Chapter 3 Systems Security 1 - Operating Systems and Networks

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IT Auditing, Hall, 3e



Perform three main tasks:◦ translates high-level languages into the machine-level language

◦ allocates computer resources to user applications

◦manages the tasks of job scheduling and multiprogramming

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Protect against tampering by users

Prevent users from tampering with the programs of other users

Safeguard users’ applications from accidental corruption

Safeguard its own programs from accidental corruption

Protect itself from power failures and other disasters

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Log-On Procedure◦ first line of defense – user IDs and passwords

Access Token◦ contains key information about the user

Access Control List◦ defines access privileges of users

Discretionary Access Control◦ allows user to grant access to another user

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Access Privileges Audit objectives: verify that access

privileges are consistent with separation of incompatible functions and organization policies

Audit procedures: review or verify…◦ policies for separating incompatible functions◦ a sample of user privileges, especially access

to data and programs◦ security clearance checks of privileged

employees◦ formal acknowledgements to maintain

confidentiality of data◦ users’ log-on times

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Password Control Audit objectives: ensure adequacy and

effectiveness of password policies for controlling access to the operating system

Audit procedures: review or verify…◦ passwords required for all users◦ password instructions for new users◦ passwords changed regularly◦ password file for weak passwords◦ encryption of password file◦ password standards◦ account lockout policies

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Malicious & Destructive Programs

Audit objectives: verify effectiveness of procedures to protect against programs such as viruses, worms, back doors, logic bombs, and Trojan horses

Audit procedures: review or verify…◦ training of operations personnel concerning

destructive programs◦ testing of new software prior to being

implemented◦ currency of antiviral software and frequency of

upgrades

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Audit Trail Controls

Audit objectives: used to (1) detect unauthorized access, (2) facilitate event reconstruction, and/or (3) promote accountability

Audit procedures: review or verify…◦ how long audit trails have been in place

◦ archived log files for key indicators

◦ monitoring and reporting of security violations

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The communications component is a unique aspect of computer networks:◦ different than processing (applications) or data

storage (databases)

Network topologies – configurations of:◦ communications lines (twisted-pair wires,

coaxial cable, microwaves, fiber optics)

◦ hardware components (modems, multiplexers, servers, front-end processors)

◦ software (protocols, network control systems)

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Intercepting network messages◦ sniffing: interception of user IDs, passwords,

confidential e-mails, and financial data files

Accessing corporate databases◦ connections to central databases increase the

risk that data will be accessible by employees

Privileged employees ◦ override privileges may allow unauthorized

access to mission-critical data

Reluctance to prosecute◦ fear of negative publicity leads to such

reluctance but encourages criminal behavior

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How serious is the risk?◦ National Consumer League: Internet fraud rose

by 600% between 1997 and 1998

◦ SEC: e-mail complaints alleging fraud rose from 12 per day in 1997 to 200-300 per day in 1999

Major areas of concern:◦ Theft of credit card numbers

◦ Theft of passwords

◦ Consumer privacy--cookies

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IP spoofing: masquerading to gain access to a Web server and/or to perpetrate an unlawful act without revealing one’s identity

Denial of service (DOS) attacks: assaulting a Web server to prevent it from servicing users ◦ particularly devastating to business entities that

cannot receive and process business transactions

Other malicious programs: viruses, worms, logic bombs, and Trojan horses pose a threat to both Internet and Intranet users

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SYN Flood – when the three-way handshake needed to establish an Internet connection occurs, the final acknowledgement is not sent by the DOS attacker, thereby tying-up the receiving server while it waits.

Smurf – the DOS attacker uses numerous intermediary computer to flood the target computer with test messages, “pings”.

Distributed DOS (DDOS) – can take the form of Smurf or SYN attacks, but distinguished by the vast number of “zombie” computers hi-jacked to launch the attacks.

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In a DOS Attack, the sender sends hundreds of messages, receives the SYN/ACK packet, but does not response with an ACK packet. This leaves the receiver with clogged transmission ports, and legitimate messages cannot be received.

13

Sender Receiver

Step 1: SYN messages

Step 2: SYN/ACK

Step 3: ACK packet code

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Include:◦Disrupting, destroying, or corrupting transmissions between senders and receivers

◦ Loss of databases and programs stored on network servers

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Firewalls provide security by channeling all network connections through a control gateway.

Network level firewalls ◦ Low cost and low security access control ◦ Do not explicitly authenticate outside users◦ Filter junk or improperly routed messages ◦ Experienced hackers can easily penetrate the

system

Application level firewalls ◦ Customizable network security, but expensive ◦ Sophisticated functions such as logging or user

authentication

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Denial-of-service (DOS) attacks

◦ Security software searches for connections which have been half-open for a period of time.

