Title: Network Security Architecture
1Network Security Architecture
2Additional Reading
- Firewalls and Internet Security Repelling the
Wily Hacker, Cheswick, Bellovin, and Rubin. - New second edition
- Firewall and Internet Security, the Second
Hundred (Internet) Years http//www.cisco.com/war
p/public/759/ipj_2-2/ipj_2-2_fis1.html
3Overview
- Network Security Architecture
- Wireless
- Security Domains
- VPN
- Firewall Technology
- Address Translation
- Denial of Service attacks
- Intrusion Detection
- Both firewalls and IDS are introductions.
4802.11 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
network e.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 to
communicate with the Access Point
5WEP
- 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
6Frame 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 used by all devices)---you are assured of
getting repeated key sequences - And knowing when you have them! Because v is in
the clear
7Security 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 as an 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
8Network 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
8
9SSL
- Mid 90s introduced concerns over credit card
transactions over the Internet - SSL designed to respond to thse concerns, develop
e-commerce - Initially designed by Netscape, moved to IETF
standard later
9
10SSL model
- A client and a server
- Implements a socket interface
- Any socket-based application can be made to run
on top of SSL - Protect against
- Eavesdroppers
- MITM attacks
- Server has X.509 certificate
- Client may have a certificate, too
- Provides encryption, and authentication of server
10
11SSL 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 servers certificate
- Server nonce
11
12SSL Handshake, (2)
- Client verifies server cert
- Likely that cert was signed by a CA whose cert is
in the browser already - generates pre_master_secret, encrypts using
servers public key, sends it - Client and server separately compute session key
and MAC keys (these from prior random numbers
passed) - Client sends MAC of all messages it sent to
server in this handshake - Server sends MAC of all messages it sent to
client in this exchange
12
13SSL certificates
13
14SSL history
- SSLv2 1994
- SSLv3 1996
- Fixed security problems
- TLS v1.0 1999
- TLS v1.1 2006
14
15SSL key lengths
- Earlier versions used 40-bit keys for export
reasons - Later versions switched to 128-bit keys, with an
option to use 40-bit ones with legacy
servers/clients - Rollback attack
- MITM
15
16SSL sequence
- Negotiate parameters
- Key exchange
- Authentication
- Session
16
17SSL negotiation
- Choice of cipher suites, key exchange algorithms,
protocol versions - E.g. choice of 40- or 128-bit keys for export
reasons - Rollback attack MITM chooses least secure
parameters
17
18SSL key exchange
- Diffie-Hellman key exchange
- RSA-based key exchange
- Encrypt secret s with public key of server
18
19SSL session
- Use ChangeCipherSpec message to start encrypting
data - Encryption RC4, also DES, 3DES, AES, ...
- Authentication HMAC, using MD5 or SHA1
19
20SSL sessionpushing the bits
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
20
21SSL pitfalls
- Hard to set up
- Expensive certificates
- Resource-intensive
- Insufficient verification
- Do people notice the lock icon?
- Do people check the URL?
- Improper use
21
22IPsec
- 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
22
23Transport vs. Tunnel Mode
- Grand vision eventually, all IP packets will be
encrypted and authenticated - Transport mode add headers to IP to do so
- May include encryption, authentication, or both
- Reality Most computers dont support IPsec (more
on why later) - Tunnel mode use IPsec between two gateways to
relay IP packets through untrusted cloud
23
24Tunnel Mode
H1
H2
24
25AH - Authentication
- Simple design add header with authentication
data - Security parameters
- Authentication data just an HMAC with
- shared key to compute Integrity Check Value (ICV)
Different of the HMAC architecture picture
25
26AH 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
27AH diagram
HMAC applied only to fields in yellow
27
28Piggybacking AH on IPv4
- The structure allows IPSec logic to
- peel off the AH header, do verification and/or
decoding, - Modify length and next protocol fields to be
that of an AH-free IP packet - Push the packet up the stack with higher levels
none the wiser that IPSec was present
29Tunneling 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
30AH in Tunnel Mode
- How to detect
- tunnel mode
Original IP header
30
31ESP - 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---
31
32ESP diagram
SPI describes encryption
Protocol using TCP is Completely hidden
Padding and pad len support block encryption
32
33Key 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
33
34IPsec 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
34
35 Perimeter Defense
- Is it adequate?
- Locating and securing all perimeter points is
quite difficult - Less effective for large border
- Inspecting/ensuring that remote connections are
adequately protected is difficult - Insiders attack is often the most damaging
36Virtual Private Networks
- A private network that is configured within a
public network - A VPN appears to be dedicated network to
customer - The customer is actually sharing trunks and
other physical infrastructure with other
customers - Security?
- Depends on implementing protocol
37Multiple VPN Technologies
- IPSec
- Confidentiality? Yes
- Data Integrity? Yes
- User Authentication? Yes
- Network access control? Yes
- Client configuration required.
