CSCE 815 Network Security Lecture 19

1. CSCE 815 Network Security Lecture 19 Intruders April 1, 2003

2. Security Software Installed on SUNs • man crypt • man –s 2 crypt, man –s 3 crypt both fail • man –k crypt • Generates pages • Blowfish • RSA • DES • OpenSSL • EVP

3. OpenSSL on SUNs • man des • OpenSSL des(3) • NAME • des_random_key, des_set_key, des_key_sched, • des_set_key_checked, des_set_key_unchecked, • des_set_odd_parity, des_is_weak_key, des_ecb_encrypt, • des_ecb2_encrypt, des_ecb3_encrypt, des_ncbc_encrypt, • des_cfb_encrypt, des_ofb_encrypt, des_pcbc_encrypt, • des_cfb64_encrypt, des_ofb64_encrypt, des_xcbc_encrypt, • des_ede2_cbc_encrypt, des_ede2_cfb64_encrypt, • des_ede2_ofb64_encrypt, des_ede3_cbc_encrypt, • des_ede3_cbcm_encrypt, des_ede3_cfb64_encrypt, • des_ede3_ofb64_encrypt, des_read_password, • des_read_2passwords, des_read_pw_string, des_cbc_cksum, • des_quad_cksum, des_string_to_key, des_string_to_2keys, • des_fcrypt, des_crypt, des_enc_read, des_enc_write - DES • encryption • SYNOPSIS • #include <openssl/des.h>

4. Intruders • Three classes of intruders (hackers or crackers): • Masquerader • Misfeasor • Clandestine user

5. Intrusion Techniques • System maintain a file that associates a password with each authorized user. • Password file can be protected with: • One-way encryption • Access Control • Brute Force attacks if you can get access to passwords • Guess the password, encrypt and check • How long would it take? • Larger character set  more time • Longer  more time

6. Password Guessing • In 1978 16% of passwords (in one study) were 3 characters or less. • In a more recent study login name, login name in reverse, and the two concatenated, would obtain access to 8 - 30% of the accounts on a typical system. • In the CSE department what would you guess?

7. Klein’s Password Guessing Experiment • Online article containing Table 9.4 -http://wks.uts.ohio-state.edu/sysadm_course/html/sysadm-528.html • From 14,000 24% of passwords were cracked • Klien Guessed • User name, initials, login 130 permutation of such were tried for each user • Dictionary 60,000 (man –k dictionary) • Various permutations of “step 2 words,” e.g., change “o” to “0”, … • Try capitalization permuations • ftp://www-wls.acs.ohio-state.edu:/pub/security/Dan_Klein_password_security.ps.Z

8. Intrusion Techniques Techniques for guessing passwords: • Try default passwords. • Try all short words, 1 to 3 characters long. • Try all the words in an electronic dictionary(60,000). • Collect information about the user’s hobbies, family names, birthday, etc. • Try user’s phone number, social security number, street address, etc. • Try all license plate numbers (MUP103). • Use a Trojan horse • Tap the line between a remote user and the host system. • Ideas? Prevention: Enforce good password selection (Ij4Gf4Se%f#)

9. UNIX Password Scheme Loading a new password

10. UNIX Password Scheme Verifying a password file

11. Storing UNIX Passwords UNIX passwords were kept in in a publicly readable file, etc/passwords. • more /etc/passwd • Format login-name:encrypted_password:uid:gid:Name:home:shell • root:x:0:1:Super-User:/:/sbin/sh • daemon:x:1:1::/: • bin:x:2:2::/usr/bin: • … • nobody:x:60001:60001:Nobody:/: • noaccess:x:60002:60002:No Access User:/: • nobody4:x:65534:65534:SunOS 4.x Nobody:/: • Commands finger, who, … use this info. But so could crackers Now they are kept in a “shadow” directory and only visible by “root”.

