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Chapter 21. Implementing Windows Vista’s... > Avoiding Overflows with Support for ...

Avoiding Overflows with Support for the NX Bit

One common cause of system crashes, and a common technique used by makers of malicious software, is the buffer overflow. A buffer is a memory area set aside to hold data. The buffer has a fixed size, which means it can’t handle data larger than that size. A well-programmed system includes checks to ensure that the only data written to the buffer is of the correct size or smaller.

In practice, however, the desire for faster code or sheer sloppiness by the programmer can occasionally result in unprotected memory buffers. When buffer overflow occurs, either by accident or by design, the system writes the extra data to memory areas adjacent to the buffer. If these adjacent areas just hold more data, nothing terrible happens. However, if the adjacent areas contain core operating system code, the system can crash; even worse, if the adjacent areas are designed to run system control code, a clever hacker can take advantage of that to run whatever code he or she wants, usually with disastrous results.


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