The buffer overflow has long been a feature of the computer security landscape. In fact the first self-propagating Internet worm—1988’s Morris Worm—used a buffer overflow in the Unix finger daemon to spread from machine to machine. Twenty-seven years later, buffer overflows remain a source of problems. Windows infamously revamped its security focus after two buffer overflow-driven exploits in the early 2000s. And just this May, a buffer overflow found in a Linux driver left (potentially) millions of home and small office routers vulnerable to attack.

At its core, the buffer overflow is an astonishingly simple bug that results from a common practice. Computer programs frequently operate on chunks of data that are read from a file, from the network, or even from the keyboard. Programs allocate finite-sized blocks of memory—buffers—to store this data as they work on it. A buffer overflow happens when more data is written to or read from a buffer than the buffer can hold.

On the face of it, this sounds like a pretty foolish error. After all, the program knows how big the buffer is, so it should be simple to make sure that the program never tries to cram more into the buffer than it knows will fit. You’d be right to think that. Yet buffer overflows continue to happen, and the results are frequently a security catastrophe.

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How security flaws work: The buffer overflow