Encryption

◦Computer program transforms a clear message into a coded (cipher) text form using an algorithm.

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Controlling for three common forms of DOS attacks:◦ Smurf attacks—organizations can program firewalls to

ignore an attacking site, once identified

◦ SYN flood attacks—two tactics to defeat this DOS attack

Get Internet hosts to use firewalls that block invalid IP addresses

Use security software that scan for half-open connections

◦ DDos attacks–many organizations use Intrusion Prevention Systems (IPS) that employ deep packet inspection (DPI) IPS works with a firewall filter that removes malicious packets from the

flow before they can affect servers and networks

DPI searches for protocol non-compliance and employs predefined criteria to decide if a packet can proceed to its destination

(See chapter 12 for more on DOS attacks)

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The conversion of data into a secret code for storage and transmission

The sender uses an encryption algorithm to convert the original cleartext message into a coded ciphertext.

The receiver decodes / decrypts the ciphertext back into cleartext.

Encryption algorithms use keys◦ Typically 56 to 128 bits in length ◦ The more bits in the key the stronger the encryption

method.

Two general approaches to encryption are private key and public key encryption.

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Digital signature – electronic authentication technique to ensure that…◦ transmitted message originated with the authorized

sender

◦ message was not tampered with after the signature was applied

Digital certificate – like an electronic identification card used with a public key encryption system ◦ Verifies the authenticity of the message sender

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Digital Signature



Message sequence numbering – sequence number used to detect missing messages

Message transaction log – listing of all incoming and outgoing messages to detect the efforts of hackers

Request-response technique – random control messages are sent from the sender to ensure messages are received

Call-back devices – receiver calls the sender back at a pre-authorized phone number before transmission is completed

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Review firewall effectiveness in terms of flexibility, proxy services, filtering, segregation of systems, audit tools, and probing for weaknesses.

Review data encryption security procedures

Verify encryption by testing

Review message transaction logs

Test procedures for preventing unauthorized calls

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Line errors are data errors from communications noise.

Two techniques to detect and correct such data errors are:

◦ echo check - the receiver returns the message to the sender

◦ parity checks - an extra bit is added onto each byte of data similar to check digits

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Using a sample of messages from the transaction log:◦ examine them for garbled contents caused by line noise

◦ verify that all corrupted messages were successfully retransmitted

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Electronic data interchange (EDI) uses computer-to-computer communications technologies to automate B2B purchases.

Audit objectives: 1. Transactions are authorized, validated, and in

compliance with the trading partner agreement.

2. No unauthorized organizations can gain access to database

3. Authorized trading partners have access only to approved data.

4. Adequate controls are in place to ensure a complete audit trail.

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Reduction or elimination of data entry

Reduction of errors Reduction of paper Reduction of paper processing and postage

Reduction of inventories (via JIT systems)

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Authorization ◦ automated and absence of human intervention

Access◦ need to access EDI partner’s files

Audit trail◦ paperless and transparent (automatic) transactions

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Authorization◦ use of passwords and value added networks (VAN) to ensure valid partner

Access◦ software to specify what can be accessed and at what level

Audit trail◦ control log records the transaction’s flow through each phase of the transaction processing

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EDI System

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EDI System using Transaction Control Log for Audit Trail

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Tests of Authorization and Validation Controls◦ Review procedures for verifying trading partner

identification codes ◦ Review agreements with VAN◦ Review trading partner files

Tests of Access Controls◦ Verify limited access to vendor and customer files◦ Verify limited access of vendors to database◦ Test EDI controls by simulation

Tests of Audit Trail Controls◦ Verify existence of transaction logs◦ Review a sample of transactions

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PC operating systems PC systems risks & controls

In general: Relatively simple to operate and program Controlled and operated by end users Interactive data processing vs. batch Commercial applications vs. custom Often used to access data on mainframe or

network Allows users to develop their own

applications Operating Systems:

Are located on the PC (decentralized) O/S family dictates applications (e.g.,

Windows)

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Controls Risk assessment Inherent weaknesses Weak access control Inadequate segregation of duties Multilevel password control – multifaceted access

control

Risk of physical loss Laptops, etc. can “walk off”

Risk of data loss Easy for multiple users to access data End user can steal, destroy, manipulate Inadequate backup procedures

Local backups on appropriate medium Dual hard drives on PC External/removable hard drive on PC

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Risk associated with virus infection Policy of obtaining software Policy for use of anti-virus software Verify no unauthorized software on PCs

Risk of improper SDLC procedures Use of commercial software Formal software selection procedures

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Verify controls are in place to protect data, programs, and computers from unauthorized access, manipulation, destruction, and theft

Verify that adequate supervision and operating procedures exist to compensate for lack of segregation between the duties of users, programmers, and operators

Verify that backup procedures are in place to prevent data and program loss due to system failures, errors

Verify that systems selection and acquisition procedures produce applications that are high quality, and protected from unauthorized changes

Verify the system is free from viruses and adequately protected to minimize the risk of becoming infected with a virus or similar object

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Verify that microcomputers and their files are physically controlled

Verify from organizational charts, job descriptions, and observation that the programmers of applications performing financially significant functions do not also operate those systems.

