The inclusion of open-source components into large, closed-sourced applications has become a common practice in modern software. Vendors obviously benefit from this approach as it allows them to quickly add functionality for their users without the need to invest costly engineering effort. However, leveraging open source for a quick functionality boost comes with security side effects that might not be understood by the vendor until it is too late. In those cases, misunderstood or poorly implemented open source allows attackers to bypass security mechanisms that may exist elsewhere in the proprietary system. This talk provides insight into these side effects through an examination of Adobe Reader’s XSLT (Extensible Stylesheet Language Transformations) engine, which is based on the now abandoned open-source project called Sablotron – an XML processor fully implemented in C++. We focus on techniques for auditing the source code of Sablotron in order to find corresponding bugs in Adobe Reader. We also present a new source-to-binary matching technique to help you pinpoint the vulnerable conditions within Sablotron that also reside in the assembly of Reader. Real-world application of these techniques will be demonstrated through a series of code execution vulnerabilities discovered in Adobe Reader’s codebase. Finally, we’ll highlight the trends in vulnerabilities discovered in Adobe Reader’s XSLT engine over the last year.