Sometimes poor SUID scripts (shell or other programs that Set the UserID to run under another user’s privileges) that perform certain tasks can be run as root. If the scripts are writeable by an id, for example, the scripts can be edited and executed.
NMRC. The Hack FAQ. Unix Local Attacks. [Online, 2004.] NMRC Website. http://www.nmrc.org/pub/faq/hackfaq/hackfaq-29.html.