This talk explores the often overlooked vulnerabilities in build pipelines of Open Source Software packages, moving beyond typical supply chain security discussions. Learn how researchers developed large-scale data analysis infrastructure to uncover numerous zero-day vulnerabilities in critical OSS projects including AWS-managed Kubernetes Operators, Google OSS Fuzz, RedHat OS Build, hundreds of popular Terraform providers and modules, and widely-used GitHub Actions. Examine a comprehensive attack tree for GitHub Actions pipelines that provides deeper analysis than previous research, detailing both attack vectors and mitigation strategies. Discover three complementary open source projects that emerged from this research: the 'Living Off the Pipeline' (LOTP) project, the 'poutine' build pipeline scanner, and the 'messypoutine' CTF-style training tool - all designed to provide actionable insights for builders and defenders in the OSS ecosystem.