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Network Security Architecture

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Additional Reading

“Firewalls and Internet Security: Repelling theWily Hacker”, Cheswick, Bellovin, and Rubin.− New second edition

“Firewall and Internet Security, the SecondHundred (Internet) Years”http://www.cisco.com/warp/public/759/ipj_2-2/ipj_2-2_fis1.html

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Overview

Network Security Architecture− Wireless− Security Domains− VPN

Firewall Technology− Address Translation− Denial of Service attacks

Intrusion Detection

Both firewalls and IDS are introductions.

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802.11 or Wi-Fi IEEE standard for wireless communication

− Operates at the physical/data link layer

− Operates at the 2.4 or 5 GHz radio bands

Wireless Access Point is the radio base station

− The access point acts as a gateway to a wired networke.g., ethernet

− Can advertise Service Set Identifier (SSID) or not Doesn't really matter, watcher will learn active

SSIDs

Laptop with wireless card uses 802.11 tocommunicate with the Access Point

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WEP “Wired Equivalency Privacy” -- early technique for

encrypting wireless communication Authenticated devices use a key and initialization

vector to seed RC4---a stream cipher

V (initialization vector) is changed every frame− Dangers of repeated encryption using the same key stream--

XOR of ciphertexts gives XOR of plaintexts And if some of the plaintext is known, the other is recovered

v

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Frame transmission RC4(v,k) is stream generated by long-lived key k and

initialization vector v v transmitted in the clear

v is only 24 bits long---since k is long-lived (and usedby all devices)---you are assured of getting repeatedkey sequences− And knowing when you have them! Because v is in the

clear…

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Security Mechanisms MAC restrictions at the access point

− “white list” : Protects servers from unexpected clients

− Unacceptable in a dynamic environment

− No identity integrity. You can reprogram your card to pose asan “accepted” MAC.

IPSec− To access point or some IPSec gateway beyond

− Protects clients from wireless sniffers

− Used by UIUC wireless networks 802.11i

− Authentication and integrity integral to the 802.11 framework− WEP, WPA, WPA2

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Network Security Protocols

SSL/TLS− Secure sockets layer / Transport layer security− Used mainly to secure Web traffic

SSH− Secure Shell− Remote login

IPsec− IP-level security suite

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SSL

Mid ‘90s introduced concerns over credit cardtransactions over the Internet

SSL designed to respond to thse concerns,develop e-commerce

Initially designed by Netscape, moved to IETFstandard later

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SSL model

A client and a server Implements a socket interface

− Any socket-based application can be made to run on top ofSSL

Protect against:− Eavesdroppers− MITM attacks

Server has X.509 certificate− Client may have a certificate, too

Provides encryption, and authentication of server

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SSL Handshake, (1)

Client requests “https” connection with server− Passes information to server in message describing

available protocols Key exchange method (e.g., RSA, Diffie-Hellman, DSA) Cipher (e.g., Triple DES, AES) Hash (e.g., HMAC-MD5, HMAC-SHA) Compression algorithms Client nonce

Server responds with messages that− Selects (key xchg, cipher, hash, compression)− Provide server’s certificate− Server nonce

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SSL Handshake, (2) Client verifies server cert

− Likely that cert was signed by a CA whose cert is in thebrowser already

generates pre_master_secret, encrypts using server’spublic key, sends it

Client and server separately compute session key andMAC keys (these from prior random numbers passed)

Client sends MAC of all messages it sent to server inthis handshake

Server sends MAC of all messages it sent to client inthis exchange

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SSL certificates

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SSL history

SSLv2 1994 SSLv3 1996

− Fixed security problems TLS v1.0 1999 TLS v1.1 2006

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SSL key lengths

Earlier versions used 40-bit keys for exportreasons

Later versions switched to 128-bit keys, withan option to use 40-bit ones with legacyservers/clients

Rollback attack:− MITM

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SSL sequence

Negotiate parameters Key exchange Authentication Session

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SSL negotiation

Choice of cipher suites, key exchangealgorithms, protocol versions

E.g. : choice of 40- or 128-bit keys for exportreasons

Rollback attack: MITM chooses least secureparameters

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SSL key exchange

Diffie-Hellman key exchange

RSA-based key exchange− Encrypt secret s with public key of server

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SSL session

Use ChangeCipherSpec message to startencrypting data

Encryption: RC4, also DES, 3DES, AES, ... Authentication: HMAC, using MD5 or SHA1

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SSL session…pushing the bits

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Blocks, sized up to 18K

Algorithm agreed-up on in handshake

MAC added for authentication

Algorithm, key, agreed-up on in handshake

Passed on to TCP

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SSL pitfalls

Hard to set up− Expensive certificates− Resource-intensive

Insufficient verification− Do people notice the lock icon?− Do people check the URL?

