS → DNSKEY → DS forms a link
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Query: "www.example.com A?"
3
5
7
8
Reply
"com. NS a.gtld.net""a.gtld.net A 192.5.6.30"
"example.com. NS a.iana.net""a.iana.net A 192.0.34.43"
"www.example.com A 1.2.3.4"
"www.example.com A 1.2.3.4"
RRs in DNS Reply Added by DNSSEC
"com. DS""RRSIG(DS) by ."
"com. DNSKEY""RRSIG(DNSKEY) by com."
"example.com. DS""RRSIG(DS) by com."
"example.com DNSKEY""RRSIG(DNSKEY) by example.com."
"RRSIG(A) by example.com."
Last Hop?
DNSSEC Lookup
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Authenticated Denial-of-Existence
Most DNS lookups result in denial-of-existence NSEC (Next SECure)
Lists all extant RRs associated with an owner name Easy zone enumeration
NSEC3 Hashes owner names
Public salt to prevent pre-computed dictionaries NSEC3 chain in hashed order Opt-out bit for TLDs to support incremental adoption
For TLD type zones to support incremental adoption Non-DNSSEC children not in NSEC3 chain
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Insecure Sub-Namespace
NSEC3 Opt-out "Does not assert the existence or non-
existence of the insecure delegations that it may cover" (RFC 5155)
Only thing asserting this is insecure glue records
Property: Possible to insert bogus pre-pended name into otherwise secure zone. (RFC 5155)
Insecure delegation from secure zone Spoofs possible for resultant lookup results
Acceptable for TLD, bad for enterprises
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DNS Rebinding Attack
Read permitted: it’s the “same origin”F
irewall
www.evil.com
web server
ns.evil.com
DNS server
171.64.7.115
www.evil.com?
corporateweb 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]
DNSSEC cannot stop this attack
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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
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Summary of this section
Network infrastructure protocols BGP vulnerabilities and S-BGP
Security can be achieved by applying cryptography and basic network connection security to every step
Heavyweight solution, but illustrates the ways BGP can be vulnerable
DNS security, rebinding attack Domain-name security achieved by additional infrastructure Most complicated part is addressing non-existence Recommendation: do not allow opt out in enterprise networks
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Summary
Protecting network connections Wireless security – 802.11i/WPA2 IPSEC
Perimeter network perimeter defenses Firewall
Packet filter (stateless, stateful), Application layer proxies
Intrusion detection Anomaly and misuse detection
Network infrastructure security BGP vulnerability and S-BGP DNSSEC, DNS rebinding