Security fundamentals
Topic 7
Securing network communications
Agenda
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Remote access
Remote authentication
RADIUS and TACACS
VPNs and tunnelling
Terminal sessions
Wireless network security
IPSec
Remote access
• Additional security challenges with RAS
networks
• Traditional: remote access by dialling into
modems at corporate office
• VPNs use existing internet connections – no
control over how packets flow
Dial-up connections
• Bank of modems connected to remote access server
• RAS server authenticates the user
• RAS server routes packets between remote computer and
corporate network
• Uses PPP (Point-to-Point) or SLIP (Serial Line Internet Protocol)
• SLIP does not support encryption and transmits passwords in clear
text
• Connection level security from RAS server:
• CBCP (Call-Back Control Protocol) – either end calls back on a
supplied number or prearranged number (access only from specific
location)
– Accepts or reject calls based on caller ID or automatic number
identification
Dial-up connections
• Advantages:
– Limited access for attackers (long distance rates)
– Low likelihood of eavesdropping (requires access to
telephone lines/exchange)
• Disadvantages:
– Cost – equipment, line rental, call charges
– Low productivity – limited connection speed
– War dialling – dialling numbers to determine if modem
attached
Virtual Private Networks
• Virtual Private Networks
– Both sides establish an encrypted session with
each other over the internet
– Connection is assigned IP addresses and uses a
virtual adapter
• VPN tunnel – a secure connection through the
internet
Virtual Private Networks
• Advantages:
– Low costs: modems are not required, local call to ISP
– High productivity: Broadband connection at remote users end means
faster connection speed
– Fewer external connection points: A single device (firewall) to handle
multiple client connections – limits the attack surface – only one
connection point
• Disadvantages:
– Risk of attack: potential attacks from Internet to compromise RAS
server
– Risk of eavesdropping: potential to intercept traffic and decrypt it
– High exposure to attackers: if remote user’s computer is not secure
there is a risk to the corporate network
Remote connection requirements
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Common protocols at physical and data link layers
Secure communications
RAS - authenticate remote user
Common data-link layer protocols:
– Point-to-point connections: PPP or SLIP
– Network and transport protocols: TCP/IP
– Remote computer is assigned an IP address and other parameters for
virtual adapter – generally DHCP
• Host and remote software:
– Remote user requires a VPN client program
– Host or server requires VPN server service
• Security:
– Control access to resources – only authenticated and authorised users
– Limit access of remote users to only those resources required
Authentication methods
• PAP: (Password Authentication Protocol) – password sent in
clear text
• SPAP: (Shiva Password Authentication Protocol) – a reversible
encryption mechanism more secure but does not prevent
attacker impersonating a legitimate RAS server
• CHAP: (Microsoft® Challenge Handshake Authentication
Protocol) – uses MD5 hashing algorithm – Server sends
session ID and random value (challenge) to client – client
creates a hash from session ID, challenge and password and
sends to server. Password is never sent.
Authentication methods
• MS-CHAP: (Microsoft® Challenge Handshake Authentication
Protocol) support domain login capabilities – MD4 hash
comprising challenge string, session ID and password
• MS-CHAPv2: – Version 2, larger initial encryption key and
support for bidirectional challenge so that client also sends a
challenge to the server to authenticate itself
• EAP: (Extensible Authentication Protocol)
– Supports multitude of authentication mechanisms: passwords,
tokens, Kerberos and smart cards
– Highly secure and flexible
Centralised authentication
• RAS authenticates users against a user
accounts database such as ADS
• RAS forwards the authentication messages to
a central server that authenticates the user
against a central database
Centralised authentication
• Steps:
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RAS queries the user for credentials
Client sends authentication information
RAS forwards authentication information to a central authentication
server
Central server checks users credentials against user account
database: ADS, Novell e-directory, Unix NIS
Authentication server returns success or failure to RAS
If success, RAS allows client onto the network
• RAS servers contain no user account data – less risk of
compromise
• Protocols:
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RADIUS: Remote Authentication Dial-In User Service
TACACS+: Terminal Access Controller Access Control Service
RADIUS
• Provides AAA – authentication, authorisation and accounting for RAS
servers
• RADIUS protocol used to establish connection from RAS to authentication
server or RADIUS Server
• Separation of remote access and user authentication to allow:
• RADIUS client (RAS server) and Server can use different OS and
hardware architectures – different vendor implementation of RADIUS
work with each other
• RADIUS client and server can be geographically separated
• User accounts are secure because they are located on internal
corporate network
• Authentication traffic between RADIUS client and server can be
encrypted
• Can outsource RAS servers to third party organisation
RADIUS
• RADIUS separates authentication from remote access functions:
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Can use variety of authentication protocols: CHAP, MS-CHAP
Encrypt with variety of algorithms: MPPE, DES
Create connections with a variety of protocols: TCP/IP, IPX/SPX
Allow connections by using a variety of technologies: dialup, DSL, ISDN
• RADIUS servers can maintain remote access policies, inform users
of such and enforce policies:
– Includes protocols the user is allowed to use
– Length of time that the user can be connected
– Operating system that the user can use
• Case-sensitive shared secret password is set on RADIUS Server and
RADIUS Client – never sent over the network – hashing algorithm
used to verify shared secret, to authenticate RADIUS server and
client and to encrypt authentication information.
