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Malware:
Malicious Software
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Viruses, Worms, Trojans, Rootkits
• Malware can be classified into several categories, depending
on propagation and concealment
• Propagation
– Virus: human-assisted propagation (e.g., open email attachment)
– Worm: automatic propagation without human assistance
• Concealment
– Rootkit: modifies operating system to hide its existence
– Trojan: provides desirable functionality but hides malicious operation
• Various types of payloads, ranging from annoyance to crime
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Insider Attacks
• An insider attack is a security breach that is
caused or facilitated by someone who is a part
of the very organization that controls or builds
the asset that should be protected.
• In the case of malware, an insider attack refers
to a security hole that is created in a software
system by one of its programmers.
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Backdoors
• A backdoor, which is also sometimes called a
trapdoor, is a hidden feature or command in a
program that allows a user to perform actions he
or she would not normally be allowed to do.
• When used in a normal way, this program
performs completely as expected and advertised.
• But if the hidden feature is activated, the program
does something unexpected, often in violation of
security policies, such as performing a privilege
escalation.
• Benign example: Easter Eggs in DVDs and software
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Logic Bombs
• A logic bomb is a program that performs a malicious action as
a result of a certain logic condition.
• The classic example of a logic bomb is a programmer coding
up the software for the payroll system who puts in code that
makes the program crash should it ever process two
consecutive payrolls without paying him.
• Another classic example combines a logic bomb with a
backdoor, where a programmer puts in a logic bomb that will
crash the program on a certain date.
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The Omega Engineering Logic Bomb
• An example of a logic bomb that was actually
triggered and caused damage is one that
programmer Tim Lloyd was convicted of using
on his former employer, Omega Engineering
Corporation. On July 31, 1996, a logic bomb
was triggered on the server for Omega
Engineering’s manufacturing operations,
which ultimately cost the company millions of
dollars in damages and led to it laying off
many of its employees.
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The Omega Bomb Code
• The Logic Behind the Omega Engineering Time Bomb included the
following strings:
• 7/30/96
– Event that triggered the bomb
• F:
– Focused attention to volume F, which had critical files
• F:\LOGIN\LOGIN 12345
– Login a fictitious user, 12345 (the back door)
• CD \PUBLIC
– Moves to the public folder of programs
• FIX.EXE /Y F:\*.*
– Run a program, called FIX, which actually deletes everything
• PURGE F:\/ALL
– Prevent recovery of the deleted files
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Defenses against Insider Attacks
• Avoid single points of failure.
• Use code walk-throughs.
• Use archiving and reporting tools.
• Limit authority and permissions.
• Physically secure critical systems.
• Monitor employee behavior.
• Control software installations.
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Computer Viruses
• A computer virus is computer code that can
replicate itself by modifying other files or
programs to insert code that is capable of further
replication.
• This self-replication property is what
distinguishes computer viruses from other kinds
of malware, such as logic bombs.
• Another distinguishing property of a virus is that
replication requires some type of user assistance,
such as clicking on an email attachment or
sharing a USB drive.
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Biological Analogy
• Computer viruses share some properties with
Biological viruses
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Attack Penetration
Replication and assembly
Release
Early History
1972 sci-fi novel “When HARLIE Was One” features a
program called VIRUS that reproduces itself
First academic use of term virus by PhD student Fred
Cohen in 1984, who credits advisor Len Adleman with
coining it
In 1982, high-school student Rich Skrenta wrote first
virus released in the wild: Elk Cloner, a boot sector
virus
(c)Brain, by Basit and Amjood Farooq Alvi in 1986,
credited with being the first virus to infect PCs
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Virus Phases
• Dormant phase. During this phase, the virus just exists—
the virus is laying low and avoiding detection.
• Propagation phase. During this phase, the virus is
replicating itself, infecting new files on new systems.
• Triggering phase. In this phase, some logical condition
causes the virus to move from a dormant or propagation
phase to perform its intended action.
• Action phase. In this phase, the virus performs the
malicious action that it was designed to perform, called
payload.
– This action could include something seemingly innocent, like
displaying a silly picture on a computer’s screen, or something
quite malicious, such as deleting all essential files on the hard
drive.
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Infection Types
• Overwriting
– Destroys original code
• Pre-pending
– Keeps original code, possibly
compressed
• Infection of libraries
– Allows virus to be memory
resident
– E.g., kernel32.dll
• Macro viruses
– Infects MS Office documents
– Often installs in main
document template
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virus
compressed
original code
Degrees of Complication
• Viruses have various degrees of complication in how
they can insert themselves in computer code.
