CS526: Information SecurityChris Clifton

November 13, 2003

Network Security

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Network Security:What is the Problem?

• Name examples of security breaches– Morris Worm– Email viruses (many)– …

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Network Security:What is it?

• What is the purpose of a network?– Move bits– from A– to B– Securely…

NetworkNetwork

A

B

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Securely means

• Confidentiality– Only A and B see bits

• Integrity– Message intact– Really from A– Order?

• Availability– B gets it in time

NetworkNetwork

A

B

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Network Security:What is the Problem?

• Were our examples Network security failures?– Morris Worm– Email viruses (many)– …

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Network Security:Mostly Solved!

• Confidentiality– Encryption

• Integrity– Digital Signatures– Retransmission– Order?

• Availability– Quality of Service

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Network Security:What’s all the fuss?

• Firewalls– Solve poor internal security using the network

• Intrusion Detection– Detect non-network security breaches

accomplished via the network– Early start on forensics

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Network Security:What is interesting?

• Distributed Authentication– Scaling issues– Autonomy

• Distributed Cooperation– Commit– Fault tolerance

• Availability– Denial of service

But first: State of practice

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“Typical” corporate network

Web Server

Mail forwarding

FirewallMail server DNS (internal)

DNS (DMZ)

InternetInternet

File Server

User machinesUser machines

User machines

Web Server

Firewall DemilitarizedZone (DMZ)

Intranet

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Typical network:Terms

• Network Regions– Internet– Intranet– DMZ

• Network Boundaries– Firewall

• Filtering firewall: Based on packet headers• Audit mechanism

– Proxy• Proxy firewall: Gives external view that hides intranet

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Issues

• IP: Intranet hidden from outside world– Internal addresses can be real

• Proxy maps between real address and firewall

– Fake addresses: 10.b.c.d, 172.[16-31].c.d, 192.168.c.d

• Network Address Translation Protocol maps internal to assigned address

• Mail Forwarding– Hide internal addresses– Map incoming mail to “real” server– Additional incoming/outgoing checks

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Firewalls: Configuration

• External Firewall– What traffic allowed

• External source: IP restrictions• What type of traffic: Ports (e.g., SMTP, HTTP)

– Proxy between DMZ servers and internet– Proxy between inner and outer firewall

• Internal Firewall– Traffic restrictions: Ports, From/to IP– Proxy between intranet and outside

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DMZ Administration

• Direct console access required?– Real hassle

• “Special” access– SSH connections allowed from internal to

DMZ “administration” connections– Only from specified internal IPs– Only through internal firewall

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Distributed Authentication

• Authentication has been covered– But does it scale?

• Scaling Issues:– Repeat authentication– Multiple administrators

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Repeat authentication:Kerberos

• Kerberos developed at MIT in the 1980s– Project Athena: clusters of publicly available

computers for student/faculty use– Shared file service – log in anywhere– Problem: how to ensure user logging in at A

authorized to use resources at B?• Solution: ticket as credential

– Ticket server– Client– Client address– Valid time– Session key

Encrypted with ticket server’s key

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KerberosAuthentication Example

• Client sends to Authentication Server– Client name, Ticket server name

• Authentication Server returns– Client: {kclient,ticket server} kclient password, ticketclient, ticket server

• Client performs similar exchange with ticket server– Ticket contains session key, only ticket server can

decrypt• Now client has session key and ticket for service

– Message includes ticket and encrypted generation time as “signature”

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Kerberos:Problems

• Subject to replay attacks– Relies on clock synchronization– “Window of opportunity” based on maximum

message delay

• Standard password attacks– Password used to decrypt initial authenticator

ticket– Some fixed fields enable verifying if password

broken

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The Next Problem:Multiple Administration Domains

• Problem: Many users– We know how to authenticate– But how to administer?

• Solution: Hierarchical directories– X.500, LDAP, Active Directory

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X.500: What is it?