Confirm that reports of processed transactions, listings of updated accounts, and control totals are prepared, distributed, and reconciled by appropriate management at regular and timely intervals.

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Determine that multilevel password control or multifaceted access control is used to limit access to data and applications, where applicable.

Verify that the drives are removed and stored in a secure location when not in use, where applicable.

Verify that backup procedures are being followed.

Verify that application source code is physically secured (such as in a locked safe) and that only the compiled version is stored on the microcomputer.

Review systems selection and acquisition controls

Review virus control techniques.



Internet Technologies

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Packet switching◦ messages are divided into small packets◦ each packet of the message takes a different routes

Virtual private network (VPN)◦ a private network within a public network

Extranets◦ a password controlled network for private users

World Wide Web ◦ an Internet facility that links users locally and globally

Internet addresses◦ e-mail address◦ URL address◦ IP address

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facilitate the physical connection between the network devices.

synchronize the transfer of data between physical devices.

provide a basis for error checking and measuring network performance.

promote compatibility among network devices.

promote network designs that are flexible, expandable, and cost-effective.

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Transfer Control Protocol/Internet Protocol (TCP/IP) - controls how individual packets of data are formatted, transmitted, and received

Hypertext Transfer Protocol (HTTP) - controls web browsers

File Transfer Protocol (FTP) - used to transfer files across the internet

Simple Network Mail Protocol (SNMP) - e-mail

Secure Sockets Layer (SSL) and Secure Electronic Transmission (SET) - encryption schemes

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A federation of computers located close together (on the same floor or in the same building) linked together to share data and hardware

The physical connection of workstations to the LAN is achieved through a network interface card (NIC) which fits into a PC’s expansion slot and contains the circuitry necessary for inter-node communications.

A server is used to store the network operating system, application programs, and data to be shared.

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File Server

LAN

LAN

Node

Node

Node

Node

Printer Server

Files

Printer

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A WAN is a network that is dispersed over a wider geographic area than a LAN. It typically requires the use of:◦ gateways to connect different types of

LANs◦ bridges to connect same-type LANs

WANs may use common carrier facilities, such as telephone lines, or they may use a Value Added Network (VAN).

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LANLAN

Bridge

Gateway

Gateway

LAN

WAN

WAN

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A network of IPUs with a large central computer (the host)

The host computer has direct connections to smaller computers, typically desktop or laptop PCs.

This topology is popular for mainframe computing.

All communications must go through the host computer, except for local computing.

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Local Data Local Data

Local Data

Local Data

Central Data

POS

POS

POS

POS

POS

Topeka St. Louis

Kansas

City

DallasTulsa

Star Network

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A host computer is connected to several levels of subordinate smaller computers in a master-slave relationship.

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Production

Planning System

Production

Scheduling

System

Regional

Sales System

Warehouse

System

Warehouse

System

Production

System

Production

System

Sales

Processing

System

Sales

Processing

System

Sales

Processing

System

Corporate

Level

Regional

Level

Local

Level

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This configuration eliminates the central site. All nodes in this configuration are of equal status (peers).

Responsibility for managing communications is distributed among the nodes.

Common resources that are shared by all nodes can be centralized and managed by a file server that is also a node.

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Ring Topology

Figure 12-10

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The nodes are all connected to a common cable - the bus.

Communications and file transfers between workstations are controlled by a server.

It is generally less costly to install than a ring topology.

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Bus Topology

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This configuration distributes the processing between the user’s (client’s) computer and the central file server.

Both types of computers are part of the network, but each is assigned functions that it best performs.

This approach reduces data communications traffic, thus reducing queues and increasing response time.

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Client-Server Topology

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establish a communications sessionbetween the sender and the receiver

manage the flow of data across the network

detect errors in data caused by line failure or signal degeneration

detect and resolve data collisions between competing nodes

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A random access technique that detects collisions when they occur

This technique is widely used--found on Ethernets. The node wishing to transmit listens to the line to

determine if in use. If it is, it waits a pre-specified time to transmit.

Collisions occur when nodes listen, hear no transmissions, and then simultaneously transmit. Data collides and the nodes are instructed to hang up and try again.

Disadvantage: The line may not be used optimally when multiple nodes are trying to transmit simultaneously.

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Documents

Chapter 3 Systems Security 1 - Operating Systems and Networks