Introduction #

Application control is one of the most effective mitigation strategies in ensuring the security of systems. As such, application control forms part of the Essential Eight from the Strategies to Mitigate Cyber Security Incidents.

This publication provides guidance on what application control is, what application control is not, and how to implement application control.

What application control is #

Application control is a security approach designed to protect against malicious code (also known as malware) executing on systems. When implemented robustly, it ensures only approved applications (e.g. executables, software libraries, scripts, installers, compiled HTML, HTML applications, control panel applets and drivers) can be executed.

While application control is primarily designed to prevent the execution and spread of malicious code, it can also prevent the installation or use of unapproved applications.

What application control is not #

The following approaches are not considered to be application control:

• providing a portal or other means of installation for approved applications
• using web or email content filters to prevent users from downloading applications from the internet
• checking the reputation of an application using a cloud-based service before it is executed
• using a next-generation firewall to identify whether network traffic is generated by an approved application.

How to implement application control #

Implementing application control involves the following high-level steps:

• identifying approved applications
• developing application control rules to ensure only approved applications are allowed to execute
• maintaining the application control rules using a change management program
• validating application control rules on an annual or more frequent basis.

When determining how to enforce application control, the following methods are considered suitable if implemented correctly:

• cryptographic hash rules
• publisher certificate rules (combining both publisher names and product names)
• path rules (ensuring file system permissions are configured to prevent unauthorised modification of folder and file permissions, folder contents and individual files).

Conversely, the use of file names, package names or any other easily changed application attribute is not considered suitable as a method of application control.

In addition to preventing the execution of unapproved applications, application control can contribute to the identification of attempts by an adversary to execute malicious code. This can be achieved by configuring application control to generate event logs for allowed and blocked executions. Such event logs should ideally include information such as the name of the file, the date/time stamp and the username of the user attempting to execute the file.

Finally, it is important that application control does not replace antivirus and other security software already in place on systems. Using multiple security solutions together can contribute to an effective defence-in-depth approach to preventing the compromise of systems.

Implement application control using Windows Defender Application Control

Windows Defender Application Control (WDAC), a security feature of Microsoft Windows 10, uses a code integrity policies to restrict what code can run in both kernel mode and on the desktop. WDAC can also use virtualisation to protect itself from being disabled by an adversary that has obtained administrative privileges.

If WDAC is used for application control, the following Group Policy settings can be implemented, noting additional hardware requirements for the optional use of virtualisation-based security.

Additional information on WDAC is available from Microsoft in their WDAC design guide, WDAC deployment guide and WDAC operational guide. Furthermore, the WDAC policy wizard can assist organisations in creating WDAC policies.

Finally, to ensure application control has been robustly implemented, testing should be undertaken on a regular basis to check for misconfigurations of file system permissions. In addition, known ways of bypassing application control rules should be prevented by implementing Microsoft’s recommend block rules and recommend driver block rules.

Implement application control using Microsoft AppLocker

If Microsoft AppLocker (the predecessor of WDAC) is used for application control, the following rules can be used as a sample path-based implementation. In support of this, the rules, enforcement of rules and the automatic starting of the Application Identity service should be set via Group Policy at a domain level.

Note, for those organisations using the latest version of Microsoft Windows 10, the following paths no longer need to be included as exceptions:

• %WinDir%\servicing\Packages\*
• %WinDir%\servicing\Sessions\*.

Implementing application control within Linux environments

Implementing application control within Linux environments can be achieved using the File Access Policy daemon (fapolicyd). The fapolicyd framework allows Linux system administrators to control which applications are allowed (or denied) execution based on either path, hash, MIME type or if they are trusted (i.e. properly installed by the system package manager and registered in the RPM database). The Red Hat Security Hardening publication provides advice on how to configure and manage the use of the fapolicyd framework within Red Hat Enterprise Linux 8.

Further information #

The Information Security Manual is a cyber security framework that organisations can apply to protect their systems and data from cyber threats. The advice in the Strategies to Mitigate Cyber Security Incidents, along with its Essential Eight, complements this framework.