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CIT 380: Securing Computer Systems

CIT 380: Securing Computer Systems

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CIT 380: Securing Computer Systems

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  1. CIT 380: Securing Computer Systems Malware CIT 380: Securing Computer Systems

  2. Topic • Types of Malware • Trojan Horses • Viruses • Worms • Backdoors • Rootkits • Self-Protection Mechanisms. • Payloads. • Malware Interactions. • Detecting Malware. • Defending against Malware. • The changing Malware environment. CIT 380: Securing Computer Systems

  3. 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. CIT 380: Securing Computer Systems

  4. What about Spyware? Malware by any other name… • Corporate malware. • Presents legal issues for anti-malware software. CIT 380: Securing Computer Systems

  5. 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 Trojan Horse CIT 380: Securing Computer Systems

  6. 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 CIT 380: Securing Computer Systems

  7. Types of Viruses • Boot Sector • 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 • Infects executable programs (e.g., COM, EXE). • Executes when infected program is run. • Virus usually runs first, then runs original code. • Dynamic Library • Infected dynamicly linked libraries (DLLs.) • Executed when any program uses infected DLL. CIT 380: Securing Computer Systems

  8. Types of Viruses • Device Driver • Infects loadable device driver. • Executes in kernel mode. • Virtual Machine (.NET) • Infects .NET MSIL binaries. • Portable: compiled to native code by CLR. • Archive Infectors • Inserts Trojan horse into ZIP files. • Uses social engineering techniques to get user to run. CIT 380: Securing Computer Systems

  9. Types of Viruses • Macro Virus • Infects embedded interpreted code. • Needs interpreter like sh, MS Word macro. • Can infect executables or data files • Executables must invoke appropriate interpreter. • Most modern data formats support some type of scripting, including • Microsoft Office • Windows Help files • HTML: VBScript, JScript CIT 380: Securing Computer Systems

  10. Infection Methods • Overwriting • Overwrites program code with virus. • Breaks infected program. • Appending • Append virus code to executable. • Insert JMP at beginning of executable. • Prepending • Insert virus code at beginning of executable. • Shift original code to follow virus. CIT 380: Securing Computer Systems

  11. Infection Methods • Parasitic • Inserts virus code at beginning of executable. • Shifts beginning of program to end of file. • Cavity • Insert virus code into unused blocks of file. • Insert JMP at beginning of executable. • Fractionated Cavity • Fragment virus; inject into multiple cavities. • Loader reads fragments into continuous memory. CIT 380: Securing Computer Systems

  12. Infection Methods • Compressing • Compresses executable to make space. • Inserts virus and decompression code. • Fragmenting • Dynamically fragment virus. • Insert fragments by overwriting or shifting code. • Fragments JMP/CALL each other. • Companion • Infects COM file of same name as EXE file. • Infects alternate data stream of Win32 file. CIT 380: Securing Computer Systems

  13. In-Memory Strategies Direct Action • Virus runs only when infected code is run. Memory Resident • Remains active in memory after application terminates. • Interrupt hook (TSR) in DOS. • Kernel-mode rootkit techniques under modern OSes. • Can infect any program that runs after virus. • Example: Jerusalem Virus (Danube variant) • Multipartite TSR virus. • Infects all executables except • Also infects boot sector. • Deletes files on Friday the 13th. CIT 380: Securing Computer Systems

  14. 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 CIT 380: Securing Computer Systems

  15. History of Worms CIT 380: Securing Computer Systems

  16. Worm Components • Vector • Propagation Engine • Target Selection • Scanning Engine • Payload CIT 380: Securing Computer Systems

  17. 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. CIT 380: Securing Computer Systems

  18. Propagation Engine Transfers worm to host exploited by vector. • Small worms like Slammer included in vector. Worm Propagation Methods: • FTP • HTTP • SMB • TFTP CIT 380: Securing Computer Systems

  19. 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. CIT 380: Securing Computer Systems

  20. 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. CIT 380: Securing Computer Systems

  21. 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. CIT 380: Securing Computer Systems

  22. Morris Worm • First Internet Worm: November 1988 • Multi-architecture: Sun, VAX • Multi-vector • sendmail (debug backdoor) • fingerd (buffer overflow) • rsh (open .rhosts; password cracking) CIT 380: Securing Computer Systems

  23. 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. CIT 380: Securing Computer Systems

  24. 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. CIT 380: Securing Computer Systems

  25. 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. CIT 380: Securing Computer Systems

  26. 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. CIT 380: Securing Computer Systems

  27. 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. CIT 380: Securing Computer Systems

  28. 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. CIT 380: Securing Computer Systems

  29. 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. • Ex: SUB -1 instead of ADD 1 • Randomize names in macro viruses. CIT 380: Securing Computer Systems

  30. 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. CIT 380: Securing Computer Systems

  31. 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. CIT 380: Securing Computer Systems

  32. 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. CIT 380: Securing Computer Systems

  33. 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. CIT 380: Securing Computer Systems

  34. 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. CIT 380: Securing Computer Systems

  35. 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. CIT 380: Securing Computer Systems

  36. 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. CIT 380: Securing Computer Systems

  37. 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. CIT 380: Securing Computer Systems

  38. 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 CIT 380: Securing Computer Systems

  39. 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. • Development difficult when compilers can’t produce executable code. CIT 380: Securing Computer Systems

  40. Defences: Information Flow Limit Information Flow • Virus executes with user’s identity. • Soln: Limit information flow between users. • 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? CIT 380: Securing Computer Systems

  41. Defences: Least Privilege Limit programs to least privilege needed example: SELinux Mail virus example • 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 • Mail application cannot create virus binaries. • Mail application cannot write to Startup folder. • Mail application cannot write to Office documents. CIT 380: Securing Computer Systems

  42. 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. CIT 380: Securing Computer Systems

  43. 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. CIT 380: Securing Computer Systems

  44. Defences: Counter-worms Worm that removes other worms from net. Nachi/Welchia • 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 • Infected Diebold ATMs • Worm traffic DOSed Internet, esp Microsoft. CIT 380: Securing Computer Systems

  45. 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. CIT 380: Securing Computer Systems

  46. 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.) CIT 380: Securing Computer Systems

  47. 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. CIT 380: Securing Computer Systems

  48. 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. CIT 380: Securing Computer Systems

  49. 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. CIT 380: Securing Computer Systems

  50. 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. CIT 380: Securing Computer Systems