By combining permission features of UNIX operating system and features of the C programming language, it is possible for an unprivileged user or process to gain unrestricted system privilege. Common to many high profile UNIX security incidents, this page contains links that analyze how these exploits are constructed, why they work and what can be done to prevent the problem.
Download a copy of Nate's rant on Stack Smashing "Stack Smashing Vulnerabilities in the UNIX Operating System" in postscript format. Or, download this version if that postscript is too toxic for you :-)