TLS 1.3 requirement: New Protocols Using TLS Must Require TLS 1.3

Internet Engineering Task Force (IETF)                           R. Salz
Request for Comments: 9852                           Akamai Technologies
BCP: 195                                                       N. Aviram
Updates: 9325                                                  July 2026
Category: Best Current Practice
ISSN: 2070-1721


              New Protocols Using TLS Must Require TLS 1.3

Abstract

   TLS 1.3 is widely used, has had comprehensive security proofs, and
   improves both security and privacy deficiencies in TLS 1.2.
   Therefore, new protocols that use TLS must require TLS 1.3.  As DTLS
   1.3 is not widely available or deployed, this prescription does not
   pertain to DTLS (in any DTLS version); it pertains to TLS only.

   This document updates RFC 9325.  It discusses post-quantum
   cryptography and the security and privacy improvements in TLS 1.3 as
   the rationale for the update.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9852.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
   2.  Conventions
   3.  Implications for Post-Quantum Cryptography (PQC)
   4.  TLS Use by Other Protocols and Applications
   5.  Changes to RFC 9325
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Authors' Addresses

1.  Introduction

   This document specifies that new protocols that use TLS must assume
   that TLS 1.3 is available and require its use.  As DTLS 1.3 is not
   widely available or deployed, this prescription does not pertain to
   DTLS (in any DTLS version); it pertains to TLS only.

   TLS 1.3 [TLS13] is in widespread use and fixes most known
   deficiencies with TLS 1.2.  Examples of this include encrypting more
   of the traffic so that it is not readable by outsiders and removing
   most cryptographic primitives now considered weak.  Importantly, the
   protocol has had comprehensive security proofs and should provide
   excellent security without any additional configuration.

   TLS 1.2 [TLS12] is in use and can be configured such that it provides
   good security properties.  However, TLS 1.2 suffers from several
   deficiencies, as described in Section 6.  Addressing them usually
   requires bespoke configuration.

   This document updates [RFC9325].  It discusses post-quantum
   cryptography and the security and privacy improvements in TLS 1.3 as
   the rationale for the update.  See Section 5.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Implications for Post-Quantum Cryptography (PQC)

   Cryptographically Relevant Quantum Computers (CRQCs), once available,
   will have a huge impact on TLS traffic (see, e.g., Section 3 of
   [RFC9958]).  To mitigate this, TLS applications will need to migrate
   to Post-Quantum Cryptography (PQC) [PQC].  Detailed considerations of
   when an application requires PQC or when a CRQC is a threat that an
   application needs to protect against are beyond the scope of this
   document.

   It is important to note that the TLS Working Group is focusing its
   efforts on TLS 1.3 or later; TLS 1.2 will not be supported (see
   [TLS12FROZEN]).  This is one more reason for new protocols to require
   TLS to default to TLS 1.3, where PQC is actively being standardized,
   as this gives new applications the option to use PQC.

4.  TLS Use by Other Protocols and Applications

   Any new protocol that uses TLS MUST specify TLS 1.3 as its default.
   For example, QUIC [QUICTLS] requires TLS 1.3 and specifies that
   endpoints MUST terminate the connection if an older version is used.

   If deployment considerations are a concern, the protocol MAY specify
   TLS 1.2 as an additional, non-default option.  As a counter example,
   the Usage Profile for DNS over TLS [DNSTLS] specifies TLS 1.2 as the
   default, while also allowing TLS 1.3.  For newer specifications that
   choose to support TLS 1.2, those preferences are to be reversed.

   The initial TLS handshake allows a client to specify which versions
   of TLS it supports, and the server is intended to pick the highest
   version that it also supports.  This is known as "TLS version
   negotiation"; protocol and negotiation details are discussed in
   Section 4.2.1 of [TLS13] and Appendix E of [TLS12].  Many TLS
   libraries provide a way for applications to specify the range of
   versions they want, including an open interval where only the lowest
   or highest version is specified.

   If the application is using a TLS implementation that supports TLS
   version negotiation and if it knows that the TLS implementation will
   use the highest version supported, then clients SHOULD specify just
   the minimum version they want.  This MUST be TLS 1.3 or TLS 1.2,
   depending on the circumstances described in the above paragraphs.