RADIUS accounting
• Tracks the following:
– Length of time the remote user is connected
– Remote user authentication success or failure
– Situation when the RADIUS server cannot
authenticate a RADIUS client
• ISPs use RADIUS to bill customers
RADIUS authentication
1. RAS receives a connection request from remote access client
2. RAS configured as RADIUS client, creates an Access-Request message and sends it
to the RADIUS server
3. RADIUS server processes message
• If required RADIUS Server sends an Access-Challenge to RAS
• RAS server or remote access client process challenge and sends a new AccessRequest to RADIUS server
• User credentials are verified – connection to ADS or user accounts DB
• Remote access policies are evaluated – is client allowed access to network
4. If connection is authenticated and authorised, RADIUS server sends an AccessAccept message to remote access server or sends Access-Reject message
5. RAS server receives Access-Accept message – sends an accounting request
message to RADIUS server
6. RADIUS server processes accounting request message and sends an accounting
response message to RAS (RADIUS client)
7. Client connection is completed
TACACS and TACACS+
• Cisco AAA protocol
• Standard method to manage dissimilar NAS network
access servers
• NAS provides connections to single user, network or
interconnected networks
• TACACS+ has replaced TACACS – not compatible
TACACS and TACACS+
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Supports:
– Authentication: forwards username and password in these formats: ARA
(Apple Remote Access), SLIP, PAP, CHAP, telnet, KCHAP (Kerberos CHAP)
• Authentication data hashed with MD5
• Supports multiple challenge and responses as required for token based
authentication (OTP password)
– Authorisation:
• Mechanism to give an access server a pointer to an ACL associated with a
user. ACL specifies which resources can be accessed once authenticated
– Accounting:
• Accounting protocol part of TACACS+ contains network address of user,
username, service attempted, protocol used, time and date and packet
filter – all logged
• Auditing information includes commands and arguments used and the
connection associated with it.
Protocol can provide intruder detection routines, reporting statistics, number of
packets and number of bytes
RADIUS and TACACS differences
• RADIUS runs over UDP, TACACS+ runs over TCP
• RADIUS provides a user profile with
authentication that defines user specific
parameters, TACACS+ separates the
authentication and authorisation functions
• TACACS+ typically used for network devices
such as routers and switches
• RADIUS used for computers and network
devices
Virtual Private Networks
• Protocols used to establish VPN tunnel:
– PPTP: Point-to-Point Tunnelling Protocol
– L2TP: Layer 2 Tunnelling Protocol
– IPSec: Internet Protocol Security: architecture,
protocol and IKE (Internet Key Exchange) protocol
PPTP
• Point-to-Point Tunnelling Protocol
• PPTP client computer establishes a control connection to server using TCP
port 1723 > PPTP tunnel is established
• GRE (Generic Routing Encapsulation) protocol used to send data
• Characteristics:
– Layer 2 protocol encapsulates PPP frames in IP datagrams
– PPTP uses PAP, CHAP, MS-CHAP for authentication
– Requires an IP based network and does not support header
compression – provide encryption
– IP packet is encapsulation into a PPP frame which is encapsulated into
a VPN IP packet
– All data sent through the tunnel is encrypted
– PPTP supports many protocols and multicast – can be established
across networks that use NAT
L2TP
• IETF standard
• L2TP encapsulates PPP frames for sending over IP, X.25, frame
relay or ATM networks
• L2TP over IP uses UDP port 1701 and includes control
messages for tunnel maintenance
• Encapsulated frames can be encrypted or compressed
however L2TP has no mechanism for encryption or
authentication – use with another protocol - IPSec
• L2TP over IPSec: L2TP provides encapsulation and IPSec in
transport mode provides strong integrity, replay, authenticity
and privacy protection
• Authentication options: CHAP, MSCHAP, MSCHAPv2, EAP
• Some NAT implementations cannot use IPSec tunnel mode,
use NAT-T transversal technology to encapsulate IPSec in UDP
Comparing VPN solutions
• L2TP/IPSec:
– Multivendor interoperability
– Client to gateway and gateway to gateway
– IPSec may have issues with NAT
• PPTP:
– Early implementations have lower security
– Security of PPTP depends on strength of users
password
– No NAT limitations
Securing VPN connections
• Choose a protocol that provides a secure
authentication method and encrypts the data
• Monitor traffic that originates from VPNs after it has
been decrypted on RAS
• Require multifactor authentication – such as smart card
• Ensure computers connecting with VPN meet
corporate security policies (antivirus, quarantine if
configuration not sufficient)
• Ensure that after VPN is established, no routes to
internet exist in client computer that bypass your
network – hacker may gain access via VPN
Terminal sessions
• Remotely access a single computer: all processes run
on remote computer but session takes input from
another computer and redirects output to that
computer: terminal services, remote desktop and VNC
• Terminal services also allows multiple users to
simultaneously establish sessions and run programs on
a single computer.
