Level 5 CMMC - CMMC Practices

AC.4.023  

Reference: CMMC 1.02

Family: AC

Level Introduced: 4

Practice:
Control information flows between security domains on connected systems.

CMMC Clarification:
This practice is not concerned with classified security domains. It addresses information flow among domains containing CUI and those that do not. While access control is concerned with controlling access to information by users and processes, controlling information flow (information flow control) is concerned with where information is allowed to move within a system and between systems. In general, information flow control can apply to any needed flow restrictions. For this CMMC practice the flows of concern are primarily between CUI authorized and CUI not-authorized components/systems. Any attempt to move CUI to a domain that has not been designated as a domain allowed to store or process CUI must be blocked.

Example 1
You are the IT administrator for your organization. You have designed the network in each of the regional offices to have two zones: one zone that can store and process CUI data and a second zone where CUI information is not permitted. A firewall separates the two zones in the office so staff cannot access files and resources within the office, and a site-to-site VPN over the corporate WAN allows the CUI zones to communicate. To ensure separation between CUI projects, staff are given file access permissions to project servers and file stores by project. To facilitate the transfer of CUI files and data between the same project team working in each regional office, you install a SharePoint server on the CUI zone of the headquarters office. Authorized staff have accounts and use their MFA token to log into the SharePoint server to view or modify projects files stored there.

3.1.3e

Employ [Assignment: organization-defined secure information transfer solutions] to control information flows between security domains on connected systems.

Discussion:
Organizations employ information flow control policies and enforcement mechanisms to control the flow of information between designated sources and destinations within systems and between connected systems. Flow control is based on the characteristics of the information and/or the information path. Enforcement occurs, for example, in boundary protection devices that employ rule sets or establish configuration settings that restrict system services, provide a packet-filtering capability based on header information, or provide a message-filtering capability based on message content. Organizations also consider the trustworthiness of filtering and inspection mechanisms (i.e., hardware, firmware, and software components) that are critical to information flow enforcement.

Transferring information between systems in different security domains with different security policies introduces the risk that the transfers violate one or more domain security policies. In such situations, information owners or information stewards provide guidance at designated policy enforcement points between connected systems. Organizations mandate specific architectural solutions when required to enforce logical or physical separation between systems in different security domains. Enforcement includes prohibiting information transfers between connected systems, employing hardware mechanisms to enforce one-way information flows, verifying write
permissions before accepting information from another security domain or connected system, and implementing trustworthy regrading mechanisms to reassign security attributes and labels.

Secure information transfer solutions often include one or more of the following properties: use of cross-domain solutions when traversing security domains, mutual authentication of the sender and recipient (using hardware-based cryptography), encryption of data in transit and at rest, isolation from other domains, and logging of information transfers (e.g., title of file, file size, cryptographic hash of file, sender, recipient, transfer time and IP address, receipt time, and IP address).

Source: NIST Special Publication 800-172 (Draft)

AC-4

INFORMATION FLOW ENFORCEMENT

Description:
The information system enforces approved authorizations for controlling the flow of information within the system and between interconnected systems based on [Assignment: organization-defined information flow control policies].

Supplemental Guidance:
Information flow control regulates where information is allowed to travel within an information system and between information systems (as opposed to who is allowed to access the information) and without explicit regard to subsequent accesses to that information. Flow control restrictions include, for example, keeping export-controlled information from being transmitted in the clear to the Internet, blocking outside traffic that claims to be from within the organization, restricting web requests to the Internet that are not from the internal web proxy server, and limiting information transfers between organizations based on data structures and content. Transferring information between information systems representing different security domains with different security policies introduces risk that such transfers violate one or more domain security policies. In such situations, information owners/stewards provide guidance at designated policy enforcement points between interconnected systems. Organizations consider mandating specific architectural solutions when required to enforce specific security policies. Enforcement includes, for example: (i) prohibiting information transfers between interconnected systems (i.e., allowing access only); (ii) employing hardware mechanisms to enforce one-way information flows; and (iii) implementing trustworthy regrading mechanisms to reassign security attributes and security labels.
Organizations commonly employ information flow control policies and enforcement mechanisms to control the flow of information between designated sources and destinations (e.g., networks, individuals, and devices) within information systems and between interconnected systems. Flow control is based on the characteristics of the information and/or the information path. Enforcement occurs, for example, in boundary protection devices (e.g., gateways, routers, guards, encrypted tunnels, firewalls) that employ rule sets or establish configuration settings that restrict information system services, provide a packet-filtering capability based on header information, or message-filtering capability based on message content (e.g., implementing key word searches or using document characteristics). Organizations also consider the trustworthiness of filtering/inspection mechanisms (i.e., hardware, firmware, and software components) that are critical to information flow enforcement. Control enhancements 3 through 22 primarily address cross-domain solution needs which focus on more advanced filtering techniques, in-depth analysis, and stronger flow enforcement mechanisms implemented in cross-domain products, for example, high-assurance guards. Such capabilities are generally not available in commercial off-the-shelf information technology products. Related controls: AC-3, AC-17, AC-19, AC-21, CM-6, CM-7, SA-8, SC-2, SC-5, SC-7, SC-18.

