no, buffer overflows do not modify excecutable code. A buffer overflow is what happens when a chunk of allocated memory devoted to storing transient data (i.e.: keystrokes, filesystem caches) before it can be processed "overflows" it's allocated space and "spills over" either into other data, or excecutable code, corrupting it, or the OS catches the error and kills the application. Not all buffer overflows are security exploits - most are just badly written crashy code. However, sometimes a buffer overflow can cause a security feature to fail while the application stays up (at least for a short little while) and excecute code. Under linux that is only a serious problem if the application is running as root.

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