• Goal: Global “white pages”– Lookup anyone, anywhere– Developed by Telecommunications Industry– ISO standard directory for OSI networks

• Idea: Distributed Directory– Application uses Directory User Agent to access a Directory Access

Point

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Issues

• How is name used?– Access resource given the name– Build a name to find a resource– Information about resource

• Do humans need to use name?– Construct and Recall

• Is resource static?– Resource may move– Change in location may change name

• Performance requirements– Human-scale

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Directory Information Base(X.501)

• Tree structure– Root is entire directory– Levels are “groups”

• Country• Organization• Individual

• Entry structure– Unique name

• Build from tree– Attributes: Type/value

pairs– Schema enforces type

rules• Alias entries

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Directory Entry

• Organization level– CN=Purdue University– L=West Lafayette– …

• Person level– CN=Chris Clifton– SN=Clifton– TITLE=Associate

Professor– …

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Directory Operations(X.511)

• Query:– Read – get selected attributes of an entry– Compare – does an entry match a set of attributes– List – children of an entry– Search – portion of directory for matching entries– Abandon request

• Modification – add, remove, modify entry– Modify distinguished name

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Distributed Directory(X.518)

• Directory System Agent– May have local data– Can forward requests to other system agents– Can process requests from user agents and

other system agents

• Referrals– If DSA can’t handle request, can make

request to other DSA– Or tell DUA to ask other DSA

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Access Control

• Directory information can be protected

• Two issues:– Authentication (X.509)– Access control (X.501)

• Standards specify basic access control– Individual DSA’s can define their own

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Replication(X.525)

• Single entries can be replicated to multiple DSAs– One is “master” for that entry

• Two replication schemes:– Cache copies – On demand– Shadow copies – Agreed in advance

• Copies required to enforce access control– When entry sent, policy must be sent as well

• Modifications at Master only• Copy can be out of date

– Each entry must be internally consistent– DSA giving copy must identify as copy

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Protocols(X.519)

• Directory Access Protocol– Request/response from DUA to DSA

• Directory System Protocol– Request/response between DSAs

• Directory Information Shadowing Protocol– DSA-DSA with shadowing agreement

• Directory Operational binding management Protocol– Administrative information between DSAs

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Uses

• Look-up– Attributes, not just Distinguished Name– Context

• Humans can construct likely names• Browsing• Yellow pages

– Aliases

• Search restriction/relaxation• Groups

– Multi-valued “member” attribute

• Authentication information contained in directory– E.g., password attribute

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LDAP vs. X.500

• Lightweight Directory Access Protocol– Supports X.500 interface– Doesn’t require OSI protocol– IETF RFC 2251, 2256X.500 for the internet crowd

• Useful as generic addressing interface– Netscape address book– System logon identification/authentication– …

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The Next Problem:Multiple Administration Domains

• Problem: Many users– We know how to authenticate– But how to administer?

• Solution: Hierarchical directories– X.500, LDAP, Active Directory

• Still not enough– Is every administrator in the hierarchy enforcing our

policy?– Think this is an interesting area of research?

Talk to Prof. Ninghui Li

CS526: Information SecurityChris Clifton

November 18, 2003

Network Security

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Attacks and Defense

• Confidentiality on the network manageable– Encryption to protect transmission– Public key cryptography / key management to

verify recipient

• Integrity reducible to single system– Digital signatures verify source– Commit protocols handle network failure

• What about Availability?

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Network Attacks

• Flooding– Overwhelm TCP stack on target machine– Prevents legitimate connections

• Routing– Misdirect traffic

• Spoofing– Imitate legitimate source– But we know how to handle this!

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What is a Flood attack?

• Limit availability by– Overwhelming service– by following service’s protocol

• Perhaps not exactly

• Examples– SYN flood

• Overwhelms TCP stack

– Email attacks

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Syn Flood

• TCP connection multi-step– SYN to initiate– SYN+ACK to respond– ACK gets agreement

• Sequence numbers then incremented for future messages– Ensures message order– Retransmit if lost– Verifies party really initiated

connection– We’ll get back to this

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Syn Flood

• Implementation– Receive SYN– Allocate connection– Acknowledge– Wait for response

• See the problem?– What if no response– And many SYNs

• All space for connections allocated– None for legitimate ones

Time?

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Solution Ideas

• Limit connections from one source?– But source is in packet, can be faked

• Ignore connections from illegitimate sources– If you know who is legitimate– Can figure it quickly– And the attacker doesn’t know this

• Drop oldest connection attempts– Adaptive timeout

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Network Solutions

• TCP intercept– Router establishes connection to client– When connected establish with server

• Synkill– Monitor machine as “firewall”– Good addresses: history of successful connections– Bad addresses: previous timeout attempt– Block and terminate attempts from bad addresses

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Protocol solutions

• Problem: Server maintaining state– Runs out of space

• Solution: Don’t maintain state on server– How does it know sequence numbers?– Encrypt in SYN response

• h(source,destination,random)+sequence+time

– Client increments this and ACKs– Server subtracts h(), time to get sequence

• Knows if this is in valid range

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Service-Level Flooding

• Overload server– Processing– Storage

• Typically “garbage” requests using legitimate protocol– Large emails to victim– Many http connections– Heavy use of scripts

