Why SaaS Developers Should Avoid Building Custom Authentication Systems

A software developer shared how building a custom authentication system from scratch for their SaaS product consumed three weeks of development time, only to require ongoing bug fixes and security patches for months afterward. The author argues that authentication goes far beyond a simple login form, encompassing session management, multi-factor authentication, token rotation, account recovery, and compliance with data privacy regulations like GDPR and CCPA. Each of these requirements diverts engineering effort away from the core product features that actually deliver user value. The article recommends separating user identity from application data, using short-lived JWTs, and storing only a unique identifier that maps to an external auth provider rather than managing sensitive credentials in-house. For the vast majority of SaaS projects, the author concludes that adopting a managed authentication service or a battle-tested open-source library is more cost-effective than maintaining a custom-built system.