This morning the DHS ICS-CERT published an advisory for a protection mechanism failure vulnerability in the Siemens SIMATIC S7-1200 CPU. The vulnerability was reported by Maik Brüggemann and Ralf Spenneberg from Open Source Training. The newest version (December 2014) of the firmware does not include this vulnerability.
ICS-CERT reports that a relatively unskilled attacker could remotely exploit this vulnerability to circumvent user program block protection. The Siemens Security Advisory notes that the attacker must have network access to an affected device, and the PLC’s access protection must be disabled for this vulnerability to be exploited.