An allow list, sometimes called a whitelist, is a cybersecurity mechanism that specifies trusted entities - such as email addresses, IP addresses, domains, or applications - that are granted access to a system, network, or service. Unlike blocklists, which deny access to known threats, allow lists only permit explicitly approved sources, blocking all others by default. This approach is commonly used in spam filters, firewalls, and application control, where only pre-approved senders, sites, or programs are allowed. Allow lists help reduce the risk of unauthorized access and minimize exposure to malicious content by ensuring that only vetted and legitimate sources can interact with protected resources. However, maintaining an effective allow list requires regular updates and careful management to avoid inadvertently blocking legitimate users or allowing new threats.