What is an Allowlist and Blocklist?

Allow lists and block lists are the simplest tools in content moderation: a set of entities that are always permitted and a set that are always rejected. They have been around since the earliest email spam filters, and most platforms still run some version of them today, usually in addition to machine learning rather than instead of it. The terms have largely replaced the older "whitelist" and "blacklist" labels.

What an allow list is

An allow list, sometimes called a pass list or safe list, is a set of entities that are pre-approved for access or publication. In moderation, that usually means trusted users, known-good domains, partner accounts, verified publishers, or specific content types that should always get through. The point is to keep routine traffic from known-good sources out of the review queue so moderators can focus on the ambiguous cases.

What a block list is

A block list, sometimes called a deny list or ban list, is the inverse: a set of entities that are denied outright.

Typical entries include banned users, compromised IP addresses, phishing domains, specific slurs, spam signatures, or hash-matched copies of known harmful media like CSAM via PhotoDNA. When an entry matches, the content is blocked or quarantined without further analysis.

Where lists fall short

Allow lists and block lists are useful, but they are brittle. They only catch what is already on them, and harmful content evolves faster than any manually curated list can keep up. A slur misspelled with a zero-width space slips past a word block. A new scam domain registered this morning is not in yesterday's list. A legitimate publication that happens to share an IP with a flagged account gets blocked by mistake. The result is the familiar pair of failure modes: false positives, where legitimate content is blocked, and false negatives, where harmful content gets through. Neither gets better as the platform grows.

Where modern moderation picks up

This is why most moderation stacks layer machine learning on top of the lists rather than relying on lists alone.

Classifiers and large language models can read context, handle misspellings and code-mixed language, and adapt to new patterns as they are retrained. Lists still have a role, especially for high-confidence deterministic decisions like hash-matched CSAM or a known-bad domain, but the judgment calls now sit with the models and the human reviewers behind them.

Why the terminology changed

The shift from "whitelist" and "blacklist" to "allow list" and "block list" is part of a wider move toward more neutral technical language. The older terms were criticized for associating "white" with safe and "black" with dangerous, a framing that many organizations decided they did not want baked into their codebases and documentation.

Major technology companies, standards bodies, and open source projects including the Linux kernel, Google, GitHub, and the IETF have adopted the newer terms in recent years.