12. ”Salt” • The salt serves three purposes: • Prevents duplicate passwords. • Effectively increases the length of the password. • Prevents the use of hardware implementations of DES

13. Access Control • /etc/password in old Unix was readable by everybody to support finger, who etc. This is a bad idea • /etc/shadow readable only by root • Setuid bit in permissions • Bit set in permissions • Process changes to owner of the file in execution • For the passwd command, the owner is root, thus it changes to root and then can read /etc/shadow • Getpwname – function that returns the password entry ? excluding password ?

14. Password Selecting Strategies • User education • Computer-generated passwords • FIPS PUB 181 • Reactive password checking • Peridocially run a password cracker and shoot the violators • Resource intensive • Proactive password checking • Allow the user to select but at the time of selection analyze the choice • Enforcing Rules • PINS can be no more than 4 decimal digits ?!! • Must have at least x characters, from several standard sets, i.e., letters, digits, punctuation, uppercase, lowercase …

15. Password Selecting Strategies (cont) • Build dictionary of bad passwords • Problems: space and Time • Two techniques for proactive password checking based on a rejection list: • Markov model for generation of guessable passwords [DAVI93] • Bloom Filters [Spaf92b]

16. Markov Model • Figure 9.5 is simplified Markov model (next slide) • Alphabet only three characters • State is the identity of most recent character • The value on the arc(transition) is the probability that one character follows another in the list • Markov Model M = (m, A, T, k) • M = number of states • A = state space • T = Transition matrix • k = order of the model – for a k order model probability of making a transition depends on k previous characters

17. Markov Model

18. Markov Model Technique Details • Markov Model M = (m, A, T, 2) • note k = 2  2nd order model • M = ?? • A • Construction of transition Matrix T (next slide) • Transforms “Is this a bad password?” to “was this password generated by the Markov model?” • From a proposed new password generate trigrams and check frequencies then see how likely?

19. Transition Matrix Determine the frequency matrix f, where f(i,j,k) is the number of occurrences of the trigram consisting of the ith, jth and kth character. For each bigram ij, calculate f(i,j, ) as the total number of trigrams beginning with ij. Compute the entries of T as follows:

20. Spafford (Bloom Filter) • where • Bloom filter consists of k independent hash functions • The following procedure is then applied to the dictionary: • A hash table of N bits is defined, with all bits initially set to 0. • For each password, its k hash values are calculated, and the corresponding bits in the hash table are set to 1 • If the bit is already set it remains set.

21. Spafford’s use of Bloom Filters (cont) • When a new password is entered • the k hash values are computed • If all corresponding bits of the hash table are 1 the password is rejected • Example: • dictionary contains “undertaker” and “hulkhogan” by not “xG%#jj98” • And suppose that • H1(undertaker)= 25, H1(hulkhogan) = 83 • H2(undertaker)= 917, H2(hulkhogan) = 432 • And also that when “xG%#jj98” is entered we calculate • H1(“xG%#jj98”)= 917, H2(“xG%#jj98”) = 432 • The we reject “xG%#jj98” -- a false positive

22. Spafford (Bloom Filter) • Design the hash scheme to minimize false positives. Why? • Probability of false positive:

23. Performance of Bloom Filter

24. The Stages of a Network Intrusion 1. Scan the network to: • locate which IP addresses are in use, • what operating system is in use, • what TCP or UDP ports are “open” (being listened to by Servers). 2. Run “Exploit” scripts against open ports 3. Get access to Shell program which is “suid” (has “root” privileges). 4. Download from Hacker Web site special versions of systems files that will let Cracker have free access in the future without his cpu time or disk storage space being noticed by auditing programs. 5. Use IRC (Internet Relay Chat) to invite friends to the feast. 24

25. Intrusion Detection • The intruder can be identified and ejected from the system. • An effective intrusion detection can prevent intrusions. • Intrusion detection enables the collection of information about intrusion techniques that can be used to strengthen the intrusion prevention facility.