Improper use

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IPsec Designed as part of IPv6 suite

− One of the key features v6 was supposed to bring

Backported to IPv4 Two options: AH (authentication) and ESP

(encapsulated security) Two modes: transport and tunnel Readable resource

http://www.unixwiz.net/techtips/iguide-ipsec.html

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Transport vs. Tunnel Mode Grand vision: eventually, all IP packets will be

encrypted and authenticated

Transport mode: add headers to IP to do soMay include encryption, authentication, or both

Reality: Most computers don’t support IPsec (more onwhy later)

Tunnel mode: use IPsec between two gateways torelay IP packets through “untrusted cloud”

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Tunnel Mode

H1H2

PP PP PP

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AH - Authentication Simple design: add header with authentication data

− Security parameters− Authentication data : just an HMAC with

shared key to compute Integrity Check Value (ICV)

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Different of the HMACarchitecture picture

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AH Header Next hdr is protocol type of the following header AH Length gives size of AH header SPI -- sort of a switch code indicating which set

of security parameters apply Sequence number --- basically a nonce to

prevent replay attacks HMAC field

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AH diagram

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HMAC applied onlyto fields in yellow

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Piggybacking AH on IPv4 The structure allows IPSec logic to

− peel off the AH header, do verification and/ordecoding,

− Modify “length” and “next protocol” fields to be that ofan AH-free IP packet

− Push the packet up the stack with higher levels nonethe wiser that IPSec was present

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Tunneling in IPSec Change the source and destination addresses

to be the tunnel endpoints IPSec tunnel endpoints strip off AH header, to

authentication and endcoding Original IP packet is part of the payload, just

released into the local network

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AH in Tunnel Mode

How todetect

tunnel mode

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Original IPheader

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ESP - Encapsulated Security Payload

Encapsulate data− Encapsulate datagram rather than add a header− Encrypt & authenticate

Authentication header based only on encapsulation---not Iaddresses---hold that thought---

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ESP diagram

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Protocol using TCP is Completely hidden

SPI describes encryption

Padding and pad len supportblock encryption

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Key management

ESP and AH use session keys Sessions are called Security Associations

− Indexed by protocol, IP address, SPI ISAKMP: Internet Security Association Key

Management Protocol− Authenticates parties− Establishes session keys

Authentication− Big global PKI (DNSSEC??)− Manual configuration

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IPsec redux

Deployment of IPsec limited

Some reasons

− Global PKI infrastructure hard to set up− Fixes a “solved” problem

SSL & SSH work well IPsec success: VPNs

− Use tunnel mode of IPsec

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Perimeter Defense

Is it adequate?− Locating and securing all perimeter points is quite

difficult Less effective for large border

− Inspecting/ensuring that remote connections areadequately protected is difficult

− Insiders attack is often the most damaging

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Virtual Private Networks

A private network that is configured within apublic network

A VPN “appears” to be dedicated network tocustomer

The customer is actually “sharing” trunks andother physical infrastructure with othercustomers

Security?− Depends on implementing protocol

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Multiple VPN Technologies

SSL• Confidentiality? Yes• Data integrity? Yes• User authentication?

Yes• Network access

control? No• In addition, limited

traffic

IPSec• Confidentiality? Yes• Data Integrity? Yes• User Authentication?

Yes• Network access

control? Yes• Client configuration

required.

VLAN – Layer 2 tunnellingtechnology

• Confidentiality? No• Data Integrity? No• User authentication?