5.  Changes to RFC 9325

   [RFC9325] provides recommendations for ensuring the security of
   deployed services that use TLS and, unlike this document, DTLS as
   well.  [RFC9325] describes TLS 1.3 as "widely available", and the
   transition to TLS 1.3 has further increased since publication of that
   document.  This document thus makes two changes to the
   recommendations in Section 3.1.1 of [RFC9325]:

   *  That section says that TLS 1.3 SHOULD be supported; this document
      mandates that TLS 1.3 MUST be supported for new protocols using
      TLS.

   *  That section says that TLS 1.2 MUST be supported; this document
      says that TLS 1.2 MAY be supported as described above.

   Again, these changes only apply to TLS, and not DTLS.

6.  Security Considerations

   TLS 1.2 was specified with several cryptographic primitives and
   design choices that have, over time, become significantly weaker.
   The purpose of this section is to briefly survey several such
   prominent problems that have affected the protocol.  It should be
   noted, however, that TLS 1.2 can be configured securely; it is merely
   much more difficult to configure it securely as opposed to using its
   modern successor, TLS 1.3.  See [RFC9325] for a more thorough guide
   on the secure deployment of TLS 1.2.

   First, without any extensions, TLS 1.2 is vulnerable to renegotiation
   attacks (see [RENEG1] and [RENEG2]) and the Triple Handshake attack
   (see [TRIPLESHAKE]).  Broadly, these attacks exploit the protocol's
   support for renegotiation in order to inject a prefix chosen by the
   attacker into the plaintext stream.  This is usually a devastating
   threat in practice (e.g., it allows an attacker to obtain secret
   cookies in a web setting).  In light of the above problems, [RFC5746]
   specifies an extension that prevents this category of attacks.  To
   securely deploy TLS 1.2, either renegotiation must be disabled
   entirely, or this extension must be used.  Additionally, clients must
   not allow servers to renegotiate the certificate during a connection.

   Second, the original key exchange methods specified for TLS 1.2,
   namely RSA key exchange and finite field Diffie-Hellman, suffer from
   several weaknesses.  To securely deploy the protocol, most of these
   key exchange methods must be disabled.  See [RFC10015] for details.

   Third, symmetric ciphers that are widely used in TLS 1.2, namely RC4
   and Cipher Block Chaining (CBC) cipher suites, suffer from several
   weaknesses.  RC4 suffers from exploitable biases in its key stream;
   see [RFC7465].  CBC cipher suites have been a source of
   vulnerabilities throughout the years.  A straightforward
   implementation of these cipher suites inherently suffers from the
   Lucky13 timing attack [LUCKY13].  The first attempt to implement the
   cipher suites in constant time introduced an even more severe
   vulnerability [LUCKY13FIX].  Refer to [CBCSCANNING] for another
   example of a vulnerability with CBC cipher suites and a survey of
   similar works.

   In addition, TLS 1.2 was affected by several other attacks that TLS
   1.3 is immune to: BEAST [BEAST], Logjam [WEAKDH], FREAK [FREAK], and
   SLOTH [SLOTH].

   Finally, while application-layer traffic in TLS 1.2 is always
   encrypted, most of the content of the handshake messages is not.
   Therefore, the privacy provided is suboptimal.  This is a protocol
   issue that cannot be addressed by configuration.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9325]  Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [TLS12]    Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [TLS12FROZEN]
              Salz, R. and N. Aviram, "TLS 1.2 is in Feature Freeze",
              RFC 9851, DOI 10.17487/RFC9851, July 2026,
              <https://www.rfc-editor.org/info/rfc9851>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 9846, DOI 10.17487/RFC9846, July 2026,
              <https://www.rfc-editor.org/info/rfc9846>.

8.2.  Informative References

   [BEAST]    Duong, T. and J. Rizzo, "Here Come the XOR Ninjas", May
              2011, <http://www.hpcc.ecs.soton.ac.uk/dan/talks/bullrun/
              Beast.pdf>.

   [CBCSCANNING]
              Merget, R., Somorovsky, J., Aviram, N., Young, C.,
              Fliegenschmidt, J., Schwenk, J., and Y. Shavitt, "Scalable
              Scanning and Automatic Classification of TLS Padding
              Oracle Vulnerabilities", 28th USENIX Security Symposium
              (USENIX Security 19), August 2019,
              <https://www.usenix.org/system/files/sec19-merget.pdf>.

