Cisco ASA VPN Security Technical Implementation Guide

If the system were to continue processing after audit failure, actions can be taken on the system that cannot be tracked and recorded for later forensic analysis. Because of the importance of ensuring mission/business continuity, organizations may determine that the nature of the audit failure is not so severe that it warrants a complete shutdown of the application supporting the core organizational missions/business operations. In those instances, partial application shutdowns or operating in a degraded mode with reduced capability may be viable alternatives. This requirement only applies to components where this is specific to the function of the device (e.g., IDPS sensor logs, firewall logs). This does not apply to audit logs generated on behalf of the device itself (management).

It is critical for the appropriate personnel to be aware if a system is at risk of failing to process audit logs as required. Without a real-time alert, security personnel may be unaware of an impending failure of the audit capability and system operation may be adversely affected. Alerts provide organizations with urgent messages. Automated alerts can be conveyed in a variety of ways, including, for example, telephonically, via electronic mail, via text message, or via websites. Log processing failures include software/hardware errors, failures in the log capturing mechanisms, and log storage capacity being reached or exceeded. While this requirement also applies to the event monitoring system (e.g., Syslog, Security Information and Event Management [SIEM], or SNMP servers), the VPN Gateway must also be configured to generate a message to the administrator console. The VPN daemon facility and log facility are messages in the log, which capture actions performed or errors encountered by system processes. The ISSM or ISSO may designate the firewall/system administrator or other authorized personnel to receive the alert within the specified time, validate the alert, then forward only validated alerts to the ISSM and ISSO.

Without path validation, an informed trust decision by the relying party cannot be made when presented with any certificate not already explicitly trusted. To meet this requirement, the information system must create trusted channels between itself and remote trusted authorized IT product (e.g., syslog server) entities that protect the confidentiality and integrity of communications. The information system must create trusted paths between itself and remote administrators and users that protect the confidentiality and integrity of communications. A trust anchor is an authoritative entity represented via a public key and associated data. It is most often used in the context of public key infrastructures, X.509 digital certificates, and DNSSEC. However, applications that do not use a trusted path are not approved for non-local and remote management of DoD information systems. When there is a chain of trust, usually the top entity to be trusted becomes the trust anchor; it can be, for example, a Certification Authority (CA). A certification path starts with the subject certificate and proceeds through a number of intermediate certificates up to a trusted root certificate, typically issued by a trusted CA. This requirement verifies that a certification path to an accepted trust anchor is used for certificate validation and that the path includes status information. Path validation is necessary for a relying party to make an informed trust decision when presented with any certificate not already explicitly trusted. Status information for certification paths includes certificate revocation lists or online certificate status protocol responses. Validation of the certificate status information is out of scope for this requirement.

In order to prevent unauthorized connection of devices, unauthorized transfer of information, or unauthorized tunneling (i.e., embedding of data types within data types); organizations must disable or restrict unused or unnecessary physical and logical ports/protocols on information systems. Use of IKEv2 leverages DoS protections because of improved bandwidth management and leverages more secure encryption algorithms.

Use of weak or untested encryption algorithms undermines the purposes of utilizing encryption to protect data. The VPN gateway must implement cryptographic modules adhering to the higher standards approved by the federal government since this provides assurance they have been tested and validated.

PFS generates each new encryption key independently from the previous key. Without PFS, compromise of one key will compromise all communications. The Phase 2 (Quick Mode) Security Association (SA) is used to create an IPsec session key. Hence, its rekey or key regeneration procedure is very important. The Phase 2 rekey can be performed with or without Perfect Forward Secrecy (PFS). With PFS, every time a new IPsec Security Association is negotiated during the Quick Mode, a new Diffie-Hellman (DH) exchange occurs. The new DH shared secret will be included with original keying material (SYKEID_d, initiator nonce, and responder nonce from Phase 1 for generating a new IPsec session key. If PFS is not used, the IPsec session key will always be completely dependent on the original keying material from Phase 1. Hence, if an older key is compromised at any time, it is possible that all new keys may be compromised. The DH exchange is performed in the same manner as was done in Phase 1 (Main or Aggressive Mode). However, the Phase 2 exchange is protected by encrypting the Phase 2 packets with the key derived from the Phase 1 negotiation. Because DH negotiations during Phase 2 are encrypted, the new IPsec session key has an added element of secrecy.

