A vulnerability has been identified in PostNuke, which may be exploited by attackers to read arbitrary files outside of the webroot directory. This flaw is due to an input validation error in the Blocks Module when handling a specially crafted "func" variable containing "..\" sequences, which may be exploited remotely to conduct directory traversal attacks.
Postnuke released a patch that covers this, and several other vulnerabilities, yesterday, May 20th. It's available for download from their main downloads section of postnuke.com.