In computing, a blacklist or block list is a basic access control mechanism that allows through all elements (email addresses, users, passwords, URLs, IP addresses, domain names, file hashes, etc.), except those explicitly mentioned. Those items on the list are denied access. The opposite is a whitelist, which means only items on the list are let through whatever gate is being used. A greylist contains items that are temporarily blocked (or temporarily allowed) until an additional step is performed.
Blacklists can be applied at various points in a security architecture, such as a Host (network), Web proxy, DNS servers, Email server, Firewall (computing), directory servers or application authentication gateways. The type of element blocked is influenced by the access control location. DNS servers may be well-suited to block domain names, for example, but not URLs. A firewall is well-suited for blocking IP addresses, but less so for blocking malicious files or passwords.
Example uses include a company that might prevent a list of software from running on its network, a school that might prevent a list of web sites from being accesses on its computers, or a business that wants to ensure their computer users are not choosing easily-guessed, poor passwords.
Example systems to protect
Blacklists are used to protect a variety of systems in computing. The content of the blacklist is likely needs to be targeted to the type of system defended.
An information system includes end-point hosts like user machines and servers. A blacklist in this location may include certain types of software that are not allowed to run in the company environment. For example, a company might blacklist peer to peer file sharing on its systems. In addition to software, people, devices and Web sites can also be blacklisted. 
Most email providers have a anti-spam feature that essentially blacklists certain email addresses if they are deemed unwanted. How this happens is when a successful phishing attack (from an address that is forged from reliable accounts to try to recover personal information) is executed, then the email device deems the address to be spam, and proceeds to blacklist the address.
An e-mail spam filter may keep a blacklist of email addresses, any mail from which would be prevented from reaching its intended destination. It may also use sending domain names or sending IP addresses to implement a more general block.
In addition to private email blacklists, there are lists that are kept for public use, examples are
- China Anti-Spam Alliance 
- Fabel Spamsources 
- Spam and Open Relay Blocking System
- The DrMX Project
The goal of a blacklist in a web browser is to prevent the user from visiting a malicious or deceitful web page via filtering locally. A common web browsing blacklist is Google's Safe Browsing, which is installed by default in Firefox, Safari, and Chrome.
Usernames and Passwords
Blacklisting can also apply to user credentials. It is common for systems or websites to blacklist certain reserved usernames that are not allowed to be chosen by the system or websites user populations. These reserved usernames are commonly associated with built-in system administration functions.
Password blacklists are very similar to username blacklists but typically contain significantly more entries that username blacklists. Password blacklists are applied to prevent users from choosing passwords that are easily guessed or are well known and could lead to unauthorized access by malicious parties. Password blacklists are deployed as an additional layer of security, usually in addition to a password policy, which sets the requirements of the password length and/or character complexity. This is because there are a significant number of password combinations that fulfill many password policies but are still easily guessed (i.e., Password123, Qwerty123).
Blacklists are distributed in a variety of ways. Some use simple mailing lists. A DNSBL is a common distribution method that leverages the DNS itself. Some lists make use of rsync for high-volume exchanges of data. Web-server functions may be used; either simple GET requests may be used or more complicated interfaces such as a RESTful API.
- For a list of some DNS-based blacklists, see the Comparison of DNS blacklists.
- Companies like Google, Symantec and Sucuri keep internal blacklists of sites known to have malware and they display a warning before allowing the user to click them.
- Content-control software such as DansGuardian and SquidGuard may work with a blacklist in order to block URLs of sites deemed inappropriate for a work or educational environment.
- A firewall or IDS may also use a blacklist to block known hostile IP addresses and/or networks. An example for such a list would be the OpenBL project.
- Many copy protection schemes include software blacklisting.
- The company Password RBL offers a password blacklist for Microsoft's Active Directory, web sites and apps, distributed via a RESTful API.
- Members of online auction sites may add other members to a personal blacklist. This means that they cannot bid on or ask questions about your auctions, nor can they use a "buy it now" function on your items.
- Yet another form of list is the yellow list which is a list of email server IP addresses that send mostly good email but do send some spam. Examples include Yahoo, Hotmail, and Gmail. A yellow listed server is a server that should never be accidentally blacklisted. The yellow list is checked first and if listed then black list tests are ignored.
- In Linux modprobe, the
blacklist modulenameentry in a modprobe configuration file indicates that all of the particular module's internal aliases are to be ignored. There are cases where two or more modules both support the same devices, or a module invalidly claims to support a device.
- Many web browsers have the ability to consult anti-phishing blacklists in order to warn users who unwittingly aim to visit a fraudulent website.
- Many peer-to-peer file sharing programs support blacklists that block access from sites known to be owned by companies enforcing copyright. An example is the Bluetack blocklist set.
Considerations of Usage
As expressed in a recent conference paper focusing on blacklists of domain names and IP addresses used for Internet security, "these lists generally do not intersect. Therefore, it appears that these lists do not converge on one set of malicious indicators." This concern combined with an economic model means that, while blacklists are an essential part of network defense, the need to be used in concert with whitelists and greylists, for example.
- Shimeall, Timothy; Spring, Jonathan (2013-11-12). Introduction to Information Security: A Strategic-Based Approach. Newnes. ISBN 9781597499729.
- "Domain Blacklist Ecosystem - A Case Study". insights.sei.cmu.edu. Retrieved 2016-02-04.
- Rainer, Watson (2012). Introduction to Information Systems. Wiley Custom Learning Solutions. ISBN 978-1-118-45213-4.
- "Guidelines". www.surbl.org. Retrieved 2016-02-04.
- "B.I.S.S. Forums - FAQ - Questions about the Blocklists". Bluetack Internet Security Solutions. Archived from the original on 2008-10-20. Retrieved 2015-08-01.
- Metcalf, Leigh; Spring, Jonathan M. (2015-01-01). "Blacklist Ecosystem Analysis: Spanning Jan 2012 to Jun 2014". Proceedings of the 2nd ACM Workshop on Information Sharing and Collaborative Security (ACM): 13–22.
- Kührer, Marc; Rossow, Christian; Holz, Thorsten (2014-09-17). Stavrou, Angelos; Bos, Herbert; Portokalidis, Georgios, eds. Paint It Black: Evaluating the Effectiveness of Malware Blacklists. Lecture Notes in Computer Science. Springer International Publishing. pp. 1–21. doi:10.1007/978-3-319-11379-1_1. ISBN 9783319113784.
- Spring, Jonathan M. (2013-01-01). "Modeling malicious domain name take-down dynamics: Why eCrime pays". eCrime Researchers Summit (eCRS), 2013 (IEEE): 1–9.
- Multi-RBL Checker Tool - Multi-RBL Checker Tool (Check to see if your IP is showing up one or more blacklists)
- Password RBL - A Real-time Blacklist of Passwords for Active Directory, Web sites and apps
- OpenBL.org Blacklisting and Abuse Reporting
- Squidblacklist.org Blacklists For Squid Proxy
- MxToolbox.com MxToolbox Blacklist Lookup Tool