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Buffer Overflow

"A buffer overflow is the computing equivalent of trying to pour two liters of water into a one-liter pitcher: some water is going to spill out and make a mess."(1) A buffer overflow, sometimes called a buffer overrun, occurs when a program that is writing data to a buffer (a sequential section of memory) writes data past the buffer's boundary and into adjacent memory. The Multics operating system created in the 1960s was particularly secure against buffer overflows due to a variety of checks built into the system's software (written in PL/I) and hardware. By contrast, some computing languages such as Pascal, C, and C++ do not require programmers to pre-define the buffer sizes, which means that the compiler has no way to know the proper boundaries of memory addresses. Even in programming languages that do define buffer sizes, such as C#, Java, and Visual Basic, there is no way to check every single circumstance owing to the heavy use of "pointers" in modern programming.(2)

Overflows may cause serious problems. When an overflow occurs, the damage it causes depends on what exactly is in the memory that gets overwritten. If the program overflows into either the user's data space or the user's program area, the consequences are limited solely to the user and to the program in question. If the space contained already-used data or an already-performed program instruction, there might be no detectable effect. Errors or inaccuracies in the program might result, depending on the nature of the data overwritten.

In more serious cases, however, the program overflows into parts of the computer's memory that contain data or code for the operating system, causing errors in the operating system. Malicious persons sometimes seek to take advantage of this vulnerability, using it to place unauthorized instructions (as it were) inside the operating system. The operating system of a computer generally has higher privileges than normal programs, so any malicious code masquerading as the operating system will have identical privileges. This masquerading might allow a malicious programmer to gain control over the system. Because they are difficult to identify, buffer overflows are the target of many forms of malicious code, including the SQLSlammer worm (sometimes called the Sapphire worm).(3)

Notes

1 , 2 , 3 : Charles P. Pfleeger and Shari Lawrence Pfleeger, Security in Computing. 3rd Edition, New Jersey: Prentice Hall, 2003.


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