FIPS 140-2/140-3 precludes the use of invalidated cryptography for the cryptographic protection of sensitive or valuable data within federal systems. Unvalidated cryptography is viewed by NIST as providing no protection to the information or data. In effect, the data would be considered unprotected plain text. If the agency specifies that the information or data be cryptographically protected, then FIPS 140-2/140-3 is applicable. In essence, if cryptography is required, it must be validated. Cryptographic modules that have been approved for classified use may be used in lieu of modules that have been validated against the FIPS 140-2/140-3 standard. The cryptographic module used must have at least one validated hash algorithm. This validated hash algorithm must be used to generate cryptographic hashes for all cryptographic security function within the product being evaluated.

FIPS 140-2/140-3 precludes the use of invalidated cryptography for the cryptographic protection of sensitive or valuable data within federal systems. Unvalidated cryptography is viewed by NIST as providing no protection to the information or data. In effect, the data would be considered unprotected plain text. If the agency specifies that the information or data be cryptographically protected, then FIPS 140-2/140-3 is applicable. In essence, if cryptography is required, it must be validated. Cryptographic modules that have been approved for classified use may be used in lieu of modules that have been validated against the FIPS 140-2/140-3 standard. The cryptographic module used must have one FIPS-validated encryption algorithm (i.e., validated Advanced Encryption Standard [AES]). This validated algorithm must be used for encryption for cryptographic security function within the product being evaluated.

Without cryptographic integrity protections, information can be altered by unauthorized users without detection. Although allowed by SP800-131Ar2 for some applications, SHA-1 is considered a compromised hashing standard and is being phased out of use by industry and government standards. Unless required for legacy use, DOD systems should not be configured to use SHA-2 for integrity of remote access sessions.

Unrestricted traffic may contain malicious traffic which poses a threat to an enclave or to other connected networks. Additionally, unrestricted traffic may transit a network, which uses bandwidth and other resources. VPN traffic received from another enclave with different security policy or level of trust must not bypass being inspected by the firewall before being forwarded to the private network.

Without identifying devices, unidentified or unknown devices may be introduced, thereby facilitating malicious activity. For distributed architectures (e.g., service-oriented architectures), the decisions regarding the validation of identification claims may be made by services separate from the services acting on those decisions. In such situations, it is necessary to provide the identification decisions (as opposed to the actual identifiers) to the services that need to act on those decisions. This requirement applies to applications that connect either locally, remotely, or through a network to an endpoint device (including, but not limited to, workstations, printers, servers (outside a datacenter), VoIP Phones, and VTC CODECs). Gateways and SOA applications are examples of where this requirement would apply.

The IPsec SA and its corresponding key will expire either after the number of seconds or amount of traffic volume has exceeded the configured limit. A new SA is negotiated before the lifetime threshold of the existing SA is reached to ensure that a new SA is ready for use when the old one expires. The longer the lifetime of the IPsec SA, the longer the lifetime of the session key used to protect IP traffic. The SA is less secure with a longer lifetime because an attacker has a greater opportunity to collect traffic encrypted by the same key and subject it to cryptanalysis. However, a shorter lifetime causes IPsec peers to renegotiate Phase 2, more often resulting in the expenditure of additional resources. Specify the lifetime (in seconds) of an Internet Key Exchange (IKE) security association (SA). When the SA expires, it is replaced by a new SA, the Security Parameter Index (SPI), or terminated if the peer cannot be contacted for renegotiation.