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Concealment
• Encrypted virus
– Decryption engine + encrypted body
– Randomly generate encryption key
– Detection looks for decryption engine
• Polymorphic virus
– Encrypted virus with random variations of the decryption engine (e.g.,
padding code)
– Detection using CPU emulator
• Metamorphic virus
– Different virus bodies
– Approaches include code permutation and instruction replacement
– Challenging to detect
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Computer Worms
• A computer worm is a malware program that spreads
copies of itself without the need to inject itself in other
programs, and usually without human interaction.
• Thus, computer worms are technically not computer
viruses (since they don’t infect other programs), but
some people nevertheless confuse the terms, since
both spread by self-replication.
• In most cases, a computer worm will carry a malicious
payload, such as deleting files or installing a backdoor.
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Early History
First worms built in the labs of John Shock and Jon
Hepps at Xerox PARC in the early 80s
CHRISTMA EXEC written in REXX, released in
December 1987, and targeting IBM VM/CMS
systems was the first worm to use e-mail service
The first internet worm was the Morris Worm,
written by Cornell student Robert Tappan Morris
and released on November 2, 1988
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a
th
Worm Development
• Identify vulnerability still
unpatched
• Write code for
– Exploit of vulnerability
– Generation of target list
• Random hosts on the internet
• Hosts on LAN
• Divide-and-conquer
– Installation and execution of
payload
– Querying/reporting if a host is
infected
• Initial deployment on a private
network
• Worm template
– Generate target list
– For each host on target
list
• Check if infected
• Check if vulnerable
• Infect
• Recur
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Worm Propagation
• Worms propagate by finding and infecting vulnerable hosts.
– They need a way to tell if a host is vulnerable
– They need a way to tell if a host is already infected.
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initial infection
Propagation: Theory
Classic epidemic model
– N: total number of vulnerable
hosts
– I(t): number of infected hosts
at time t
– S(t): number of susceptible
hosts at time t
– I(t) + S(t) N
– : infection rate
Differential equation for I(t):
dI dt I(t) S(t)
More accurate models adjust
propagation rate over time
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Source:
Cliff C. Zou,WeiboGong, Don Towsley,
and LixinGao. The Monitoring and Early
Detection of Internet Worms, IEEE/ACM
Transactions on Networking, 2005.
Propagation: Practice
• Cumulative total of unique IP addresses infected by the first
outbreak of Code-RedI v2 on July 19-20, 2001
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Source:
David Moore, Colleen
Shannon, and Jeffery
Brown. Code-Red: a
case study on the spread
and victims of an Internet
worm, CAIDA, 2002
Trojan Horses
• A Trojan horse (or Trojan) is a malware program that
appears to perform some useful task, but which also
does something with negative consequences (e.g.,
launches a keylogger).
• Trojan horses can be installed as part of the payload of
other malware but are often installed by a user or
administrator, either deliberately or accidentally.
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Current Trends
• Trojans currently have largest infection potential
– Often exploit browser vulnerabilities
– Typically used to download other malware in multi-stage attacks
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Source:
Symantec Internet
Security Threat
Report, April 2009
Rootkits
• A rootkit modifies the operating system to hide its
existence
– E.g., modifies file system exploration utilities
– Hard to detect using software that relies on the OS itself
• RootkitRevealer
– By Bryce Cogswell and Mark Russinovich (Sysinternals)
– Two scans of file system
– High-level scan using the Windows API
– Raw scan using disk access methods
– Discrepancy reveals presence of rootkit
– Could be defeated by rootkit that intercepts and modifies
results of raw scan operations
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Malware Zombies
• Malware can turn a computer in to a zombie, which is
a machine that is controlled externally to perform
malicious attacks, usually as a part of a botnet.
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BotnetController (Attacker)
Victim
Botnet:
Attack Commands
Attack Actions
ontroller
Financial Impact
Malware often affects a large user
population
Significant financial impact, though
estimates vary widely, up to $100B
per year (mi2g)
Examples
LoveBug (2000) caused $8.75B in
damages and shut down the
British parliament
In 2004, 8% of emails infected by
W32/MyDoom.A at its peak
In February 2006, the Russian
Stock Exchange was taken down by
a virus.
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Economics of Malware
• New malware threats
have grown from 20K to
1.7M in the period 2002-
2008
• Most of the growth has
been from 2006 to 2008
• Number of new threats
per year appears to be
growing an exponential
rate.
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Source:
Symantec Internet
Security Threat
Report, April 2009
Professional Malware
• Growth in professional cybercrime
and online fraud has led to demand
for professionally developed
malware
• New malware is often a customdesigned
variations of known
exploits, so the malware designer
can sell different “products” to
his/her customers.
• Like every product, professional
malware is subject to the laws of
supply and demand.
– Recent studies put the price of a
software keystroke logger at $23 and
a botnet use at $225.
