Apply deny-by-exception (blacklisting) policy to prevent the use of unauthorized software or deny-all, permit-by-exception (whitelisting) policy to allow the execution of authorized software.
DiscussionThe process used to identify software programs that are not authorized to execute on systems is commonly referred to as blacklisting. The process used to identify software programs that are authorized to execute on systems is commonly referred to as whitelisting. Whitelisting is the stronger of the two policies for restricting software program execution. In addition to whitelisting, organizations consider verifying the integrity of whitelisted software programs using, for example, cryptographic checksums, digital signatures, or hash functions. Verification of whitelisted software can occur either prior to execution or at system startup. NIST SP 800-167 provides guidance on application whitelisting.
Further DiscussionOrganizations should determine their blacklisting or whitelisting policy and configure the system to manage software that is allowed to run. Blacklisting or deny-by-exception allows all software to run except if on an unauthorized software list such as what is maintained in antivirus solutions. Whitelisting or permit-by-exception does not allow any software to run except if on an authorized software list. The stronger policy of the two is whitelisting. This practice, CM.L2-3.4.8, requires the implementation of allow-lists and deny-lists for application software. It leverages CM.L2-3.4.1, which requires the organization to establish and maintain software inventories. This practice, CM.L2-3.4.8, also extends CM.L2-3.4.9, which only requires control and monitoring of any user installed software.
ExampleTo improve your company’s protection from malware, you have decided to allow only designated programs to run. With additional research you identify a capability within the latest operating system that can control executables, scripts, libraries, or application installers run in your environment [c]. To ensure success you begin by authorizing digitally signed executables. Once they are deployed, you then plan to evaluate and deploy whitelisting for software libraries and scripts [c].
Potential Assessment Considerations
- Is the information system configured to only allow authorized software to run [a,b,c]?
- Is the system configured to disallow running unauthorized software [a,b,c]?
- Is there a defined list of software programs authorized to execute on the system [b]?
- Is the authorization policy a deny-all, permit by exception for software allowed to execute on the system [a,b,c]?
- Are automated mechanisms used to prevent program execution in accordance with defined lists (e.g., white listing) [a,b,c]?