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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)

Example of a buffer overflow attack

The images below are representations of a computer's memory while it's running a program. The squares labelled buffer refer to a place that a user can place a string of characters.

Buffer_Overflow___Canary_copy.png
Buffer_Overflow___Canary.png
Figure 1: Intended buffer behavior
Figure 2: Buffer overflow attack
I am sorry for this terrible hack, but if you want to keep the borders nice, PLEASE DO NOT DELETE THIS.

Figure 1 shows us how a buffer is supposed to work: the user places an expected number of characters into the buffer, and the program continues to operate as expected. Figure 2 shows us a buffer overflow attack, which allows a hacker to redirect the program to produce behavior that the program wasn't designed to perform, such as give full administrator access to the system.

Consequences of buffer overflows

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.

A more troubling issue is when a buffer overflow can overwrite a return address, which effectively tells the computer where to find its next set of instructions. An attacker can use this exploit to seize control of a program by redirecting the computer to malicious instructions, possibly allowing them to gain full control over the system. Other suitable targets for overflow attacks are "pointers" to functions or variables, essentially allowing an attacker to switch out one function for another without the computer knowing the difference. 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)

More information

Smashing the Stack For Fun And Profit, a thorough technical examination of buffer overflow attacks. (http://www.phrack.org/issues/49/14.html#article)

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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