Ralph Langner, an independent security researcher, presented his analysis of specialized code used by the Stuxnet worm to an exploit Siemens Simatic S7-417 programmable logic controllers (PLCs) to an audience of his peers: industrial control security experts at the annual S4 Conference. Langner said the code, while incapable of running on its own, was designed to leverage components of the specific applications used at Natanz possibly a list of input/output addresses for that application and would have worked perfectly in the environment.
Source: https://threatpost.com/stuxnet-expert-langner-analysis-shows-design-flaw-not-vulnerability-sunk-siemens-011912/76115/