Open-source software OSS is widely adopted in enterprise settings, but standalone tools often lack native support for protocols like SAML or OIDC, creating a critical security integration gap. This paper introduces and formalizes the Auth Shim, a lightweight architectural pattern designed to solve this problem. The Auth Shim is a minimal, external proxy service that acts as a compatibility layer, translating requests from an enterprise Identity Provider IdP into the native session management mechanism of a target application. A key prerequisite for this pattern is that the target application must expose a programmatic, secure administrative API. We present a case study of the pattern's implementation at Adobe to integrate a popular OSS BI tool with Okta SAML, which enabled automated Role-Based Access Control RBAC via IAM group mapping and eliminated manual user provisioning. By defining its components, interactions, and production deployment considerations, this paper provides a reusable, secure, and cost-effective blueprint for integrating any standalone OSS tool into an enterprise SSO ecosystem, thereby enabling organizations to embrace open-source innovation without compromising on security governance.