What Is A Blacklist?

Date First Published: 23rd August 2023

Topic: Cybersecurity

Subtopic: Security Mechanisms & Technologies

Article Type: Computer Terms & Definitions

Difficulty: Medium

Difficulty Level: 4/10

CONTENTS

Uses Of Blacklists

Learn about what a blacklist is in this article.

A blacklist, also known as a blocklist, or a denylist, is a list of email addresses, IP addresses, domain names, usernames, passwords, and applications that are denied access to a specific system or protocol. With the exception of those items listed in the blacklist, all other items are allowed access.

A blacklist is the opposite of a whitelist, which refers to a list of items allowed access to a specific system or protocol, and only items on the list are allowed access. Therefore, blacklists are more suitable when a limited number of items need to be denied access and whitelists are more suitable when most items need to be denied access.

Uses Of Blacklists

Blacklists are often used to block email addresses and IP addresses when persistent spam or other malicious activity has been coming from them. This prevents any more unwanted messages, spam, or malicious activity.

For example, if an email address has been persistently sending lots of spam or viruses to a lot of other recipients using that same email provider, they may add the email address or domain name to a blacklist, preventing any users from receiving any emails from that email address or domain name. If someone's email address or domain name has been added to a blacklist, their email may bounce back to them or the recipient may simply not get it.

Specific usernames and passwords can be blacklisted. Systems and websites often blacklist usernames that are not allowed to be chosen by the system or other users. These usernames often include names commonly associated with built-in system administration functions and offensive or inappropriate usernames. Password blacklists often contain much more entries than username blacklists and are applied to prevent users from choosing easily guessed or overused passwords.

Web servers often include a blacklist of IP addresses that denies access from specific IP addresses or ranges of IP addresses. This is a useful method of banning someone from accessing a website if they have been engaging in spam or other malicious activities. Usually, anyone visiting from a blacklisted IP address will receive a 403 error, meaning that access to the requested page is forbidden.

Firewalls use blacklists to deny access to individual computer systems located in specific countries, IP addresses, and IP ranges, to block web threats and malicious activity. Firewalls are well-suited for blocking IP addresses, but not as suitable for blocking malicious files or passwords. Firewalls can also block access to specific domain names to prevent websites from being reached. This is often for parents to block age-inappropriate web content from being accessed by their children and for schools and workplaces to block undesirable or distracting content from being viewed.