The problem stems from a telnet debug shell glitch that can allow an unauthenticated user to connect to the printer and in turn, glean data. HP is advising affected customers to download updated firmware for printers impacted by the bug from the company s Support Center site. German security researcher Christop von Wittich with Hentschke Bau GmbH was credited with discovering the flaw. A handful of security vulnerabilities have been discovered on printers, along with other Internet-enabled devices over the last few years.
Source: https://threatpost.com/hp-cert-warn-critical-hole-laserjet-printers-031113/77611/