TACACS+ TLS 1.3

The TACACS+ Protocol provides device administration for routers, network access servers and other networked computing devices via one or more centralized servers. The protocol provides authentication, authorization and accounting services for TACACS+ clients. While the content of the protocol is highly sensitive, TACACS+ lacks modern and/or effective confidentiality, integrity, and authentication of the connection and network traffic between the server and client. The existing mechanisms of TACACS+ are extremely weak and the Security Considerations section of the TACACS+ Protocol adequately describes this. To address these deficiencies, this document updates the TACACS+ Protocol to use TLS 1.3 authentication and encryption, and obsoletes the use of its former mechanisms.

The Technical Definitions section of the TACACS+ Protocol is fully applicable here and will not be repeated. The following terms are also used in this document.

Inferior form of encryption used in TACACS+, referred to as obfuscation in to indicate that it is not encryption and is utterly insufficient.

This is another term for a Connection as defined in TACACS+ Protocol. It is a Connection without TLS and therefore being plaintext or possibly using unsecure TACACS+ authentication and obfuscation.

A TLS Connection is a TCP/IP connection with TLS authentication and encryption used by TACACS+ for transport. The TLS Connection for TACACS+ is always between one Client and one Server as defined in TACACS+ Protocol.

In this document, we clarify that a TACACS+ Server is an instance of the Server as defined in that responds to TACACS+ traffic on a specific port in a host, and does not refer to the host itself. A host may have multiple TACACS+ Servers installed, listening to different ports.

In the context of a TLS Connection, the peer of a TACACS+ Client is the Server, and the peer of a TACACS+ Server is the Client. Together, the ends of a TACACS+ Connection are referred as the peers.

TACACS+ connections are TCP/IP connections initiated by the Client to the Server. The well-known TCP/IP port 49 on the Server is used for unobfuscated and obfuscated connections as defined in the TACACS+ Protocol. A connection might be used for only a single Session or the multiplexing of multiple Sessions in TACACS+ Single Connection Mode. TLS is introduced into TACACS+ to fulfill the following requirements:

Confidentiality and Integrity: The MD5 Obfuscation specified in the original protocol definition is not fit for purpose, requiring that TACACS+ be deployed over a secured network. Securing TACACS+ protocol with TLS is intended to provide confidentiality and integrity without requiring the provision of a secured network.
Peer authentication: The authentication capabilities of TLS replace the pre-shared keys of Obfuscation to authenticate Peers.

All data exchanged by TACACS+ Peers MUST be encrypted, including the authentication of the Peers. Therefore, TLS Hello MUST be initiated by the client immediately upon the establishment of the TCP/IP connection. This document favors the predictable use of TLS security for a deployment, see (). TACACS+ TLS will therefore follow , where a different well-known system TCP/IP port is assigned by IANA, port [TBD] with the service name [TBDN], for TLS connections. TACACS+ TLS could use any other TCP port by operator configuration, though should still be considered.

A TACACS+ Client initiates a TLS connection by making a TCP connection to a configured Server on the TACACS+ TLS well-known port ([TBD]) (). Once the TCP connection is established, the Client MUST immediately begin the TLS negotiation before sending any TACACS+ protocol data. Implementations MUST support TLS 1.3 and MAY permit TLS 1.3 session resumption. If resumption is supported, the resumption ticket_lifetime SHOULD be configurable, including a zero seconds lifetime. Once the TLS connection is established, the exchange of TACACS+ data proceeds as normal, except that it is transmitted over TLS as TLS application data and without TACACS+ obfuscation (see ) The connection persists until the Server or Client closes it. It might be closed due to an error or at the conclusion of the TACACS+ Session. If Single Connection Mode has been negotiated, it might remain open after a successful Session, until an error or an inactivity timeout occurs. Why it closed has no bearing on TLS resumption, unless closed by a TLS error, in which case the ticket might be invalidated.

Implementations MUST support the TLS 1.3 mandatory cipher suites (See TLS 1.3 Section 9.1). The cipher suites offered or accepted SHOULD be configurable so that operators can adapt. This document makes no cipher suite recommendations, but recommendations can be found in the TLS Cipher Suites section of the .

Implementations MUST support certificate-based TLS authentication and certificate revocation bi-directionally for authentication, identity verification and policy purposes. Certificate path verification as described in MUST be supported. If this succeeds, the authentication is successful and the connection is permitted. Policy MAY impose further constraints upon the Peer, allowing or denying the connection based on certificate fields or any other parameters exposed by the implementation. Unless disabled by configuration, a Peer MUST disconnect a Peer that offers an invalid TLS Certificate.