• Often exploit flaws in service implementation– Self-replicating attacks

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Solutions

• Limit traffic from any source– Still open to distributed attacks

• Quality of Service– Guarantee service to known good sites

• Careful Programming– Don’t allow service to defeat itself

• Throttling– Limit traffic to any service– Protects other services on same host

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IP-Spoofing

• Start with SYN flood to spoofed client

• Open connection from spoofing client to server

• Real client fails to respond– Should terminate

• Spoofing client sends ACK– Must guess Sequence

Number

S

CS526: Information SecurityChris Clifton

November 18, 2003

Network Security

(slides courtesy Wenke Lee)

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Note to 1st year SFS students(only)

• Interested in Summer internship?

• Independent study or thesis next year?

• Did the ITSEC/Common Criteria lectures interest you?

• If so, please talk to me after class

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Routing

I want to knowthe shortest path

Routers/Switches

So, the routers must exchange local information!

SRC

DST

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IP Routing

• Routing is based on network addresses• Routers use forwarding table

– Destination, next hop, network interface, metric

– Table look-up for each packet– Need to recognize address structure

• Routing information exchange allows computation of new routes, which is used to update the forwarding table

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Routing Protocol Framework - Information Model

FIB

RIB

NPDU Header (Network Protocol Data Unit)

(Dest, NextHop, Routing Metrics)

Forwarding Algorithm

OSPF

RIBRIB

RIPv2 BGP4

FIB

ForwardingDecision

ForwardingInformationBase

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Routing Information

• Link State: I have these links to XYZ (routers or networks); their current status is … (e.g., delay)

• Distance Vector: I can get to XYZ (networks) in m hops

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A

B

C

Every node sends its neighbor a vector: the # of hops of reaching each other node.

Distribution of Routes - Distance Vector

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A node sends to itsA node sends to itsneighbors the state of itsneighbors the state of itsdirectly connected links: up/down anddirectly connected links: up/down andcosts. Each node that receives the costs. Each node that receives the information forwards it to all its information forwards it to all its neighbors.neighbors.

Link State

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Routing Information ExchangeRouting Information Exchange correctness of Routing Information Basecorrectness of Routing Information Base

Interface between RIB and FIBInterface between RIB and FIB configuration, FIB update etc.configuration, FIB update etc.

Kernel-Level (IP): Packet ForwardingKernel-Level (IP): Packet Forwarding Is the packet forwarded according to the FIB?Is the packet forwarded according to the FIB?

Routing Security

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RIP

• Routing information protocol is a simple distance vector protocol

• Initialization:– When the routing daemon is initialized, it sends

requests through each network interface– Neighboring routers will reply with routing table

information

• Updates:– Routers advertise tables with neighbors periodically

(~30 seconds); or triggered by route changes.

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RIP - Cont’d

• To prevent route oscillation, existing routes are retained until a new one is discovered with strictly lower cost

• Split-Horizon Update– Routers do not advertise a route on an

interface from which it learned of the route in the first place!

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Properties: RIP

• Good news travels fast; Bad news travels slowly

• Routing loop, routing inconsistency, and slow convergence

• Security:– Ripv2 provides simple password

authentication– Black hole routers possible

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Route Convergence - good news

A: 0 A: 1 A: 2 A: 3 A: 4 A: 51 1 1 1 1

G1G1 G2G2 G3G3 G4G4 G5G5

•G1 happily advertises route to network A with distance 1G1 happily advertises route to network A with distance 1•G2-G5 quickly learns the good news and install the routesG2-G5 quickly learns the good news and install the routes

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A: 0 A: 3 A: 2 A: 3 A: 4 A: 51 1 1 18

G1G1 G2G2 G3G3 G4G4 G5G5

• G1’s link to A goes downG1’s link to A goes down• G1 learns a ‘better’ route via G2G1 learns a ‘better’ route via G2• Packets going to A through G2 will loop between G2 and G1Packets going to A through G2 will loop between G2 and G1• G1 and G2 will find the cost of their routes to A slowly count G1 and G2 will find the cost of their routes to A slowly count to to infinityinfinity• Use a number, e.g., 16, to approximate infinityUse a number, e.g., 16, to approximate infinity• Split horizon only prevents loops involving two nodesSplit horizon only prevents loops involving two nodes

Route Convergence - bad news

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A: 3 hopsB: 2 hopsC: 2 hops

Black Hole

A

B

C

You: G

YourNeighbor: H

A: 1 hop

•C lies easily about routes to A C lies easily about routes to A •Your neighbor and you look into the ‘routes’Your neighbor and you look into the ‘routes’

Who won’t jump on a better route?Who won’t jump on a better route?