Yes• Network access

control? Yes• Not viable over non-

VLAN internetworks

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Security Domains with VPNs

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“Typical” corporate network

Web Server

Mail forwarding

Mail server DNS (internal)

DNS (DMZ)

Internet

File Server

User machinesUser machines

User machines

Web Server

DemilitarizedZone (DMZ)

IntranetFirewall

Firewall

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VPN using IPSec

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ESP does theencryption

Difficulty with NATmeans ESP+Auth intunnel mode

Requires VPNgateway---view is atunnel between twotrusted networks

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VPN using IPSec

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Firewall Goal

Insert after the fact security by wrapping orinterposing a filter on network traffic

Inside Outside

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Application Proxy Firewall

Firewall software runs in application space on thefirewall

The traffic source must be aware of the proxyand add an additional header

Leverage basic network stack functionality tosanitize application level traffic− Block java or active X− Filter out “bad” URLs− Ensure well formed protocols or block suspect aspects

of protocol

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Packet Filter Firewall

Operates at Layer 3 in router or HW firewall Has access to the Layer 3 header and Layer 4

header Can block traffic based on source and destination

address, ports, and protocol Does not reconstruct Layer 4 payload, so cannot

do reliable analysis of layer 4 or higher content

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Stateful Packet Filters Evolved as packet filters aimed for proxy functionality

In addition to Layer 3 reassembly, it can reconstruct layer 4 traffic

Some application layer analysis exists, e.g., for HTTP, FTP, H.323

− Called context-based access control (CBAC) on IOS

− Configured by fixup command on PIX

Some of this analysis is necessary to enable address translation anddynamic access for negotiated data channels

Reconstruction and analysis can be expensive.

− Must be configured on specified traffic streams

− At a minimum the user must tell the Firewall what kind of traffic toexpect on a port

− Degree of reconstruction varies per platform, e.g. IOS does not do IPreassembly

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Traffic reconstruction

X Y

FTP: X to YGET /etc/passwd

GET command causes firewall to dynamically

open data channel initiate from Y to X

Might have filter for files to block, like /etc/passwd

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Access Control Lists (ACLs) Used to define traffic streams

− Bind ACL’s to interface and action

Access Control Entry (ACE) contains

− Source address

− Destination Address

− Protocol, e.g., IP, TCP, UDP, ICMP, GRE

− Source Port

− Destination Port

ACL runtime lookup

− Linear

− N-dimensional tree lookup (PIX Turbo ACL)

− Object Groups

− HW classification assists

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Ingress and Egress Filtering

Ingress filtering

− Filter out packets from invalid addresses before entering yournetwork

Egress filtering

− Filter out packets from invalid addresses before leaving yournetwork

Inside Outside

Owns network X

Egress FilteringBlock outgoing traffic not sourced from network X

Ingress FilteringBlock incoming traffic from

one of the set of invalid networks

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Denial of Service

Example attacks− Smurf Attack− TCP SYN Attack− Teardrop

DoS general exploits resource limitations− Denial by Consumption− Denial by Disruption− Denial by Reservation

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TCP SYN Attack

Exploits the three-way handshake

S D

SYNx LISTEN

SYNy , ACKx+1 SYN_RECIEVED

ACKy+1 CONNECTED

Figure 1. Three-way Handshake

S D

Nonexistent (spoofed) SYN LISTEN

SYN SYN SYN_RECEIVED

SYN+ACK

Figure 2. SYN Flooding Attack

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TCP SYN Attack Solutions

Intermediate Firewall/Router− Limit number of half open connections

Ingress and egress filtering to reduce spoofedaddresses− Does not help against DDoS bot networks

Reactively block attacking addresses− Generally expensive to acquire technology to do

fast enough Fix Protocol - IPv6

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Teardrop Attack

Send series of fragments that don't fit together− Poor stack implementations would crash− Early windows stacks

Offset 0, len 60

Offset 30, len 90

Offset 41, len 173

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Address Translation Traditional NAT RFC 3022 Reference RFC

Map real address to alias address

− Real address associated with physical device, generally anunroutable address

− Alias address generally a routeable associated with thetranslation device

Originally motivated by limited access to publicly routable IPaddresses

− Folks didn’t want to pay for addresses and/or hassle withgetting official addresses

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Address Translation Later folks said this also added security