   [DNSTLS]   Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018,
              <https://www.rfc-editor.org/info/rfc8310>.

   [FREAK]    Beurdouche, B., Bhargavan, K., Delignat-Lavaud, A.,
              Fournet, C., Kohlweiss, M., Pironti, A., Strub, P.-Y., and
              J. K. Zinzindohoue, "A Messy State of the Union: Taming
              the Composite State Machines of TLS", IEEE Symposium on
              Security & Privacy 2015, HAL ID: hal-01114250, May 2015,
              <https://inria.hal.science/hal-01114250/file/messy-state-
              of-the-union-oakland15.pdf>.

   [LUCKY13]  Al Fardan, N. J. and K. G. Paterson, "Lucky Thirteen:
              Breaking the TLS and DTLS record protocols", February
              2013, <http://www.isg.rhul.ac.uk/tls/TLStiming.pdf>.

   [LUCKY13FIX]
              Somorovsky, J., "Systematic Fuzzing and Testing of TLS
              Libraries", CCS '16: Proceedings of the 2016 ACM SIGSAC
              Conference on Computer and Communications Security, pp.
              1492-1504, DOI 10.1145/2976749.2978411, October 2016,
              <https://nds.rub.de/media/nds/
              veroeffentlichungen/2016/10/19/tls-attacker-ccs16.pdf>.

   [PQC]      NIST, "What Is Post-Quantum Cryptography?", June 2025,
              <https://www.nist.gov/cybersecurity/what-post-quantum-
              cryptography>.

   [QUICTLS]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/info/rfc9001>.

   [RENEG1]   Rescorla, E., "Understanding the TLS Renegotiation
              Attack", Wayback Machine archive, 5 November 2009,
              <https://web.archive.org/web/20091231034700/
              http://www.educatedguesswork.org/2009/11/
              understanding_the_tls_renegoti.html>.

   [RENEG2]   Ray, M., "Authentication Gap in TLS Renegotiation",
              Wayback Machine archive,
              <https://web.archive.org/web/20091228061844/
              http://extendedsubset.com/?p=8>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <https://www.rfc-editor.org/info/rfc5746>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <https://www.rfc-editor.org/info/rfc7465>.

   [RFC9958]  Banerjee, A., Reddy.K, T., Schoinianakis, D., Hollebeek,
              T., and M. Ounsworth, "Post-Quantum Cryptography for
              Engineers", RFC 9958, DOI 10.17487/RFC9958, June 2026,
              <https://www.rfc-editor.org/info/rfc9958>.

   [RFC10015] Aviram, N., "Deprecating Obsolete Key Exchange Methods in
              TLS 1.2 and DTLS 1.2", RFC 10015, DOI 10.17487/RFC10015,
              July 2026, <https://www.rfc-editor.org/info/rfc10015>.

   [SLOTH]    Bhargavan, K. and G. Leurent, "Transcript Collision
              Attacks: Breaking Authentication in TLS, IKE, and SSH",
              Network and Distributed System Security Symposium - NDSS
              2016, DOI 10.14722/ndss.2016.23418, HAL ID: hal-01244855,
              February 2016, <https://inria.hal.science/hal-
              01244855/file/SLOTH_NDSS16.pdf>.

   [TRIPLESHAKE]
              "Triple Handshakes Considered Harmful: Breaking and Fixing
              Authentication over TLS", Wayback Machine archive,
              <https://web.archive.org/web/20250804151857/
              https://mitls.org/pages/attacks/3SHAKE>.

   [WEAKDH]   Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P.,
              Green, M., Halderman, J. A., Heninger, N., Springall, D.,
              Thome, E., Valenta, L., VanderSloot, B., Wustrow, E.,
              Zanella-Beguelin, S., and P. Zimmerman, "Imperfect Forward
              Secrecy: How Diffie-Hellman Fails in Practice", CCS '15:
              Proceedings of the 22nd ACM SIGSAC Conference on Computer
              and Communications Security, pp. 5-17,
              DOI 10.1145/2810103.2813707, October 2015,
              <https://dl.acm.org/doi/pdf/10.1145/2810103.2813707>.

Authors' Addresses

   Rich Salz
   Akamai Technologies
   Email: rsalz@akamai.com


   Nimrod Aviram
   Email: nimrod.aviram@gmail.com