Implementations MUST support certificate Path verification as described in . Because a Peer could be isolated from a remote Peer's Certificate Authority (CA), implementations MUST support certificate chains (aka. bundles or chains of trust), where the entire chain of the remote's certificate is stored on the local Peer.

In addition to authentication of TLS certificates, implementations MUST support policy consideration of Peer-identifying certificate fields and policy used to verify that the Peer is a valid source for the received certificate and that it is permitted access to TACACS+. Implementations MUST support either: Network location based validation methods as described in . or Device Identity based validation methods where the peer’s identity is used in the certificate subjectName. This is applicable in deployments where the device securely supports an identity which is shared with its peer. This approach allows a peer's network location to be reconfigured without issuing a new client certificate. Only the local server mapping needs to be updated. Implementations SHOULD support the TLS Server Name Indication extension (). Policy can be applied to this attribute and it can be useful for load balancing or multiplexing at the server.

An ALPN number is registered for Secured TACACS+ . Implementations MUST use it, in accordance to the guideleines in

The original draft of TACACS+ described the Obfuscation mechanism, documented in . It is insufficient for modern purposes. The introduction of TLS PSK, certificate Peer authentication, and TLS encryption to TACACS+ replaces these former mechanisms and so Obfuscation is hereby obsoleted. This section describes how the TACACS+ client and servers MUST operate with regards to the obfuscation mechanism. Peers MUST NOT use Obfuscation with TLS. A TACACS+ client initiating a TACACS+ TLS connection MUST set the TAC_PLUS_UNENCRYPTED_FLAG bit, thereby asserting that Obfuscation is not used for the Session. All subsequent packets MUST have the TAC_PLUS_UNENCRYPTED_FLAG set. A TACACS+ server that receives a packet with the TAC_PLUS_UNENCRYPTED_FLAG not set (cleared) over a TLS connection, MUST return an error of TAC_PLUS_AUTHEN_STATUS_ERROR, TAC_PLUS_AUTHOR_STATUS_ERROR, or TAC_PLUS_ACCT_STATUS_ERROR as appropriate for the TACACS+ message type, with the TAC_PLUS_UNENCRYPTED_FLAG set, and terminate the Session. This behavior corresponds to that defined in RFC8907 Section 4.5. Data Obfuscation for TAC_PLUS_UNENCRYPTED_FLAG or key mismatches. A TACACS+ client that receives a packet with the TAC_PLUS_UNENCRYPTED_FLAG not set (cleared), MUST terminate the Session, and SHOULD log this error.

This document improves the confidentiality, integrity, and authentication of the connection and network traffic between TACACS+ Peers by adding TLS support. Simply adding TLS support to the protocol does not guarantee the protection of the server and clients. It is essential for the operators and equipment vendors to adhere to the latest best practices for ensuring the integrity of network devices and selecting secure TLS key and encryption algorithms. . offers substantial guidance for implementing protocols that use TLS and their deployment. Those implementing and deploying Secure TACACS+ must adhere to the recommendations relevant to TLS 1.3 outlined in , or its subsequent versions. This document outlines additional restrictions permissible under . For example, any recommendations referring to TLS 1.2, including the mandatory support, are not relevant for Secure TACACS+ as TLS 1.3 or above is mandated.

TLS encryption SHOULD be used in deployments where both the clients and servers support it. TACACS+ servers that have TLS support MUST NOT allow non-TLS connections from clients that do not support TLS, because of the threat of downgrade attacks, as described in Section 5.2. Instead, separate non-TLS TACACS+ servers can be set up to cater for these clients. Further, TLS TACACS+ Servers and non-TLS TACACS+ servers SHOULD NOT be deployed on the same host. Non-TLS Connections would be better served by deploying the required Non-TLS TACACS+ Servers on separate hosts. It is NOT RECOMMENDED to deploy TACACS+ without TLS authentication and encryption, including TLS using the NULL algorithm, except for within test and debug environments. Also see .

TLS 1.3 resumption and PSK techniques make it possible to send Early Data, aka. 0-RTT data, data that is sent before the TLS handshake completes. Replay of this data is possible. Given the sensitivity of TACACS+ data, a Client MUST NOT send data until the full TLS handshake completes; that is, Clients MUST NOT send 0-RTT data and Servers MAY abruptly disconnect Clients that do.

Implementations MAY support TLS authentication with Pre-Shared Keys (PSKs), also known as external PSKs in TLS 1.3, which are not resumption PSKs. PSKs SHOULD NOT be shared among Clients or Servers to limit exposure of a compromised key and to ease key rotation. Also see and . PSKs are otherwise considered out-of-scope for this document.