− By hiding structure of internal network

− Obscuring access to internal machines

Adds complexity to firewall technology

− Must dig around in data stream to rewrite references to IPaddresses and ports

− Limits how quickly new protocols can be firewalled

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Address Hiding (NAPT) NAPT = Network Address Port Translation

Many to few dynamic mapping

− Packets from a large pool of private addresses aremapped to a small pool of public addresses at runtime

Port remapping makes this sharing morescalable

− Two real addresses can be rewritten to the same aliasaddress

− Rewrite the source port to differentiate the streams

Traffic must be initiated from “inside”, e.g. theprivate address

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NAT example

EnforcingDevice

192.168.1.0/24128.128.1.0/26

10.10.10.0/24

Internet

Hide from inside to outside192.168.1.0/24 behind 128.274.1.1

Static map from inside to DMZ192.168.1.5 to 128.274.1.5

inside

DMZ

outside

Src=192.168.1.1Dst=microsoft.com

Src=128.274.1.1Dst=microsoft.com

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Static Mapping

One-to-one fixed mapping− One real address is mapped to one alias address at

configuration time− Traffic can be initiated from either side

Used to statically map out small set of serversfrom a network that is otherwise hidden

Static port remapping is also available

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NAT example

EnforcingDevice

192.168.1.0/24128.128.1.0/26

10.10.10.0/24

Internet

Hide from inside to outside192.168.1.0/24 behind 128.274.1.1

Static map from inside to DMZ192.168.1.5 to 128.274.1.5

inside

DMZ

outside

Src=192.168.1.5Dst=10.10.10.1

Src=128.274.1.5Dst=10.10.10.1

192.168.1.5

128.274.15

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NAT and IPSec AH don’t mix Recall the diagram illustrating the fields covered by AH AH header created at the sender, src/dest IP

addresses changed by NAT

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FW Runtime Characteristics

Firewalls track streams of traffic

− TCP streams are obvious

− Creates pseudo UDP streams for UCP packets betweenthe same addresses and ports that arrive near enough toeach other

Processing first packet in stream is moreexpensive

− Must evaluate ACLs and calculate address translations

− Subsequent packets get session data from a table

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Multi-legged Firewalls Historically firewalls have protected inside from outside

− Still true for the most part with personal and home firewalls

− No longer sufficient for larger enterprises

PIX security level solution

− Outbound = traffic from low security level interface to high securitylevel interface

− Inbound = traffic from high security level interface to low securitylevel interface

− Different requirements for inbound and outbound traffic

IOS divides interfaces into inside and outside groups

− Address translation can only be defined between inside and outsidegroups

Routing conflicts with address translation

− Address translation specifies both interfaces

− Must be evaluated before the routing, better be consistent

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Four Legged FW

Static translation from DMZ to Customer

− 10.10.10.10.1 to 128.1.1.1

But routing table wants to route 128.1.1.1 from DMZ to outside interface

− Static translation interface selection will winEnforcingDevice192.168.1.1

10.10.10.0/24

Internet

InsideSL=100

DMZSL=50

OutsideSL=0

10.10.20.0/2410.10.30.0/24

PartnerSL=75

CustomerSL=25

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Identity Aware Firewall Use TACACS+ or Radius to authenticate,

authorize, account for user with respect to FW− For administration of FW

− For traffic passing through FW

PIX cut-through proxy allows authentication on oneprotocol to cover other protocols from same source

Authorization for executing commands on thedevice

Download or enable ACL’s

XAuth to integrate AAA with VPN authenticationand other security mechanisms

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AAA Scenario

X Y

outside Inside

TACACS or RadiusAAA Server

Traffic from X must be authenticated via HTTP

User Joe should use ACL EngAccess

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Is the Firewall Dead? End-to-end security (encryption) renders firewalls useless

− Tunnels hide information that firewalls would filter or sanitize

− With IPSec decrypting and re-encrypting is viable

Blurring security domain perimeters

− Who are you protecting from whom

− Dynamic entities due to DHCP and laptops

− More dynamic business arrangements, short termpartnerships, outsourcing

Total Cost of Ownership (TCO) is too high

− Managing firewalls for a large network is expensive

Perhaps personal or distributed firewalls are the answer?

