Details on Directory Traversal bugs

[Neuronix <neuronix () bigfoot com>]

Hello,

  We've see every week some kind of directory traversal bug in a
multitude of softwares... but anyway, I'd like to know the roots of this
kind of bug. Where did it come from? What kind of strings can be used
to test for if a particular software is vulnerable? I intend to automate
the process of checking, in case there's a particular pattern to be
followed. 

  Anyway, any detailed information is appreciated.

Thanks,
-- Neuronix