When a VPN gateway creates an IPsec Security Association (SA), resources must be allocated to maintain the SA. These resources are wasted during periods of IPsec endpoint inactivity, which could result in the gateway’s inability to create new SAs for other endpoints, thereby preventing new sessions from connecting. The Internet Key Exchange (IKE) idle timeout may also be set to allow SAs associated with inactive endpoints to be deleted before the SA lifetime has expired, although this setting is not recommended at this time. The value of one hour or less is a common best practice.

The VPN interacts directly with public networks and devices and should not contain user authentication information for all users. AAA network security services provide the primary framework through which a network administrator can set up access control and authorization on network points of entry or network access servers. It is not advisable to configure access control on the VPN gateway or remote access server. Separation of services provides added assurance to the network if the access control server is compromised.

Protecting authentication communications between the client, the VPN Gateway, and the authentication server keeps this critical information from being exploited. In distributed information systems, authorization processes and access control decisions may occur in separate parts of the systems. In such instances, authorization information is transmitted securely so timely access control decisions can be enforced at the appropriate locations. To support the access control decisions, it may be necessary to transmit as part of the access authorization information, supporting security attributes. This is due to the fact that in distributed information systems, there are various access control decisions that need to be made and different entities (e.g., services) make these decisions in a serial fashion, each requiring some security attributes to make the decisions. This applies to VPN gateways that have the concept of a user account and have the login function residing on the VPN gateway.

To assure accountability and prevent unauthenticated access, organizational users must be identified and authenticated to prevent potential misuse and compromise of the system. Organizational users include organizational employees or individuals the organization deems to have equivalent status of employees (e.g., contractors). Organizational users (and any processes acting on behalf of users) must be uniquely identified and authenticated for all accesses except the following: (i) Accesses explicitly identified and documented by the organization. Organizations document specific user actions that can be performed on the information system without identification or authentication; and (ii) Accesses that occur through authorized use of group authenticators without individual authentication. Organizations may require unique identification of individuals' in-group accounts (e.g., shared privilege accounts) or for detailed accountability of individual activity. This requirement only applies to components where this is specific to the function of the device or has the concept of an organizational user (e.g., VPN or proxy capability). This does not apply to authentication for the purpose of configuring the device itself (i.e., device management).

Without mapping the certificate used to authenticate to the user account, the ability to determine the identity of the individual user or group will not be available for forensic analysis. This requirement only applies to components where this is specific to the function of the device or has the concept of a user (e.g., VPN or ALG). This does not apply to authentication for the purpose of configuring the device itself (i.e., device management).

Display of a standardized and approved use notification before granting access to the network ensures privacy and security notification verbiage used is consistent with applicable federal laws, Executive Orders, directives, policies, regulations, standards, and guidance. In most VPN implementations, the banner is configured in the management backplane (NDM SRG) and serves as the presentation for the VPN client connection as well as for administrator logon to the device management tool/backplane. System use notifications are required only for access via logon interfaces with human users and are not required when such human interfaces do not exist. This requirement applies to VPN gateways that have the concept of a user account and have the logon function residing on the VPN gateway. The banner must be formatted in accordance with applicable DoD policy. Use the following verbiage for VPN gateways that can accommodate banners of 1300 characters: "You are accessing a U.S. Government (USG) Information System (IS) that is provided for USG-authorized use only. By using this IS (which includes any device attached to this IS), you consent to the following conditions: -The USG routinely intercepts and monitors communications on this IS for purposes including, but not limited to, penetration testing, COMSEC monitoring, network operations and defense, personnel misconduct (PM), law enforcement (LE), and counterintelligence (CI) investigations. -At any time, the USG may inspect and seize data stored on this IS. -Communications using, or data stored on, this IS are not private, are subject to routine monitoring, interception, and search, and may be disclosed or used for any USG-authorized purpose. -This IS includes security measures (e.g., authentication and access controls) to protect USG interests--not for your personal benefit or privacy. -Notwithstanding the above, using this IS does not constitute consent to PM, LE or CI investigative searching or monitoring of the content of privileged communications, or work product, related to personal representation or services by attorneys, psychotherapists, or clergy, and their assistants. Such communications and work product are private and confidential. See User Agreement for details." Use the following verbiage for operating systems that have severe limitations on the number of characters that can be displayed in the banner: "I've read & consent to terms in IS user agreem't."

