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VIRUSES

VIRUSES. - Janhavi Naik. Overview. Structure Classification Categories. VIRUS:- STRUCTURE ( page no-652-654 s). Virus can be prepended, postpended or embedded to an executable program.

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VIRUSES

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  1. VIRUSES - JanhaviNaik

  2. Overview • Structure • Classification • Categories

  3. VIRUS:- STRUCTURE (page no-652-654 s) • Virus can be prepended, postpended or embedded to an executable program. • The infected program when invoked will first invoke the virus code and then execute the original code of the program.

  4. Depiction of virus structure (page no-653) Program v := (go to main; 1234567; subroutine infect-executable := { loop: file :=get-random-executable file; if (first-line-of-file=1234567) then goto loop

  5. Cont.. Else prepend v to file; } Subroutine do-damage := {whatever damage is to be done} Subroutine trigger-pulled := { return true if some condition holds} Main: main-program:= { infect-executable ;

  6. Cont.. If trigger-pulled then do-damage; Goto next;} Next: }

  7. program invoked The following steps:- • For uninfected file ,the virus first compresses that file. • Copy of virus is prepended to compressed program.

  8. Cont.. • The compressed version of infected program is uncompressed. • The uncompressed original program is executed.

  9. Compression logic (page no-654 s) • Program cv:= (go to main; 01234567; subroutine infect-executable := { loop:file :=get-random-executable file; if (first-line-of-file=01234567) then goto loop;

  10. Cont.. (1) compress file; (2) prepend cv to file; } Main: main-program {if ask permission then infect executable; (3) uncompress rest-of-file; (4) run compressed file;} }

  11. Virus classification(page no-655 s) Based upon following : • Type of target virus tries to infect • Method the virus uses to conceal itself from detection by users and antivirus softwares.

  12. CLASSIFICATION BY TARGET(page no-655 s) • Boot Sector Infector:- infects a master boot record or boot record and spreads when a system is booted from the disk containing virus. • File Infector:- infects the file that the OS or shell consider to be executable

  13. Cont.. • Macro Virus:- infects files with macro code that is interpreted by an application. • Infect a document or template rather than executable code. • Can infect at different points during a file’s use.

  14. CLASSIFICATION BY CONCEALMENT STRATERGY(page no-655,666) • Encrpted virus:- It creates random encryption key and decrypts the remainder of virus. • When infected program is invoked ,virus uses stored random key to decrypt the virus.

  15. Cont.. • When virus replicates ,different random key is selected because bulk of virus is encrypted with different key for each instance. • Stealth virus:- Its designed to hide itself from detection by antivirus software.

  16. Cont.. • Polymorphic virus:- Its mutates with every infection ,making detection by “signature” of virus. • It insert superflous instructions or interchange order of instructions. • Metamorphic virus:- It mutates with every infection but at each iteration rewrite itself completely increasing difficulty of detection.

  17. Example of stealth virus(page no-655) • Virus uses compression so that infected and uninfected program are of same length. • Virus places intercept logic in I/o routine when we attempt to read suspected portion of virus,present back uninfected program.

  18. Points to remember (page no-666) • Strategy of encryption virus where portion of virus is responsible for generating keys performing encryption or decryption is referred as “mutation engine”. • Virus kits enable to create number of different viruses and create problem for antivirus schemas.

  19. Macro viruses (page no-666) • It infects ms word document or ms office . • It infects documents ,information introduced onto computer in form of document. • It easily spreads.(electronic mail) • Traditonal file system acess controls of limited use in preventing spread.

  20. Email viruses (page no-656,657) • First rapidly spreading email virus is “Melissa” (made of ms word macro embedded in attachment). • The email virus sends itself to everyone on mailing list in user’s email package. • It does local damage on users system.

  21. Cont.. • Powerful version of virus appeared in 1999 that can be activated by opening email rather than attachment that used scripting language.

  22. Questions • What is virus?Explain different phases of virus.Also with help of symbolic code explain virus structure. (7 mks) • Write short notes on viruses.(4 mks) • Explain nature of viruses and different types of viruses. (10 mks) • Write short notes on computer viruses. (6 mks)

  23. Any questions ?

  24. Thank you

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