The primary threat to computer systems has traditionally been the insider attack. Insiders are likely to have specific goals and objectives, and have legitimate access to the system. Insiders can plant trojan horses or browse through the file system. This type of attack can be extremely difficult to detect or protect against.
The insider attack can affect all components of computer security. Browsing attacks the confidentiality of information on the system. Trojan horses are a threat to both the integrity and confidentiality of the system. Insiders can affect availability by overloading the system's processing or storage capacity, or by causing the system to crash.
These attacks are possible for a variety of reasons. On many systems, the access control settings for security-relevant objects do not reflect the organization's security policy. This allows the insider to browse through sensitive data or plant that trojan horse. The insider exploits operating system bugs to cause the system to crash. The actions are undetected because audit trails are inadequate or ignored.