In multi-tenant systems—whether you’re managing an API gateway, identity platform, or SaaS product—access control is essential. Two of the most widely used tools for managing that access are allowlists and denylists. These mechanisms define who or what is permitted or rejected, helping isolate tenants, control risk, and enforce trust boundaries. But despite their simplicity, both lists can easily become operational liabilities if not carefully managed. This article explores real-world examples of allowlists and denylists, how to store and govern them, and why every list needs a plan to die.
What Are Allowlists and Denylists?
An allowlist is a list of explicitly approved entities—users, IPs, tenants, apps, or domains—that are permitted to access a resource. Everything else is denied by default. A denylist is the opposite: a list of explicitly blocked entities; everything else is allowed. In simple terms, allowlists implement default-deny behavior, while denylists implement default-allow with overrides. Choosing between them depends on the nature of what you’re protecting, how dynamic your environment is, and how clearly you can define trust.