wdiff rfc9852v1.txt rfc9852.txt

Internet Engineering Task Force (IETF) R. Salz
Request for Comments: 9852 Akamai Technologies
BCP: 195 N. Aviram
Updates: 9325 January 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 over 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 over in TLS 1.2 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.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

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, since TLS 1.3 use is widespread, that new protocols that use TLS must require and assume
that TLS 1.3 is available and require its existence. It
updates [RFC9325] as described in Section 5. 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 fixed weaknesses security and privacy improvements in TLS 1.2 1.3 as a
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 2 of
[PQC-FOR-ENGINEERS]). 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.

For TLS, it

It is important to note that the focus of these efforts
within 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 this 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 availability of TLS 1.3 as "widely
available" at available", and the time of its publication. The
transition and
adoption mentioned in that document to TLS 1.3 has grown, and this further increased since publication of that
document. This document now 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 that allows, e.g., obtaining (e.g., it allows an attacker to obtain secret
cookies in a web setting. 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 the protocol, 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 [KEY-EXCHANGE] for
details.

Third, symmetric ciphers that are widely used in the protocol, 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]. There have been further similar
vulnerabilities throughout the years exploiting CBC cipher suites;
refer Refer to [CBCSCANNING] for an 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 are 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, January 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, January
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>.

[KEY-EXCHANGE]
Aviram, N., "Deprecating Obsolete Key Exchange Methods in
(D)TLS 1.2", Work in Progress, Internet-Draft, draft-ietf-
tls-deprecate-obsolete-kex-07, 13 November 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
deprecate-obsolete-kex-07>.

[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>.

[PQC-FOR-ENGINEERS]
Banerjee, A., Reddy, T., Schoinianakis, D., Hollebeek, T.,
and M. Ounsworth, "Post-Quantum Cryptography for
Engineers", Work in Progress, Internet-Draft, draft-ietf-
pquip-pqc-engineers-14, 25 August 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-pquip-
pqc-engineers-14>.

[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>.

[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