Native Client is Google's attempt at bringing millions of lines of existing C/C++ code to the Chrome web browser in a secure sandbox through a combination of software fault isolation, a custom compiler toolchain and a secure plugin architecture. Sound challenging? It is! Native Client isn't a typical browser extension and it certainly isn't ActiveX. Native Client allows for all sorts of applications to run inside in your browser, everything from games to PDF readers. In this talk I will cover the basics of the Native Client sandbox and general security relevant architecture including PPAPI (the replacement for NPAPI), vulnerabilities I discovered via source review in the PPAPI interface and finally a tool that dynamically generates code to fuzz the Native Client PPAPI interfaces based on the IDL (Interface Description Language) files found in the Chrome source tree.