1. Security

2. Security
Since the operating system oversees the activities in a computer, it is natural for it to play a vital role in maintaining security as well.
First ,we focus on security problems that are more closely related to the specifics of operating systems.

3. Security Attacks from the outside
An important task performed by operating system is to protect the computer’s resources from access by unauthorized personnel. In the case of computers used by multiple people, this is usually approached by means of establishing “accounts” for the various authorized users— an account being essentially a record within the operating system containing such entries as the user’s name, password, and privileges to be granted to the user.

4. Security
Accounts are established by a person known as the super user or the administrator. This person gain highly privileged access to the operating system by identifying him-or herself as the administrator during the login procedure. Once this contact is established, the administrator can alter settings within the operating system, modify critical software packages, adjust the privileges granted to other users.

5. Security
With all the technical complexities associated with computer security, it is surprising to many that one of the major obstacles to the security of computer system is the carelessness of the users themselves.

6. Security Attacks from within
If , however, access is through a general user’s account, it becomes necessary to trick the operating system into allowing the intruder to reach beyond the privileges granted to that user. For example, the intruder may try to trick the memory manager into allowing a process to access main memory cells outside its allocated area, or the prowler may try to trick the file manager into retrieving files whose access should be denied.

7. Security
CPUs are enhanced with features that are designed to foil such attempts. As a example, consider the need to restrict a process to the area of main memory assigned to it by the memory manager. Without such restrictions, a process could erase the operating system from main memory and take control of the computer itself. To counter such attempts, CPUs designed for multiprogramming systems typically contain special-purpose registers in which the operating system can store the upper and lower limits of a process’s allocated memory area.

8. Security
Embedded in this illustration is a subtle but significant problem. Without further security features, a process could still gain access to memory cells outside of its designated area merely by changing the special-purpose register that contain its memory limits.

9. Security
To protect against such actions, CPUs for multiprogramming systems are designed to operate in one of two privilege levels;
privileged mode nonprivileged mode

10. Security
When a computer is connected to a network, it becomes subject to unauthorized access and vandalism.
There are numerous ways that a computer system and its contents can be attacked via network connections. Many of these incorporate the use of malicious software (collectively called malware).

11. Security
virus
a virus is software that infects a computer by inserting itself into programs that already reside in the machine. Then when the “host” program is executed, the virus is also executed.

12. Security
Worm
A worm is an autonomous program that transfers itself through a network, taking up residence in computers and forwarding copies of itself to other computers. A characteristic consequence of a worm is an explosion of the worm’s replicated copies that degrades the performance of legitimate applications and can ultimately overload an entire network or internet.

13. Security
Trojan horse
A Trojan horse is a program that enters a computer system disguised as a desirable program, such as a game or useful utility package, that is willingly imported by the victim. Once in the computer, however, the Trojan horse performs additional activities that might have harmful effects. Trojan horses often arrive in the form of attachments to enticing messages.

14. Security Protection and Cures
The old adage “an ounce of prevention is worth of a pound of cure” is certainly true in the context of controlling vandalism over network connections. A primary prevention technique is to filter traffic passing through a point in the network, usually with a program called a firewall. For instance, a firewall might be installed at the gateway of an organization’s intranet to filter messages passing in and out of the region.

15. Security Summary Attacks from the outside Attacks from within virus
worm
Trojan horse
protection and cures