• Provide an alternative to remote access connections
• Useful administrator tool however also create security
risks
• Use strong encryption for the terminal session and
prevent users from accessing other users data
Secure shell protocol
• SSH lets users log onto a remote computer, execute
commands and moves files from one computer to
another
• Provides strong authentication and secure
communications over unsecured channels
• SSH2 is a replacement for FTP
• Encrypts passwords and data, host and user
authentication, data compression, data
confidentiality, strong encryption, host
authentication and integrity protection
SSH
• 3 major components:
– Transport layer SSH-TRANS secure authentications, confidentiality and
network integrity – encryption is an option – typically runs over TCP/IP
but can be used over any reliable data stream
– User authentication protocol SSH-USERAUTH – authenticates the user
at the client computer to the server
– Connection protocol SSH-CONN multiplexes the encrypted tunnel into
several logical channels
• SSH uses PKI- public key encryption for user authentication,
however older methods may be used
• PKI ensures that users cant access a host for which they do
not have a public key
• SSH also provides protection from packet spoofing, IP/Host
spoofing, password sniffing and eavesdropping
Wireless threats
• Data can be intercepted without requiring physical access
• Common risks:
– Theft of service: free access to internet
– Eavesdropping: intercepted traffic
– Unauthorised access: to corporate information
• Basic defences:
– Limit the range of transmissions – do a site survey periodically to
detect signal strength and unauthorised APs
– Don't broadcast the SSID – harder for a hacker to determine existence
of WLAN
– Restrict access based on MAC address (MAC addresses can be spoofed
by hacker)
– Put all APs in a separate network segment and limit the type of traffic
allowed from network into corporate network
Wireless security
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WEP (Wired Equivalency Protocol)
WPA (Wi-Fi Protected Access)
802.1x
WAP (Wireless Application Protocol)
IPSec
• Layer 3 protocol that provides authentication and
encryption for IP traffic
• Situations for implementation:
– End- to end security for IP unicast traffic – transport mode
– Remote access VPN client to gateway – L2TP and IPSec
transport mode
– Site-to-site VPNs using L2TP/IPSec or IPSec tunnel mode
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Open protocol and interoperable
Windows has built in support for IPSec clients
Transparent protection for layer 3 and above protocols:
IPSec policy defines the types of traffic that require
encryption and/or digital signing
IPSec features
• ESP and AH
– ESP: provides encryption and signing
– AH: signs traffic but does not encrypt
• Tunnel and transport modes
– Transport mode – end to end security – protects entire data
path between sender and receiver
– Tunnel mode – secures the channel between the VPN gateways
• Protection from:
– Man-in-the-middle attacks – both parties authenticate
– Spoofing – digital signing to verify integrity
– Replay attacks – sequence numbers
IPSec features
• Before communication both parties negotiate a SA (Security
Association) – determines what authentication and
encryption methods will be used
• IPSec uses IKE (Internet Key Exchange) for authentication VPN
devices.
• IKE uses:
– Kerberos
– Certificates
– Password pre-shared authentication keys
Lesson summary
• Methods and security aspects of remote
access and authentication
• How RADIUS and TACACS are used to provide
AAA
• The security benefits and types of tunnelling
and VPNs
• The security of terminal sessions, wireless
networks and the uses and details of IPSec