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Image by User:SilverStar from http://commons.wikimedia.org/wiki/File:Supply-demand-equilibrium.svg
used by permission under the CreativeCommons Attribution ShareAlike 3.0 License
Adware
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Adware software payload
Adware engine infects
a user’s computer
Computer user
Adware agent
Adware engine requests
advertisements
from adware agent
Advertisers
Advertisers contract with
adware agent for content
Adware agent delivers
ad content to user
Spyware
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Spyware software payload
1. Spyware engine infects
a user’s computer.
Computer user
Spyware data collection agent
2. Spyware process collects
keystrokes, passwords,
and screen captures.
3. Spyware process
periodically sends
collected data to
spyware data collection
agent.
p
Signatures: A Malware Countermeasure
• Scan compare the analyzed object with a database of
signatures
• A signature is a virus fingerprint
– E.g.,a string with a sequence of instructions specific for
each virus
– Different from a digital signature
• A file is infected if there is a signature inside its code
– Fast pattern matching techniques to search for signatures
• All the signatures together create the malware
database that usually is proprietary
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Signatures Database
• Common Malware
Enumeration (CME)
– aims to provide
unique, common
identifiers to new virus
threats
– Hosted by MITRE
– http://cme.mitre.org/d
ata/list.html
•
Digital Immune
System (DIS)
– Create automatically
new signatures
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White/Black Listing
• Maintain database of cryptographic hashes for
– Operating system files
– Popular applications
– Known infected files
• Compute hash of each file
• Look up into database
• Needs to protect the integrity of the database
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Heuristic Analysis
• Useful to identify new and “zero day” malware
• Code analysis
– Based on the instructions, the antivirus can determine
whether or not the program is malicious, i.e., program
contains instruction to delete system files,
• Execution emulation
– Run code in isolated emulation environment
– Monitor actions that target file takes
– If the actions are harmful, mark as virus
• Heuristic methods can trigger false alarms
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Shield vs. On-demand
• Shield
– Background process
(service/daemon)
– Scans each time a file is
touched (open, copy,
execute, etc.)
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On-demand
• Scan on explicit user
request or according to
regular schedule
• On a suspicious file,
directory, drive, etc.
Performance test of scan techniques
o Comparative: check the number of already known viruses that are
found and the time to perform the scan
o Retrospective: test the proactive detection of the scanner for unknown
viruses, to verify which vendor uses better heuristics
Anti-viruses are ranked using both parameters:
http://www.av-comparatives.org/
Online vs Offline Anti Virus Software
Online
• Free browser plug-in
• Authentication through third
party certificate (i.e. VeriSign)
• No shielding
• Software and signatures update
at each scan
• Poorly configurable
• Scan needs internet connection
• Report collected by the company
that offers the service
Offline
• Paid annual subscription
• Installed on the OS
• Software distributed securely by
the vendor online or a retailer
• System shielding
• Scheduled software and
signatures updates
• Easily configurable
• Scan without internet connection
• Report collected locally and may
be sent to vendor
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Quarantine
• A suspicious file can be isolated in a folder called quarantine:
– E.g,. if the result of the heuristic analysis is positive and you are
waiting for db signatures update
• The suspicious file is not deleted but made harmless: the user can
decide when to remove it or eventually restore for a false positive
– Interacting with a file in quarantine it is possible only through the
antivirus program
• The file in quarantine is harmless because it is encrypted
• Usually the quarantine technique is proprietary and the details are
kept secret
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Static vs. Dynamic Analysis
Static Analysis
• Checks the code without trying to
execute it
• Quick scan in white list
• Filtering: scan with different antivirus
and check if they return same result
with different name
• Weeding: remove the correct part of
files as junk to better identify the
virus
• Code analysis: check binary code to
understand if it is an executable, e.g.,
PE
• Disassembling: check if the byte code
shows something unusual
Dynamic Analysis
• Check the execution of codes inside a
virtual sandbox
• Monitor
– File changes
– Registry changes
– Processes and threads
– Networks ports
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Virus Detection is Undecidable
• Theoretical result by Fred
Cohen (1987)
• Virus abstractly modeled
as program that
eventually executes infect
• Code for infect may be
generated at runtime
• Proof by contradiction
similar to that of the
halting problem
• Suppose program
isVirus(P) determines
whether program P is a
virus
• Define new program Q
as follows:
if (not isVirus(Q))
infect
stop
• Running isVirus on Q
achieves a contradiction
10/21/2010 Malware 39
Other Undecidable Detection Problems
• Detection of a virus
– by its appearance
– by its behavior
• Detection of an evolution of a known virus
• Detection of a triggering mechanism
– by its appearance
– by its behavior
• Detection of a virus detector
– by its appearance
– by its behavior
• Detection of an evolution of
– a known virus
– a known triggering mechanism
– a virus detector
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Resources
• Computer Emergency Response Team
– Research center funded by the US federal government
– Vulnerabilities database
• Symantec
– Reports on malware trends
– Database of malware
• Art of Computer Virus Research and Defense by Peter Szor
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