− “Implementing a Distributed Firewall”http://www1.cs.columbia.edu/~angelos/Papers/df.pdf

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Intrusion Detection Holy Grail: Detect and correct “bad” system

behavior

Detection can be viewed in two parts

− Anomaly detection: Use statistical techniques todetermine unusual behavior

− Mis-use detection: Use signatures to determineoccurrence of known attacks

Detection can be performed on host data (HIDS),network data (NIDS), or a hybrid of both

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Intrusion Handling Preparation for attack

Identification of the attack

Containment of the attack

− Gather information about the attacker

− Honeypots

Eradication

− Broadly quarantine the system so it can do no more harm

− BGP blackholing

− Tighten firewalls

− Cleanse the corrupted system

Followup phase

− Gather evidence and take action against the attacker

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Honey Pots

Reconnaissance for the good guys Deploy a fake system

− Observe it being attacked Resource management

− Cannot be completely passive Must provide enough information to keep attacker

interested− Must ensure that bait does not run away

Scale− Host, network, dark address space

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IDS Architecture

Agents run at the lowest level gathering data. Performsome basic processing.

Agents send data to a Director that performs moresignificant processing of the data. Potentially there is ahierarchy of agents and directors

− Director has information from multiple sources and canperform a time-based correlation to derive more significantactions

Directors invoke Notifiers to perform some action inresponse to a detected attack

− Popup a window on a screen

− Send an email or a page

− Send a new syslog message elsewhere.

− Adjust a firewall or some other policy to block future actionfrom the attacker

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Data Sources

Direct data− Network packets− System calls

Indirect data− Syslog data, Windows event logs− Events from other intrusion detection systems− Netflow information generated by routers about

network traffic

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Mis-use/Signature Detection

Fixed signatures are used in most deployed IDS products

− E.g., Cisco, ISS, Snort

Like virus scanners, part of the value of the product is the team ofpeople producing new signatures for newly observed malevolentbehavior

The static signature mechanism has obvious problems in that adedicated attacker can adjust his behaviour to avoid matching thesignature.

The volume of signatures can result in many false positives

− Must tune the IDS to match the characteristics of your network

− E.g., what might be unusual in a network of Unix systemsmight be normal in a network of Windows Systems (or visaversa)

− Can result in IDS tuned too low to miss real events

− Can hide real attacks in the mass of false positives

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Example Signature

Signature for port sweep− A set of TCP packets attempting to connect to a

sequence of ports on the same device in a fixedamount of time

In some environments, the admin might runnmap periodically to get an inventory of what ison the network− You would not want to activate this signature in that

case

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Anomaly/statistical detection

Seems like using statistics will result in a more adaptableand self-tuning system

− Statistics, neural networks, data mining, etc.

How do you characterize normal?

− Create training data from observing “good” runs

E.g., Forrest’s program system call analysis

− Use visualization to rely on your eyes

How do you adjust to real changes in behaviour?

− Gradual changes can be easily addressed. Gradually adjustexpected changes over time

− Rapid changes can occur. E.g., different behaviour after workhours or changing to a work on the next project

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Host Based IDS

Tripwire – Very basic detection of changes toinstalled binaries

More recent HIDS. Look at patterns of actionsof system calls, file activity, etc. to permit, deny,or query operations− Cisco Security Agent− Symantec− McAfee Entercept

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Classical NIDS deployment

NIDS Agent

Outside Inside

Management

Promiscuous Interface

NIDS Director

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NIDS Remediation Options

Log the event Drop the connection Reset the connection Change the configuration of a nearby router or

firewall to block future connections

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Intrusion Protection Systems (IPS) Another name for inline NIDS

Latest buzz among the current NIDS vendors

Requires very fast signature handling− Slow signature handling will not only miss attacks but it

will also cause the delay of valid traffic

− Specialized hardware required for high volume gateways

When IDS is inline, the intrusion detector cantake direct steps to remediate.

If you move IDS into the network processingpath, how is this different from really cleverfirewalling?

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Summary

Identification of security domains basis ofperimeter security control− Firewall is the main enforcer

Intrusion detection introduces deeper analysisand potential for more dynamic enforcement

Intermediate enforcement can handle someDenial of Service attacks