The prompt parsing in bash allows a local user to execute commands as another user by creating a directory with the name of the command to execute.
(1) bash before 1.14.7, and (2) tcsh 6.05 allow local users to gain privileges via directory names that contain shell metacharacters ( back-tick), which can cause the commands enclosed in the directory name to be executed when the shell expands filenames using the \w option in the PS1 variable.