A blacklist is a security feature that denies access to a system if the way the system is accessed is identical to the blacklist criteria. Blacklists are used to exclude certain types of access to sensitive data, systems, websites, or applications.

Terms related to Blacklist: Whitelist, Systems Access, IP Address Ranges, Spam, Firewalls, Web Servers.

A blacklist restricts access to defined systems, based on specific criteria. These criteria vary, as shown below. If a specific element is blacklisted, any access attempts that contain that element will be rejected, making access impossible.

Restrict Access via IP Address
Certain IP addresses, or ranges of IP addresses can be prevented from accessing websites, systems, or data. An IP address blacklist is typically enforced through web server or firewall rules that reject access attempts if the IP address of a specific device matches the blacklisted range.

Restrict Access via Firewall
Firewalls can be setup with sophisticated rules to blacklist certain factors and block access. For example, a firewall may block specific users, countries, regions, devices, and more. Firewall rules can also be combined to provide granular access controls.

Restrict Access via Email Address or Message Content
Spam and other filters can blacklist and prevent access via email address. Such blacklisting is often used by email providers to sort messages into spam folders.

Restrict Access Within Systems and Data
Blacklists can also be applied inside applications and data. For example, a programmer may prevent certain data from being modified, or choose to restrict access to specific parts of a dataset or system.

Compare blacklists with whitelists, where any access attempt that does not meet the whitelist criteria is rejected.