CS526: Information Security
Chris Clifton
November 13, 2003
Network Security
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…
A
Network
B
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Securely means
• Confidentiality
A
– Only A and B see bits
• Integrity
Network
– Message intact
– Really from A
– Order?
• Availability
B
– B gets it in time
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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
Demilitarized
Zone (DMZ)
Firewall
Intranet
Mail forwarding
File Server
DNS (DMZ)
Web Server
Web Server
Mail server
DNS (internal)
Firewall
Internet
User machines
User machines
User machines
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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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Kerberos
Authentication 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, 2256
X.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
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CS526: Information Security
Chris Clifton
November 18, 2003
Network Security
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 multistep
– 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?
Time?
– What if no response
– And many SYNs
• All space for connections
allocated
– None for legitimate ones
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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
S
– Should terminate
• Spoofing client sends
ACK
– Must guess Sequence
Number
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CS526: Information Security
Chris Clifton
November 18, 2003
Network Security
(slides courtesy Wenke Lee)
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
Routers/
Switches
SRC
DST
I want to know
the shortest path
So, the routers must
exchange
local
information!
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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
OSPF
RIPv2
RIB
Forwarding
Information
Base
FIB
FIB
RIB
BGP4
RIB
(Dest, NextHop, Routing Metrics)
Forwarding Algorithm
NPDU Header (Network Protocol Data Unit)
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Forwarding
Decision
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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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Distribution of Routes Distance Vector
Every node sends its neighbor
a vector: the # of hops of
B
reaching each
other node.
A
C
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Link State
A node sends to its
neighbors the state of its
directly connected links: up/down and
costs. Each node that receives the
information forwards it to all its
neighbors.
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Routing Security

Routing Information Exchange
 correctness

of Routing Information Base
Interface between RIB and FIB
 configuration,

FIB update etc.
Kernel-Level (IP): Packet Forwarding
 Is
the packet forwarded according to the FIB?
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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
1
A: 1
G1
1
A: 2
G2
1
A: 3
G3
1
A: 4
G4
1
A: 5
G5
•G1 happily advertises route to network A with distance 1
•G2-G5 quickly learns the good news and install the routes
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A: 0
8
Route Convergence - bad
news
A: 3
G1
1
A: 2
G2
1
A: 3
G3
1
A: 4
G4
1
A: 5
G5
• G1’s link to A goes down
• G1 learns a ‘better’ route via G2
• Packets 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
to infinity
• Use a number, e.g., 16, to approximate infinity
• Split horizon only prevents loops involving two nodes
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BlackD Hole
B
A: 3 hops
B: 2 hops
C: 2 hops
You: G
E
A
F
Your
Neighbor: H
Who won’t jump on a better route?
C
A: 1 hop
•C lies easily about routes to A
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•Your neighbor and you look into the ‘routes’
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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
D
A: B
A: B
B
A: B
You: G
E
A
F
A: B
A: B
A: F
C
A: B
A: 1 hop
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Your
Neighbor: H
A: B 4
B: D 3
C: C 1
D: G 2
E: G 2
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F: B 4
G: G 1
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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Questions to Ask:
How do you know one LSA is fresher than
the other?
 An LSA originated by you will be received
by every router; will you receive the LSA
originated by you?
 Will the sequence number wrap-around
cause any problem? (i.e., == 0x7fffffff)
 Age ==> 1 hour

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Sequence #: old vs. new LSAs
0x80000001
ATM
Next: 0x80000002
Only accept LSAs with
newer/larger Seq#.
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Sequence# & Self-Stabilization
(1). 0x90001112
up
(2). router crashes.
(3). 0x80000001.
ATM
(5). 0x90001113
up
(4). 0x90001112
an old copy still exists!
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Flushing via Premature Aging
Specified behavior when Seq# wraps around: (1),(2),(3)
(1) 0x7FFFFFFF
MaxSeq#
ATM
(2) 0x7FFFFFF with
MaxAge to purge
this entry.
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(3) 0x80000001.
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Attack the Routing Infrastructure
(Vicious Advertising Routers)
Flooding
up
up
EVIL!
1. up ==> down
2. not exist ==> up
up
up
Impact varies depending on how critical the link is to the world!
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Attack the Routing Infrastructure
(Vicious Intermediate Routers)
Flooding
up
All the links
can be attacked
down
EVIL!
up
down
Authentication, please come to the rescue!
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Exchanging without LSA
Signature?
If attackers can just change the content of LSAs without
being detected, the routers must use all LSAs with care!
Seq#
ATM
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Fight-Back - Originator
Reaction
Seq#
(1) 0x90001112
(3) 0x90001114
fight-back
ATM
(2) 0x90001113
seq++ attack
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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?
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