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  AC-04: Information Flow Enforcement  

base control objective:
The information system enforces approved authorizations for controlling the flow of information within the system and between interconnected systems in accordance with applicable policy.

supplemental objective information:
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. A few examples of flow control restrictions include: keeping export controlled information from being transmitted in the clear to the Internet, blocking outside traffic that claims to be from within the organization, and not passing any web requests to the Internet that are not from the internal web proxy. Information flow control policies and enforcement mechanisms are commonly employed by organizations to control the flow of information between designated sources and destinations (e.g., networks, individuals, devices) within information systems and between interconnected systems. Flow control is based on the characteristics of the information and/or the information path. Specific examples of flow control enforcement can be found in boundary protection devices (e.g., proxies, gateways, guards, encrypted tunnels, firewalls, and routers) 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 content (e.g., using key word searches or document characteristics). Mechanisms implemented by AC-4 are configured to enforce authorizations determined by other security controls.

enhancements to the base objective:

(1) The information system enforces information flow control using explicit security attributes on information, source, and destination objects as a basis for flow control decisions.
Enhancement Supplemental Guidance: Information flow enforcement mechanisms compare security attributes on all information (data content and data structure), source and destination objects, and respond appropriately (e.g., block, quarantine, alert administrator) when the mechanisms encounter information flows not explicitly allowed by the information flow policy. Information flow enforcement using explicit security attributes can be used, for example, to control the release of certain types of information.

(2) The information system enforces information flow control using protected processing domains (e.g., domain type-enforcement) as a basis for flow control decisions.

(3) The information system enforces dynamic information flow control based on policy that allows or disallows information flows based on changing conditions or operational considerations.

(4) The information system prevents encrypted data from bypassing content-checking mechanisms.

(5) The information system enforces [Assignment: organization-defined limitations on the embedding of data types within other data types].

(6) The information system enforces information flow control on metadata.

(7) The information system enforces [Assignment: organization-defined one-way flows] using hardware mechanisms.

(8) The information system enforces information flow control using [Assignment: organization-defined security policy filters] as a basis for flow control decisions.
Enhancement Supplemental Guidance: Organization-defined security policy filters include, for example, dirty word filters, file type checking filters, structured data filters, unstructured data filters, metadata content filters, and hidden content filters. Structured data permits the interpretation of its content by virtue of atomic elements that are understandable by an application and indivisible. Unstructured data refers to masses of (usually) digital information that does not have a data structure or has a data structure that is not easily readable by a machine. Unstructured data consists of two basic categories: (i) bitmap objects that are inherently non language-based (i.e., image, video, or audio files); and (ii) textual objects that are based on a written or printed language (i.e., commercial off-the-shelf word processing documents, spreadsheets, or emails).

(9) The information system enforces the use of human review for [Assignment: organization-defined security policy filters] when the system is not capable of making an information flow control decision.

(10) The information system provides the capability for a privileged administrator to enable/disable [Assignment: organization-defined security policy filters].

(11) The information system provides the capability for a privileged administrator to configure the [Assignment: organization-defined security policy filters] to support different security policies.
Enhancement Supplemental Guidance: For example, to reflect changes in the security policy, an administrator can change the list of “dirty words” that the security policy mechanism checks in accordance with the definitions provided by the organization.

(12) The information system, when transferring information between different security domains, identifies information flows by data type specification and usage.
Enhancement Supplemental Guidance: Data type specification and usage include, for example, using file naming to reflect type of data and limiting data transfer based on file type.

(13) The information system, when transferring information between different security domains, decomposes information into policy-relevant subcomponents for submission to policy enforcement mechanisms.
Enhancement Supplemental Guidance: Policy enforcement mechanisms include the filtering and/or sanitization rules that are applied to information prior to transfer to a different security domain. Parsing transfer files facilitates policy decisions on source, destination, certificates, classification, subject, attachments, and other information security-related component differentiators. Policy rules for cross domain transfers include, for example, limitations on embedding components/information types within other components/information types, prohibiting more than two-levels of embedding, and prohibiting the transfer of archived information types.

(14) The information system, when transferring information between different security domains, implements policy filters that constrain data structure and content to [Assignment: organization-defined information security policy requirements].
Enhancement Supplemental Guidance: Constraining file lengths, allowed enumerations, character sets, schemas, and other data object attributes reduces the range of potential malicious and/or unsanctioned content. Examples of constraints include ensuring that: (i) character data fields only contain printable ASCII; (ii) character data fields only contain alpha-numeric characters; (iii) character data fields do not contain special characters; or (iv) maximum field sizes and file lengths are enforced based upon organization-defined security policy.

(15) The information system, when transferring information between different security domains, detects unsanctioned information and prohibits the transfer of such information in accordance with the security policy.
Enhancement Supplemental Guidance: Actions to support this enhancement include: checking all transferred information for malware, implementing dirty word list searches on transferred information, and applying the same protection measures to metadata (e.g., security attributes) that is applied to the information payload.

(16) The information system enforces security policies regarding information on interconnected systems.
Enhancement Supplemental Guidance: Transferring information between interconnected information systems of differing security policies introduces risk that such transfers violate one or more policies. While security policy violations may not be absolutely prohibited, policy guidance from information owners/stewards is implemented at the policy enforcement point between the interconnected systems. Specific architectural solutions are mandated, when required, to reduce the potential for undiscovered vulnerabilities. Architectural solutions include, for example: (i) prohibiting information transfers between interconnected systems (i.e. implementing access only, one way transfer mechanisms);
(ii) employing hardware mechanisms to enforce unitary information flow directions; and
(iii) implementing fully tested, re-grading mechanisms to reassign security attributes and associated security labels.

(17) The information system:
(a) Uniquely identifies and authenticates source and destination domains for information transfer;
(b) Binds security attributes to information to facilitate information flow policy enforcement; and
(c) Tracks problems associated with the security attribute binding and information transfer.
Enhancement Supplemental Guidance: Attribution is a critical component of a security concept of operations. The ability to identify source and destination points for information flowing in an information system, allows forensic reconstruction of events when required, and increases policy compliance by attributing policy violations to specific organizations/individuals. Means to enforce this enhancement include ensuring that the information system resolution labels distinguish between information systems and organizations, and between specific system components or individuals involved in preparing, sending, receiving, or disseminating information.

mapping to FIPS199 baseline:

  LOW: null     MOD: base     HIGH: base  

related (regimented) controls:

AC-17   Remote Access
AC-19   Access Control for Mobile Devices
AC-21   User Based Collaboration and Information Sharing
CM-07   Least Functionality
SA-08   Security Engineering Principles
SC-02   Application Partitioning
SC-05   Denial of Service Protection
SC-07   Boundary Protection
SC-18   Mobile Code

documents referenced in SP800-53rev3 for AC-04:

Document Date Status Title
NIST SP800-113 July, 2008 current   Guide to SSL VPNs
NIST SP800-41 September, 2009 current   Guidelines on Firewalls and Firewall Policy
NIST SP800-58 January, 2005 current   Security Considerations for Voice Over IP Systems
NIST SP800-77 December, 2005 current   Guide to IPsec VPNs
NIST SP800-82 June, 2011 current   Guide to Supervisory Control and Data Acquisition (SCADA) and Industrial Control Systems Security

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