Security

1. Security • Chapter 19 discussed Protection • internal control measures to provide controlled access to system resources including files, memory, CPU, etc… • What about measures to provide controlled access for external environments such as network resources and network access? • This is handled by security • Protection can be made to be 100% (or nearly) reliable, but security can not

2. A System is Secure if • Its resources are utilized and accessed as intended under all circumstances • This is unfortunately not generally possible • Security violations are misuses of system resources • unauthorized use, access, modification or destruction of files • accidental • malicious • Protection must be offered at two levels: • physical - on-site protection • human - user safeguards

3. Why Security? • Many computer systems contain very important information: • financial information • bank files, payroll programs, etc… • software in development • sensitive data • Unauthorized access could result in • sabotage, theft, or fraud

4. Authentication • How do we ensure that only authorized users access files? • OS places read/write/execute privileges on files • Require a method of authenticating users • Passwords (knowledge) • Key or Card (physical means) • Retina pattern or finger print (physical attribute) • Passwords are the most common form • Key or Card - could be stolen or lost, requires physical presence • Physical attributes - too expensive (currently)

5. More on Passwords • Problems: • sometimes other people can easily guess someone’s password • methods may be used to make this harder but that makes it harder on users to remember passwords • if its too difficult to remember a password, then writing it down on paper is a major security problem • a program could be used to discover a password by trying all words in a dictionary or all combinations of letters up to 10 in length, etc… • passwords can be determined through surveillance methods

6. Ways to make passwords safer • Use system generated passwords • these may be hard to remember • Require some abnormal character in all passwords such as a digit or punctuation mark • protects against password cracking programs • Require that users change passwords every month • Use paired passwords • these protect against people easily guessing passwords

7. Program Threats • Trojan Horse • a piggyback program that the user is unaware of • once running, may do something unexpected such as delete, copy or move files • because the program is being run by the user, the program has the rights of the user and can perform OS activities without additional authorization • Trap Door • means of breaching security by leaving yourself a mechanism to obtain unauthorized access • could be done by adding a hidden account or in a compiler that creates hidden object code

8. System Threats • Worm • process that spawns copies of itself • can be used on a network to copy itself to other network sites • could be used to bring down a network • Virus • affects single user computers (PC’s) • propagated mostly through floppy disks • an infected computer can pass the virus onto other computers through transfer of floppy disks

9. The Internet Worm of 1988 • Unleashed by Robert Morris, a Cornell grad student to affect Sun computers on the Internet • Self-replicating program, which if it could gain access to a unix computer, would spawn processes of itself to other reachable unix computers • Two-part process: • grappling hook (to obtain access to a computer) and • the worm itself • The grappling hook was 99 lines of C code compiled to run on each machine it accessed • Grappling hook would then upload the main worm

10. Grap. Hook Methods of Access • rsh - remote task execution - if already logged into one sun, rsh would allow the program to log into any other sun that shared the same password file • finger - the grappling hook would overflow the run-time stack by sending the finger daemon an excessively long string (536 bytes) which would rewrite part of the stack to allow his program access • sendmail - similar method by using a debug option in sendmail to circumvent OS protection and obtain authorization • it also attempted to discover user passwords, first seeing if no password worked, and then trying 432 favorite password choices and finally by trying the standard Unix on-line dictionary entries

11. Internet Worm Conclusion • The worm was released on November 2, 1988 and affected thousands of computers causing many sites to shut down for at least a few hours or for as much as a day • Estimated damages in the millions of $ • By the evening of Nov 3, Unix administrators had placed solutions (halting programs) on the Internet to end the Worm • Morris’s sentence • 3 years probation, 400 hours community service, $10,000 fine, legal fees probably over $100,000!

12. Safe Computing • Protection against Viruses: • Don’t download software from unknown sources • Don’t share floppy disks • Check hard disk at bootup every time • Check every floppy disk as its inserted • Use antiviral programs to find and remove viruses • Checksum • add up all bits in a program and save it • if the program ever changes in size, it is probably because of a virus installed in it

13. Threat Monitoring • Security is difficult to impose because of the human connection. Have the OS look for • instances of unauthorized access • incorrect password attempts • short or easy-to-guess passwords • unauthorized set-uid programs • unauthorized programs in system directories • long-running programs • improper directory and file protections • changes in system programs • using checksum

14. Network Protection • In spite of the previous safeguards, networks are generally unprotected and therefore eavesdroppers can intercept files • Use encryption • Encode file • Store or transmit file • Decode file at the other end • Requires a code that is known at both ends