The whole point of NaCl is that it provides a safe sandbox for running verified native code - the Wikipedia article has more on this: http://en.wikipedia.org/wiki/Google_Native_Client

Google ran a security contest to find problems with it, which turned up a few. I'm not sure what the status is at the moment with regards to security: http://code.google.com/contests/nativeclient-security/faq.ht...