- VLAN Layer 2 tunnelling technology
- Confidentiality? No
- Data Integrity? No
- User authentication? Yes
- Network access control? Yes
- Not viable over non-VLAN internetworks
- SSL
- Confidentiality? Yes
- Data integrity? Yes
- User authentication? Yes
- Network access control? No
- In addition, limited traffic
38Security Domains with VPNs
39Typical corporate network
Firewall
Demilitarized Zone (DMZ)
Intranet
Mail forwarding
DNS (DMZ)
Web Server
File Server
Web Server
Firewall
Mail server
DNS (internal)
User machines
User machines
User machines
Internet
40VPN using IPSec
- ESP does the encryption
- Difficulty with NAT means ESPAuth in tunnel mode
- Requires VPN gateway---view is a tunnel between
two trusted networks
40
41VPN using IPSec
42Firewall Goal
- Insert after the fact security by wrapping or
interposing a filter on network traffic
43Application Proxy Firewall
- Firewall software runs in application space on
the firewall - The traffic source must be aware of the proxy and
add an additional header - Leverage basic network stack functionality to
sanitize application level traffic - Block java or active X
- Filter out bad URLs
- Ensure well formed protocols or block suspect
aspects of protocol
44Packet 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
45Stateful 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 and dynamic 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 to expect on a port - Degree of reconstruction varies per platform,
e.g. IOS does not do IP reassembly
46Traffic reconstruction
47Access Control Lists (ACLs)
- Used to define traffic streams
- Bind ACLs 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
48Ingress and Egress Filtering
- Ingress filtering
- Filter out packets from invalid addresses before
entering your network - Egress filtering
- Filter out packets from invalid addresses before
leaving your network
49Denial of Service
- Example attacks
- Smurf Attack
- TCP SYN Attack
- Teardrop
- DoS general exploits resource limitations
- Denial by Consumption
- Denial by Disruption
- Denial by Reservation
50TCP SYN Attack
- Exploits the three-way handshake
51TCP SYN Attack Solutions
- Intermediate Firewall/Router
- Limit number of half open connections
- Ingress and egress filtering to reduce spoofed
addresses - Does not help against DDoS bot networks
- Reactively block attacking addresses
- Generally expensive to acquire technology to do
fast enough - Fix Protocol - IPv6
52Teardrop 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
53Address Translation
- Traditional NAT RFC 3022 Reference RFC
- Map real address to alias address
- Real address associated with physical device,
generally an unroutable address - Alias address generally a routeable associated
with the translation device - Originally motivated by limited access to
publicly routable IP addresses - Folks didnt want to pay for addresses and/or
hassle with getting official addresses
54Address 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 IP addresses and ports - Limits how quickly new protocols can be
firewalled
55Address Hiding (NAPT)
- NAPT Network Address Port Translation
- Many to few dynamic mapping
- Packets from a large pool of private addresses
are mapped to a small pool of public addresses at
runtime - Port remapping makes this sharing more scalable
- Two real addresses can be rewritten to the same
alias address - Rewrite the source port to differentiate the
streams - Traffic must be initiated from inside, e.g. the
private address
56NAT example
57Static 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 servers
from a network that is otherwise hidden - Static port remapping is also available
58NAT example
192.168.1.5
128.274.15
59NAT and IPSec AH dont mix
- Recall the diagram illustrating the fields
covered by AH - AH header created at the sender, src/dest IP
addresses changed by NAT
60FW Runtime Characteristics
- Firewalls track streams of traffic
- TCP streams are obvious
- Creates pseudo UDP streams for UCP packets
between the same addresses and ports that arrive
near enough to each other - Processing first packet in stream is more
expensive - Must evaluate ACLs and calculate address
translations - Subsequent packets get session data from a table
61Multi-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 security level interface - Inbound traffic from high security level
interface to low security level 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 outside groups - Routing conflicts with address translation
- Address translation specifies both interfaces
- Must be evaluated before the routing, better be
consistent
62Four 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 win
63Identity 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
one protocol to cover other protocols from same
source - Authorization for executing commands on the
device - Download or enable ACLs
- XAuth to integrate AAA with VPN authentication
and other security mechanisms
64AAA Scenario
65Is 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 term
partnerships, 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
66Intrusion Detection
- Holy Grail Detect and correct bad system
behavior - Detection can be viewed in two parts
- Anomaly detection Use statistical techniques to
determine unusual behavior - Mis-use detection Use signatures to determine
occurrence of known attacks - Detection can be performed on host data (HIDS),
network data (NIDS), or a hybrid of both
67Intrusion 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
68Honey 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
69IDS Architecture
- Agents run at the lowest level gathering data.
Perform some basic processing. - Agents send data to a Director that performs more
significant processing of the data. Potentially
there is a hierarchy of agents and directors - Director has information from multiple sources
and can perform a time-based correlation to
derive more significant actions - Directors invoke Notifiers to perform some action
in response 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 action from the attacker
70Data 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
71Mis-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 of people producing new
signatures for newly observed malevolent behavior - The static signature mechanism has obvious
problems in that a dedicated attacker can adjust
his behaviour to avoid matching the signature. - 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
systems might be normal in a network of Windows
Systems (or visa versa) - Can result in IDS tuned too low to miss real
events - Can hide real attacks in the mass of false
positives
72Example Signature
- Signature for port sweep
- A set of TCP packets attempting to connect to a
sequence of ports on the same device in a fixed
amount of time - In some environments, the admin might run nmap
periodically to get an inventory of what is on
the network - You would not want to activate this signature in
that case
73Anomaly/statistical detection
- Seems like using statistics will result in a more
adaptable and self-tuning system - Statistics, neural networks, data mining, etc.
- How do you characterize normal?
- Create training data from observing good runs
- E.g., Forrests 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 adjust expected changes over time - Rapid changes can occur. E.g., different
behaviour after work hours or changing to a work
on the next project
74Host Based IDS
- Tripwire Very basic detection of changes to
installed binaries - More recent HIDS. Look at patterns of actions of
system calls, file activity, etc. to permit,
deny, or query operations - Cisco Security Agent
- Symantec
- McAfee Entercept
75Classical NIDS deployment
76NIDS Remediation Options
- Log the event
- Drop the connection
- Reset the connection
- Change the configuration of a nearby router or
firewall to block future connections
77Intrusion 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 can
take direct steps to remediate. - If you move IDS into the network processing path,
how is this different from really clever
firewalling?
78Summary
- Identification of security domains basis of
perimeter security control - Firewall is the main enforcer
- Intrusion detection introduces deeper analysis
and potential for more dynamic enforcement - Intermediate enforcement can handle some Denial
of Service attacks