AC-4 (1)

INFORMATION FLOW ENFORCEMENT | OBJECT SECURITY ATTRIBUTES

Description:
The information system uses [Assignment: organization-defined security attributes] associated with [Assignment: organization-defined information, source, and destination objects] to enforce [Assignment: organization-defined information flow control policies] as a basis for flow control decisions.

Supplemental Guidance:
Information flow enforcement mechanisms compare security attributes associated with information (data content and data structure) and source/destination objects, and respond appropriately (e.g., block, quarantine, alert administrator) when the mechanisms encounter information flows not explicitly allowed by information flow policies. For example, an information object labeled Secret would be allowed to flow to a destination object labeled Secret, but an information object labeled Top Secret would not be allowed to flow to a destination object labeled Secret. Security attributes can also include, for example, source and destination addresses employed in traffic filter firewalls. Flow enforcement using explicit security attributes can be used, for example, to control the release of certain types of information. Related control: AC-16.

AC-4 (6)

INFORMATION FLOW ENFORCEMENT | METADATA

Description:
The information system enforces information flow control based on [Assignment: organization-defined metadata].

Supplemental Guidance:
Metadata is information used to describe the characteristics of data. Metadata can include structural metadata describing data structures (e.g., data format, syntax, and semantics) or descriptive metadata describing data contents (e.g., age, location, telephone number). Enforcing allowed information flows based on metadata enables simpler and more effective flow control. Organizations consider the trustworthiness of metadata with regard to data accuracy (i.e., knowledge that the metadata values are correct with respect to the data), data integrity (i.e., protecting against unauthorized changes to metadata tags), and the binding of metadata to the data payload (i.e., ensuring sufficiently strong binding techniques with appropriate levels of assurance). Related controls: AC-16, SI-7.

AC-4 (8)

INFORMATION FLOW ENFORCEMENT | SECURITY POLICY FILTERS

Description:
The information system enforces information flow control using [Assignment: organization-defined security policy filters] as a basis for flow control decisions for [Assignment: organization-defined information flows].

Supplemental Guidance:
Organization-defined security policy filters can address data structures and content. For example, security policy filters for data structures can check for maximum file lengths, maximum field sizes, and data/file types (for structured and unstructured data). Security policy filters for data content can check for specific words (e.g., dirty/clean word filters), enumerated values or data value ranges, and hidden content. Structured data permits the interpretation of data content by applications. Unstructured data typically refers to digital information without a particular data structure or with a data structure that does not facilitate the development of rule sets to address the particular sensitivity of the information conveyed by the data or the associated flow enforcement decisions. Unstructured data consists of: (i) bitmap objects that are inherently non language-based (i.e., image, video, or audio files); and (ii) textual objects that are based on written or printed languages (e.g., commercial off-the-shelf word processing documents, spreadsheets, or emails). Organizations can implement more than one security policy filter to meet information flow control objectives (e.g., employing clean word lists in conjunction with dirty word lists may help to reduce false positives).

AC-4 (12)

INFORMATION FLOW ENFORCEMENT | DATA TYPE IDENTIFIERS

Description:
The information system, when transferring information between different security domains, uses [Assignment: organization-defined data type identifiers] to validate data essential for information flow decisions.

Supplemental Guidance:
Data type identifiers include, for example, filenames, file types, file signatures/tokens, and multiple internal file signatures/tokens. Information systems may allow transfer of data only if compliant with data type format specifications.

AC-4 (13)

INFORMATION FLOW ENFORCEMENT | DECOMPOSITION INTO POLICY-RELEVANT SUBCOMPONENTS

Description:
The information system, when transferring information between different security domains, decomposes information into [Assignment: organization-defined policy-relevant subcomponents] for submission to policy enforcement mechanisms.

Supplemental Guidance:
Policy enforcement mechanisms apply filtering, inspection, and/or sanitization rules to the policy-relevant subcomponents of information to facilitate flow enforcement prior to transferring such information to different security domains. Parsing transfer files facilitates policy decisions on source, destination, certificates, classification, attachments, and other security-related component differentiators.

AC-4 (15)

INFORMATION FLOW ENFORCEMENT | DETECTION OF UNSANCTIONED INFORMATION

Description:
The information system, when transferring information between different security domains, examines the information for the presence of [Assignment: organized-defined unsanctioned information] and prohibits the transfer of such information in accordance with the [Assignment: organization-defined security policy].

Supplemental Guidance:
Detection of unsanctioned information includes, for example, checking all information to be transferred for malicious code and dirty words. Related control: SI-3.

AC-4 (20)

INFORMATION FLOW ENFORCEMENT | APPROVED SOLUTIONS

Description:
The organization employs [Assignment: organization-defined solutions in approved configurations] to control the flow of [Assignment: organization-defined information] across security domains.

Supplemental Guidance:
Organizations define approved solutions and configurations in cross-domain policies and guidance in accordance with the types of information flows across classification boundaries. The Unified Cross Domain Management Office (UCDMO) provides a baseline listing of approved cross-domain solutions.

Source: NIST Special Publication 800-53 Rev. 4

Source: CMMC v1.02