source: https://www.securityfocus.com/bid/22997/info

Holtstraeter Rot 13 is prone to a directory-traversal vulnerability because it fails to properly sanitize user-supplied input.

An attacker can exploit this vulnerability to retrieve the contents of arbitrary files from the vulnerable system in the context of the affected application. Information obtained may aid in further attacks. 

http://www.example.com/enkrypt.php?datei=../../../../etc/passwd