D

E

F

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Food for Thought...

• RIP implicitly assumes: every router is trusted and so are routing information packets

• Every router is entitled to tell others ‘I have a short cut to Pluto that is just one hop’

• Is it possible to prevent RIP black hole attacks?

• Is is possible to detect RIP black routers?

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One possible improvement

• Predecessor is the second-to-last network is the path from the source to the destination

• Inclusion of predecessor allows implicit reconstruction of the whole path

• Helps to validate the integrity of distance

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Predecessor

A

B

C

You: G

YourNeighbor: H

A: 1 hop

D

E

F

A: BA: B

A: B

A: B

A: B

A: FA: B

A: B 4B: D 3C: C 1D: G 2E: G 2F: B 4G: G 1

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OSPF

• Link State routing protocol (RFC1583)

• Routers are organized in domains and areas

• Hello message for neighbor acquisition

• Link State information are flooded through the whole area

• A topology database is maintained by every router

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Important LSA Fields

• Advertising router ID (originator)

• Advertised link or network ID

• Sequence number [0x80000001,0x7fffffff]

• Age [0, 60 minutes]

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When to Originate a LSA?

• Upon link state changes, or

• Upon timer expiration

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How do you know one LSA is fresher than How do you know one LSA is fresher than the other?the other?

An LSA originated by you will be received An LSA originated by you will be received by every router; will you receive the LSA by every router; will you receive the LSA originated by you?originated by you?

Will the sequence number wrap-around Will the sequence number wrap-around cause any problem? (i.e., == 0x7fffffff)cause any problem? (i.e., == 0x7fffffff)

Age ==> 1 hourAge ==> 1 hour

Questions to Ask:

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ATM

0x80000001

Next: 0x80000002Only accept LSAs withnewer/larger Seq#.

Sequence #: old vs. new LSAs

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Sequence# & Self-Stabilization

ATM

(1). 0x90001112

(2). router crashes.

(3). 0x80000001.

(4). 0x90001112 an old copy still exists!

(5). 0x90001113

up

up

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ATM

(1) 0x7FFFFFFF MaxSeq#

(2) 0x7FFFFFF with MaxAge to purge this entry.(3) 0x80000001.

Specified behavior when Seq# wraps around: (1),(2),(3)Specified behavior when Seq# wraps around: (1),(2),(3)

Flushing via Premature Aging

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Attack the Routing Infrastructure(Vicious Advertising Routers)

up

up up

up

Flooding

EVIL!

1. up ==> down2. not exist ==> up

Impact varies depending on how critical the link is to the world!Impact varies depending on how critical the link is to the world!

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up

up

down

Flooding

EVIL!All the linkscan be attacked

down

Authentication, please come to the rescue!Authentication, please come to the rescue!

Attack the Routing Infrastructure(Vicious Intermediate Routers)

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ATMSeq#

If attackers can just change the content of LSAs without If attackers can just change the content of LSAs without being detected, the routers must use all LSAs with care!being detected, the routers must use all LSAs with care!

Exchanging without LSA Signature?

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ATMSeq#

(1) 0x90001112

(2) 0x90001113seq++ attack

(3) 0x90001114 fight-back

Fight-Back - Originator Reaction

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Signature - How Critical?

• Observations:– Prolonged fight-back will not happen in real attacks– What’s preventing the attacker from using

LS_seq=MaxSeq?

• Can you prevent false LSA without signature?• Can you determine who did it after you realize

that you’ve been fooled without signature?• What needs to be signed by whom anyway?

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OSPF Security Strength

• In most benign cases, if something goes wrong, the advertising router will detect it and try to correct it by generating new LSAs

• The attackers have to persistently inject bad LSAs in order for it to ‘stick’

• Self-Stabilization Protocols: force the attackers to perform persistent attacks

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Detection of Hit-and-Run vs. Persistent Attacks

• Hit-and-Run Attacks: Hard to Detect/Isolate– Inject one (or very few) bad packet but cause

lasting damaging effect

• Persistent Attacks: – Attackers have to continuously inject attack

packets in order to inflict significant damages

• OSPF type of Link State protocols are resilient to hit-and-run attacks

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Secure Protocol/system Design?

• If we can force the attackers to launch “persistent attacks,” we have a better chance to detect and isolate the attack sources

• OSPF flooding coupled with periodic LSA does a fairly good job because it is refreshing link state persistently!

• What other implications do ‘flooding’ have on security?