Without information that establishes the identity of the subjects (i.e., users or processes acting on behalf of users) associated with the events, security personnel cannot determine responsibility for the potentially harmful event.

Without establishing where events occurred, it is impossible to establish, correlate, and investigate the events leading up to an outage or attack. In order to compile an accurate risk assessment, and provide forensic analysis, it is essential for security personnel to know where events occurred, such as VPN gateway components, modules, device identifiers, node names, and functionality. Associating information about where the event occurred within the network provides a means of investigating an attack, recognizing resource utilization or capacity thresholds, or identifying an improperly configured VPN gateway.

Without information about the outcome of events, security personnel cannot make an accurate assessment as to whether an attack was successful or if changes were made to the security state of the network. Event outcomes can include indicators of event success or failure and event-specific results (e.g., the security state of the network after the event occurred). As such, they also provide a means to measure the impact of an event and help authorized personnel to determine the appropriate response.

Without integrity protection, unauthorized changes may be made to the log files and reliable forensic analysis and discovery of the source of malicious system activity may be degraded. Remote access (e.g., RDP) is access to DoD nonpublic information systems by an authorized user (or an information system) communicating through an external, non-organization-controlled network. Remote access methods include broadband and wireless. Integrity checks include cryptographic checksums, digital signatures, or hash functions. Federal Information Processing Standard (FIPS) 186-4, Digital Signature Standard (DSS), specifies three NIST-approved algorithms: DSA, RSA, and ECDSA. All three are used to generate and verify digital signatures in conjunction with an approved hash function.

Both IPsec and TLS gateways use the RNG to strengthen the security of the protocols. Using a weak RNG will weaken the protocol and make it more vulnerable. Use of a FIPS validated RNG that is not DRGB mitigates to a CAT III.

Without strong cryptographic integrity protections, information can be altered by unauthorized users without detection. SHA-1 is considered a compromised hashing standard and is being phased out of use by industry and Government standards. DOD systems must not be configured to use SHA-1 for integrity of remote access sessions. The remote access VPN provides access to DOD nonpublic information systems by an authorized user (or an information system) communicating through an external, non-organization-controlled network.

The use of PIV credentials facilitates standardization and reduces the risk of unauthorized access. DoD has mandated the use of the CAC to support identity management and personal authentication for systems covered under HSPD 12, as well as a primary component of layered protection for national security systems.

Split tunneling would in effect allow unauthorized external connections, making the system more vulnerable to attack and to exfiltration of organizational information. A VPN hardware or software client with split tunneling enabled provides an unsecured backdoor to the enclave from the internet. With split tunneling enabled, a remote client has access to the internet while at the same time has established a secured path to the enclave via an IPsec tunnel. A remote client connected to the internet that has been compromised by an attacker in the internet provides an attack base to the enclave’s private network via the IPsec tunnel. Hence, it is imperative that the VPN gateway enforces a no split-tunneling policy to all remote clients.

Without generating log records that are specific to the security and mission needs of the organization, it would be difficult to establish, correlate, and investigate the events relating to an incident, or identify those responsible for one. Log records can be generated from various components within the information system (e.g., module or policy filter). This requirement only applies to components where this is specific to the function of the device, such as application layer gateway (ALG), which provides these access control and auditing functions on behalf of an application. This does not apply to audit logs generated on behalf of the device itself (management).

A certificate's certification path is the path from the end entity certificate to a trusted root certification authority (CA). Certification path validation is necessary for a relying party to make an informed decision regarding acceptance of an end entity certificate. Certification path validation includes checks such as certificate issuer trust, time validity, and revocation status for each certificate in the certification path. Revocation status information for CA and subject certificates in a certification path is commonly provided via certificate revocation lists (CRLs) or online certificate status protocol (OCSP) responses.