26. Profiles of Behavior of Intruders and Authorized Users

27. Intrusion Detection • Statistical anomaly detection • Treshold detection • Profile based • Rule based detection • Anomaly detection • Penetration identidication

28. Measures used for Intrusion Detection • Login frequency by day and time. • Frequency of login at different locations. • Time since last login. • Password failures at login. • Execution frequency. • Execution denials. • Read, write, create, delete frequency. • Failure count for read, write, create and delete.

29. Distributed Intrusion Detection Developed at University of California at Davis

30. Distributed Intrusion Detection

31. Viruses and ”Malicious Programs” Computer “Viruses” and related programs have the ability to replicate themselves on an ever increasing number of computers. They originally spread by people sharing floppy disks. Now they spread primarily over the Internet (a “Worm”). Other “Malicious Programs” may be installed by hand on a single machine. They may also be built into widely distributed commercial software packages. These are very hard to detect before the payload activates (Trojan Horses, Trap Doors, and Logic Bombs).

32. Taxanomy of Malicious Programs Malicious Programs Need Host Program Independent Trapdoors Logic Bombs Trojan Horses Viruses Bacteria Worms

33. Definitions Virus - code that copies itself into other programs. A “Bacteria” replicates until it fills all disk space, or CPU cycles. Payload - harmful things the malicious program does, after it has had time to spread. Worm - a program that replicates itself across the network (usually riding on email messages or attached documents (e.g., macro viruses).

34. Definitions Trojan Horse - instructions in an otherwise good program that cause bad things to happen (sending your data or password to an attacker over the net). Logic Bomb - malicious code that activates on an event (e.g., date). Trap Door (or Back Door) - undocumented entry point written into code for debugging that can allow unwanted users. Easter Egg - extraneous code that does something “cool.” A way for programmers to show that they control the product.

35. Virus Phases • Dormant phase - the virus is idle • Propagation phase - the virus places an identical copy of itself into other programs • Triggering phase – the virus is activated to perform the function for which it was intended • Execution phase – the function is performed

36. Virus Protection Have a well-known virus protection program, configured to scan disks and downloads automatically for known viruses. Do not execute programs (or "macro's") from unknown sources (e.g., PS files, Hypercard files, MS Office documents, Avoid the most common operating systems and email programs, if possible.

37. Virus Structure

38. A Compression Virus

39. Types of Viruses Parasitic Virus - attaches itself to executable files as part of their code. Runs whenever the host program runs. Memory-resident Virus - Lodges in main memory as part of the residual operating system. Boot Sector Virus - infects the boot sector of a disk, and spreads when the operating system boots up (original DOS viruses). Stealth Virus - explicitly designed to hide from Virus Scanning programs. Polymorphic Virus - mutates with every new host to prevent signature detection.

40. Macro Viruses Microsoft Office applications allow “macros” to be part of the document. The macro could run whenever the document is opened, or when a certain command is selected (Save File). • Platform independent. • Infect documents, delete files, generate email and edit letters.

41. Antivirus Approaches 1st Generation, Scanners: searched files for any of a library of known virus “signatures.” Checked executable files for length changes. 2nd Generation, Heuristic Scanners: looks for more general signs than specific signatures (code segments common to many viruses). Checked files for checksum or hash changes. 3rd Generation, Activity Traps: stay resident in memory and look for certain patterns of software behavior (e.g., scanning files). 4th Generation, Full Featured: combine the best of the techniques above.

42. Advanced Antivirus Techniques • Generic Decryption (GD) • CPU Emulator • Virus Signature Scanner • Emulation Control Module • For how long should a GD scanner run each interpretation?

43. Advanced Antivirus Techniques

44. Recommended Reading and WEB Sites • Denning, P. Computers Under Attack: Intruders, Worms, and Viruses. Addison-Wesley, 1990 • CERT Coordination Center (WEB Site) • AntiVirus Online (IBM’s site)