Back Door: An entry point to a program or a system that is hidden or disguised, often created by the software's author for maintenance. A certain sequence of control characters permits access to the system manager account. If the back door becomes known, unauthorized users (or malicious software) can gain entry and cause damage.

Bandwidth: Capacity of a network or data connection, often measured in kilobits/second (kbps) for digital transmissions.

Bastion Host: A system that has been hardened to resist attack at some critical point of entry, and which is installed on a network in such a way that it is expected to come under attack. Bastion hosts are often components of firewalls, or may be 'outside" Web servers or public access systems. Generally, a bastion host is running some form of general purpose operating system (e.g., LNIX, VMS, WNT, etc.) rather than a ROM-based or firmware operating system.

Bell-La Padula Security Model: Formal-state transition model of computer security policy that describes a formal set of access controls based on information sensitivity and subject authorizations.

Benign Environment: A non-hostile environment that may be protected from external hostile elements by physical, personnel, and procedural security countermeasures.

Between-the-lines Entry: Unauthorized access obtained by tapping the temporarily inactive terminal of a legitimate user.

Bimodal Virus: A virus that infects both boot records and files. Also called bipartite or multipartite.

Binding of Security Functionality: The ability of security enforcing functions and mechanisms to work together in a way which is mutually supportive and provides an integrated and effective whole.

Boot Records: Those areas on diskettes or hard disks that contain some of the first instructions executed by a PC when it is booting. Boot records must be loaded and executed in order to load the operating system. Viruses that infect boot records change the boot records to include a copy of themselves. When the PC boots, the virus program is run and will typically install itself in memory before the operating system is loaded.

Boot-sector-infecting Virus: Some viruses infect the boot records of hard disks and diskettes. They typically do so by replacing the existing boot record with their own code. The virus is executed when the system is booted from the hard disk or diskette, and installs its own code in the system's memory so that it can infect other hard disks or diskettes later. Once that has happened, the virus will usually execute the normal boot program, which it stores elsewhere on the disk.

Biometric Access Control: Any means of controlling access through human measurements, such as fingerprinting and voiceprinting.

Block Oriented Encryption: Encryption methods where the data to be encrypted is divided into blocks of equal length, on which the key is then applied according to the respective method.

Bug: An error in the design or implementation of a program that causes it to do something that neither the user nor the program author had intended to do.

Business-Critical Applications: The vital software needed to run a business, whether custom-written or commercially packaged, such as accounting/finance, ERP, manufacturing, human resources, sales databases, etc.

Blue bomb: (also known as "the blue screen of death" or "WinNuke") a technique for causing the Windows operating system of someone you're communicating with to crash or suddenly terminate. The "blue bomb" is actually an out-of-band network packet containing information that the operating system can't process. This condition causes the operating system to "crash" or terminate prematurely. The operating system can usually be restarted without any permanent damage other than possible loss of unsaved data when you crashed. The blue bomb derives its name from the effect it sometimes causes on the display as the operating system is terminating -- a white-on-blue error screen. Blue bombs are sometimes sent by multi-player game participants who are about to lose or users of Internet Relay Chat (IRC) who are making a final comment. This is known as "nuking" someone. The program that causes the blue bomb is known as WinNuke. Many Internet service providers are filtering out the packets so they don't reach users.

Buffer Overflow: A buffer overflow occurs when a buffer was assigned by a programmer to hold variable data, and the variable data placed into that buffer exceeds the size of the initial of the buffer assignment. Depending on the operating system and exactly what the "extra" data overflowing the buffer is, this can be used by a hacker to cause portions of a system to fail, or even execute arbitrary code. Most buffer overflow exploits centre around user-supplied data exceeding a buffer, and the extra data being executed on the stack to open up additional access. Buffer overflows exist on all major network operating systems.