Implementors and operators SHOULD make use of the various RFCs to determine which TLS versions and algorithms should be supported, deprecated, obsoleted, or abandoned, in the absence of updates to this document. Recommendations in Section 4, or any RFCs which obsolete it, MUST be followed. Other useful examples are the TLS specifications themselves (TLS 1.3), which prescribes mandatory support in Section 9, and TLS Recommendations .

Operators SHOULD be cognizant of the potential of Server and/or Client isolation from their Peer's CA by network failures. Isolation from a public key certificate's CA will cause the verification of the certificate to fail and thus TLS authentication of the Peer to fail. Operators SHOULD consider loading certificate chains on devices and servers to avoid this failure. Certificate caching and Raw Public Keys are other methods to help address this, but both are out of scope for this document. Certificate fingerprints are another option.

Operators SHOULD be aware that the TLS SNI extension is part of the TLS client hello, and is therefore subject to eavesdropping. Also see . If TLS Encrypted Client Hello becomes standardized and applicable to TLS 1.3, then it SHOULD be included in Secure TACACS+ implementation.

Implementors MUST ensure that the configuration scheme introduced for enabling TLS is straightforward and leaves no room for ambiguity regarding whether TLS or non-TLS will be used between the TACACS+ client and the TACACS+ server. This document introduces a separate port that TLS enabled TACACS+ Servers will listen to. Where deployments have not overridden the defaults explicitly, TACACS+ client implementations MUST use the correct values:

for non-TLS connection TACACS+: Port 49.
for TLS connection TACACS+: (TBD).

Implementors MAY offer a single option for TACACS+ clients and servers to disable all non-TLS TACACS+ operations. When enabled on a TACACS+ server, it will not respond to any requests from non-TLS TACACS+ client connections. When enabled on a TACACS+ client, it will not establish any non-TLS TACACS+ server connections.

A new port is considered appropriate and superior to a "STARTTLS" command or other negotiation method because it allows:

ease of blocking the unobfuscated or obfuscated connections by the TCP/IP port number,
passive Intrusion Detection Systems (IDSs) monitoring the unobfuscated to be unaffected by the introduction of TLS,
avoidance of Man in the Middle (MitM) attacks that can interfere with STARTTLS,
and helps prevent the accidental exposure of sensitive information due to misconfiguration.

However, co-existence of inferior authentication and obfuscated, whether an Non-TLS Connection or deprecated parts that compose TLS, also presents opportunity for down-grade attacks. Causing failure of connections to the TLS-enabled service or the negotiation of shared algorithm support are two such down-grade attacks. The simplest way to address exposure from Non-TLS Connection methods is to refuse Non-TLS Connections at the server entirely, perhaps using separate servers for Non-TLS Connections and TLS. Another approach is mutual configuration that requires TLS. Clients and Servers SHOULD support configuration that requires Peers, globally and individually, use TLS. Furthermore, Peers SHOULD be configurable to limit offered or recognized TLS versions and algorithms to those recommended by standards bodies and implementers.

Operational and deployment considerations are spread throughout the document. While avoiding repetition, it is useful for the impatient to direct particular attention to Section 5.2 and Section 5.1.5. However, it is important that the entire Section 5 is observed.

In section 5.2, it is mentioned that for an optimal deployment of TLS TACACS+, TLS should be universally applied throughout the deployment. However, during the migration process from a non-TLS TACACS+ deployment, operators may need to support both TLS and Non-TLS TACACS+ Servers. This migration phase allows operators to gradually transition their deployments from an insecure state to a more secure one, but it is important to note that it is vulnerable to downgrade attacks. Therefore, the migration phase should be considered insecure until it is fully completed. To mitigate this hazard:

the period where any client is configured with both TLS and non-TLS servers SHOULD be minimized.
the operator MUST consider the impact of mixed TLS and Non-TLS on security.

Some TACACS+ client devices in a deployment may not implement TLS. These devices will require access to Non-TLS TACACS+ Servers. Operators MUST follow the recommendation of section and deploy separate TACACS+ Servers for these Non-TLS clients from those used for the TLS clients.

The authors request that, when this draft is accepted by the working group, the OPSAWG Chairs submit a request to IANA for an early allocation, per and , of a new well-known system TCP/IP port number for the service name "tacacss" (referenced in this document also as "TACACS+ TLS well-known port ([TBD])"), described as "TACACS+ over TLS". The service name "tacacss" follows the common practice of appending an "s" to the name given to the non-TLS well-known port name. This allocation is justified in . RFC EDITOR: this port number should replace "[TBD]" and the service name should replace "[TBDN]" within this document.

The author(s) would like to thank Russ Housley, Steven M. Bellovin, Stephen Farrell, Alan DeKok, Warren Kumari, and Tom Petch for their support, insightful review, and/or comments. was also used as a basis for the approach to TLS.