LastPyMile: Identifying the discrepancy between sources and packages

Open source packages have source code available on repositories for inspection (e.g. on GitHub) but developers use pre-built packages directly from the package repositories (such as npm for JavaScript, PyPI for Python, or RubyGems for Ruby). Such convenient practice assumes that there are no discrepancies between source code and packages. These differences pose both operational risks (e.g. making dependent projects unable to compile) and security risks (e.g. deploying malicious code during package installation) in the software supply chain. Our empirical assessment of 2438 popular packages in PyPI with an analysis of around 10M lines of code shows several differences in the wild: modifications cannot be just attributed to malicious injections. Yet, scanning again all and whole most likely good but modified' packages is hard to manage for FOSS downstream users. We propose a methodology, LastPyMile, for identifying the differences between build artifacts of software packages and the respective source code repository. We show how it can be used to extend current package scanning practices for malware injection (which only covers less than 1% of the code of deployed packages).