Authenticating Users

Objectives• Explain why authentication is a critical aspect of

network security• Explain why firewalls authenticate and how they

identify users• Describe user, client, and session authentication• List the advantages and disadvantages of popular

centralized authentication systems• Discuss the potential weaknesses of password

security systems• Discuss the use of password security tools• Describe common authentication protocols used

by firewalls

The Authentication Process in General

• The act of identifying users and providing network services to them based on their identity

• Two forms– Local authentication– Centralized authentication service (often uses

two-factor authentication)

How Firewalls Implement the Authentication Process

1. Client makes request to access a resource2. Firewall intercepts the request and prompts the

user for name and password3. User submits information to firewall4. User is authenticated5. Request is checked against firewall’s rule base6. If request matches existing allow rule, user is

granted access7. User accesses desired resources

How Firewalls Implement the Authentication Process (continued)

Firewall Authentication Methods

• User authentication

• Client authentication

• Session authentication

User Authentication

• Basic authentication; user supplies username and password to access networked resources

• Users who need to legitimately access your internal servers must be added to your access control lists (ACLs)

User Authentication (continued)

Client Authentication

• Same as user authentication but with additional time limit or usage limit restrictions

• When configuring, set up one of two types of authentication systems– Standard sign-on system– Specific sign-on system

Client Authentication (continued)

Session Authentication

• Required any time the client establishes a session with a server of other networked resource

Comparison of Authentication Methods

Centralized Authentication

• Centralized server maintains all authorizations for users regardless of where user is located and how user connects to network

• Most common methods– Kerberos– TACACS+ (Terminal Access Controller

Access Control System)– RADIUS (Remote Authentication Dial-In User

Service)

Process of Centralized Authentication

Kerberos

• Provides authentication and encryption through standard clients and servers

• Uses a Key Distribution Center (KDC) to issue tickets to those who want access to resources

• Used internally on Windows 2000/XP• Advantages

– Passwords are not stored on the system– Widely used in UNIX environment; enables

authentication across operating systems

Kerberos Authentication

TACACS+

• Latest and strongest version of a set of authentication protocols for dial-up access (Cisco Systems)

• Provides AAA services– Authentication– Authorization– Auditing

• Uses MD5 algorithm to encrypt data

RADIUS

• Centralized dial-in authentication service that uses UDP

• Transmits authentication packets unencrypted across the network

• Provides lower level of security than TACACS+ but more widely supported

TACACS+ and RADIUS Compared

• Strength of security

• Filtering characteristics

• Proxy characteristics

• NAT characteristics

Strength of Security

Filtering Characteristics

Proxy Characteristics

• RADIUS– Doesn’t work with generic proxy systems, but

a RADIUS server can function as a proxy server

• TACACS+– Works with generic proxy systems

NAT Characteristics

• RADIUS– Doesn’t work with NAT

• TACACS+– Should work through NAT systems

Password Security Issues

• Passwords that can be cracked (accessed by an unauthorized user)

• Password vulnerabilities

• Lax security habits

Passwords That Can Be Cracked

• Ways to crack passwords– Find a way to authenticate without knowing

the password– Uncover password from system that holds it– Guess the password

• To avoid the issue– Protect passwords effectively– Observe security habits

Password Vulnerabilities

• Built-in vulnerabilities– Often easy to guess– Often stored visibly– Social engineering

• To avoid the issues– Choose complicated passwords– Memorize passwords– Never give passwords out to anyone

Lax Security Habits

• To maintain some level of integrity, draw up a formal Memorandum of Understanding (MOU)

Password Security Tools

• One-time password software

• Shadow password system

One-Time Password Software

• Password is generated using a secret key• Password is used only once, when the

user authenticates• Different passwords are used for each

authentication session• Types

– Challenge-response passwords– Password list passwords

Shadow Password System

• A feature of Linux that stores passwords in another file that has restricted access

• Passwords are stored only after being encrypted by a randomly generated value and an encoding formula

Other Authentication Systems

• Single-password systems

• One-time password systems

• Certificate-based authentication

• 802.1x Wi-Fi authentication

Single-Password Systems

• Operating system password

• Internal firewall password

One-Time Password Systems

• Single Key (S/Key)

• SecurID

• Axent Pathways Defender

Single Key (S/Key)

• Uses multiple-word rather than single word passwords– User specifies single-word password and the

number of times it is to be encrypted– Password is processed by a hash function n

times; resulting encrypted passwords are stored on the server

• Never stores original password on the server

SecurID

• Uses two-factor authentication– Physical object– Piece of knowledge

• Most frequently used one-time password solution with FireWall-1

SecurID Tokens

Axent Pathways Defender

• Uses two-factor authentication and a challenge-response system

Certificate-Based Authentication

• FireWall-1 supports the use of digital certificates to authenticate users

• Organization sets up a public key infrastructure (PKI) that generates keys to users– User receives a code (public key) that is generated

using the server’s private key and uses the public key to send encrypted information to the server

– Server receives the public key and can decrypt the information using its private key

802.1x Wi-Fi Authentication

• Supports wireless Ethernet connections

• Not supported by FireWall-1

• 802.1x protocol provides for authentication of users on wireless networks

• Wi-Fi uses Extensible Authentication Protocol (EAP)

Wireless Authentication

Chapter Summary

• Overview of authentication and its importance to network security

• How and why firewalls perform authentication services

• Types of authentication performed by firewalls– User– Client– Session

Chapter Summary (continued)

• Generally, users supply:– Something they have (such as a smart card)

or– Something they know (such as a password)

or– Both

• Latest authentication systems measure or evaluate a physical attribute, such as a fingerprint or voiceprint

Chapter Summary (continued)

• In a centralized authentication system:– Firewall works with an authentication server– Authentication server handles

• Username and password maintenance/generation• Login requests• Auditing

• Examples of centralized authentication systems:– Kerberos– TACACS+– RADIUS

Chapter Summary (continued)

• Passwords– Important part of virtually every authentication

system– Take one of two general forms:

• Single-word– User password compared against database of passwords;

access granted if match is made– Vulnerable to ability of hackers to determine passwords,

to user error, and to bad security habits

• One-time passwords– Generated dynamically each time user attempts to log on

to network– Secret key used to generate single- or multiple-word

password