Application whitelisting is the practice of specifying an index of approved software applications that are permitted to be present and active on a computer system. The goal of whitelisting is to protect computers and networks from potentially harmful applications.
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In general, a whitelist is an index of approved entities. In infosec, whitelisting works best in centrally managed environments, where systems are subject to a consistent workload. The National Institute of Standards and Technology suggests using application whitelisting in high-risk environments, where it is vitally important that individual systems be secure and less important that software be useable without restrictions. To provide more flexibility, a whitelist may also index approved application components, such as software libraries, plug-ins, extensions and configuration files.
Application whitelisting vs. blacklisting
Unlike technologies that use application blacklisting, which prevents undesirable programs from executing, whitelisting is more restrictive and allows only programming that has been explicitly permitted to run. There is no consensus among security experts over which technique -- blacklisting or whitelisting -- is better. Proponents of blacklisting argue application whitelisting is too complex and difficult to manage. Compiling the initial whitelist, for example, requires detailed information about all users' tasks and all the applications they need to perform those tasks. Maintaining the list is also demanding because of the increasing complexity and interconnections of business processes and applications.
Proponents of whitelisting argue it is worth the time and effort needed to proactively protect systems and prevent malicious or inappropriate programs from entering the network. Using a whitelist that allows only applications that have been explicitly approved offers more protection against malicious software, rather than the looser standard used by application blacklists, which permit any software to run unless it has been discovered to be malicious and has been added to the blacklist.
How application whitelisting works
Implementation of application whitelisting begins with building a list of approved applications. The whitelist can be built into the host operating system, or it can be provided by a third-party vendor. The simplest form of whitelisting allows the system administrator to specify file attributes associated with whitelisted applications, such as file name, file path and file size.
Windows AppLocker, which Microsoft added to Windows 7 and Windows Server 2008 R2, allows system administrators to specify which users or groups of users are permitted to -- or not permitted to -- run particular applications. In addition to restricting access to specific applications, AppLocker can be used to restrict users from installing new software, define which versions of a piece of software are permitted to be run and provide control for running licensed software.
Risks of using application whitelisting
Attackers can replace whitelisted applications with malicious apps with relative ease by creating a version of their malware that is the same size and has the same file name as a permitted application, and then replacing the whitelisted application with the malicious one. Therefore, it is much more effective for application whitelisting software to use cryptographic hashing techniques coupled with digital signatures that are linked to the software developers.