A tiger team is an individual, or more commonly a collection of individuals, who are employed or contracted by an organization to attempt to break its security, as a means of testing the reliability of that security.

The term originated in the U.S. military, where tiger teams have been known to leave hand-lettered cardboard signs in secure areas with "BOMBS" written on them.

According to the Jargon File entry (http://www.catb.org/~esr/jargon/html/T/tiger-team.html), it's common for a tiger team penetration to be immediately followed by an unexpected security inspection; base commanders, the file notes dryly, have been known to be relieved.