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Guide to Operating
System Security
Chapter 2
Viruses, Worms, and
Malicious Software
Learning Objectives



Explain how viruses, worms, and Trojan
horses spread
Discuss typical forms of malicious software
and understand how they work
Use techniques to protect operating systems
from malicious software and to recover from
an attack
Guide to Operating System Security
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Viruses, Worms, and Trojan
Horses
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Different forms of malicious software
(malware)
Intended to
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
Cause distress to a user
Damage files or systems
Disrupt normal computer and network functions
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Viruses
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Programs borne by a disk or a file that has the
ability to replicate
Typically affect



Executable program
Script or macro
Boot or partition sector of a drive
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How Viruses Spread
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
Transported from one medium or system to
another
Replicated throughout a system (eg,
W32.Pinfi)
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Virus Classification (Continued)

How they infect systems
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Boot or partition sector
File infector
Macro
Multipartite
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Virus Classification (Continued)

How they protect themselves from detection or
from a virus scanner
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
Armored
Polymorphic
Stealth
Companion
Benign or destructive
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Worms
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Programs that replicate on the same computer
or send themselves to many other computers
Can open a back door
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How Worms Spread
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Buffer overflow (eg, Code Red and Code
Red II)
Port scanning or port flooding
Compromised passwords
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Trojan Horses and How They
Spread

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Programs that at first appear useful, but can
cause damage or provide a back door
Examples
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
Backdoor.Egghead
AOL4FREE
Simpsons AppleScript Virus
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Locations for Viruses, Worms,
and Trojan Horses (Continued)
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Locations for Viruses, Worms,
and Trojan Horses (Continued)
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Locations for Viruses, Worms,
and Trojan Horses (Continued)
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Location for a UNIX/Linux
System
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Location for a Windows XP
System
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Typical Methods Used by
Malicious Software
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Executable methods
Boot and partitions sector methods
Macro methods
E-mail methods
Software exploitation
Spyware
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Executable Methods

Files that contain lines of computer code that
can be run


Examples: .exe, .com, .bat, .bin, .btm, .cgi, .pl,
.cmd, .msi
Can infect source or execution code of a
program
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Boot and Partition Sector
Methods
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Particularly affect Windows and UNIX systems
Typically infect/replace instructions in MBR or
Partition Boot Sector
Can corrupt address of primary partition
May move boot sector to another location if size of
virus exceeds space allocated for boot sector
Eradication typically involves recreating MBR and
Partition Boot Sector instructions
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Macro Methods
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
A virus can infect a macro and spread each
time the macro is used
Software is configured so that macros are
disabled unless digitally signed by a trusted
source
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Macro Protection
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E-Mail Methods

Sent as attachments to e-mail
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Software Exploitation
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Particularly aimed at new software and new
software versions
Examples of potential vulnerabilities
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DNS services
Messaging services
Remote access services
Network services and applications
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Spyware

Software placed on a computer
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typically without user’s knowledge
reports back information about user’s activities
Some operate through monitoring cookies
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Protecting an OS from
Malicious Software
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Install updates
View what is loaded when a system is booted
Use malicious software scanners
Use digital signatures for system and driver
files
Back up systems and create repair disks
Create and implement organizational policies
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Installing Updates for Windows

Windows Update
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Provides access to patches that are regularly issued
Service packs

Address security issues and problems affecting
stability, performance, or operation of features
included with the OS
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Using Windows Update
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Using Windows Update
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Installing Updates for
Red Hat Linux (Continued)

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Issued frequently; can be downloaded from
Web site
Red Hat Network Alert Notification Tool must
be configured
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Installing Updates for
Red Hat Linux (Continued)
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Installing Updates for NetWare

Download updates and/or consolidated support
packs from Novell’s Web site
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Installing Updates for Mac OS X
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Software Update tool enables you to:
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Configure the system to automatically check for
updates at specified intervals
Manually check for updates
View currently installed updates
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Installing Updates for Mac OS X
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Viewing What Is Loaded When
a System Is Booted

Windows 2000, Windows XP Professional,
and Windows Server 2003
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Red Hat Linux and NetWare
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View information on-screen
Have a log record information (Advanced Options
menu)
Automatically display boot load information
Mac OS X

Display boot process by booting into either single
user mode or verbose mode
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Advanced Options Menu
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Using Malicious Software
Scanners



Effective way to protect operating system
Scan systems for virus, worms, and Trojan
horses
Often Called Virus Scanners
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Malicious Software Scanners:
Features to Look For (Continued)
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Scans memory and removes viruses
Continuous memory scanning
Scans hard and floppy disks and removes
viruses
Scans all know file formats
Scans HTML documents and e-mail
attachments
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Malicious Software Scanners:
Features to Look For (Continued)
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Automatically runs at a scheduled time
Manual run option
Detects known and unknown malicious
software
Updates for new malicious software
Scans files that are downloaded
Uses protected or quarantined zones for
downloaded files
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Using a Virus Scanner
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Virus Scanning Software (Continued)
Guide to Operating System Security
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Virus Scanning Software (Continued)
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Using Digital Signatures for
System and Driver Files

Digital signature

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Code placed in a file to verify its authenticity by
showing that it originated from a trusted source
Driver signing

Placing a digital signature in a device driver to
• Show that the driver is from a trusted source
• Indicate compatibility with an OS
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Backing Up Systems and
Creating Repair Disks
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Most OSs offers ways to back up your system
Some OSs enable creation of a boot disk or
repair disk

Windows 2000
• Emergency Repair Disk (ERD)

Windows XP or Windows Server 2003
• Automated System Recovery (ASR) set

Red Hat Linux
• Boot disk
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Creating a Windows 2000 ERD
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Create a new ERD each time you:
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Install software
Make a server configuration change
Install a new adapter
Add a NIC
Restructure a partition
Upgrade the OS
Enables you to fix problems with the server
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Creating a Windows 2000 ERD
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Creating an ASR Set
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Two components
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Backup of all system files (1.5 MB or more)
Backup of system settings (about 1.44 MB)
Does not back up application data files
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Creating an ASR Set
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Creating a Red Hat Linux
Boot Disk

Enables booting a system from a floppy disk
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Creating and Implementing
Organizational Policies (Continued)
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Provide users with training in security techniques
Train users about common malicious software
Require users to scan floppies and CDs before use
Establish policies about types of media that can be
brought in from outside and how they can be used
Establish policies that discourage/prevent users from
installing their own software
Guide to Operating System Security
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Creating and Implementing
Organizational Policies (Continued)
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Define policies that minimize/prevent
downloading files; require users to use a virus
scanner on any downloaded files
Create quarantine areas for files of uncertain
origin
Use virus scanning on e-mail and attachments
Discard e-mail attachments from unknown or
untrusted sources
Guide to Operating System Security
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Chapter Summary

Viruses, worms, and Trojan horses



Typical forms of malicious software



How they spread through operating systems and
across networks
What they target and why
Boot sector viruses
Viruses that attack through macros
How to set up defenses, such as operating
system patches and repair disks
Guide to Operating System Security
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