RFC 9905

Deprecating the Use of SHA-1 in DNSSEC Signature Algorithms

📅 published November 2025, from ietf

Deprecating the Use of SHA-1 in DNSSEC Signature Algorithms 💖

Okay bestie, this is RFC 9905 🎀 It's a Standards Track doc from the IETF, dated November 2025, and it's published under ISSN 2070-1721 🧚 Wes Hardaker (USC/ISI) and Warren Kumari (Google) wrote it. It updates RFC 4034 and RFC 5155, and its header doesn't list any RFCs it obsoletes. Let's get into it 🦋

Abstract 🌸

This doc deprecates the use of the RSASHA1 and RSASHA1-NSEC3-SHA1 algorithms for creating DNS Public Key (DNSKEY) and Resource Record Signature (RRSIG) records.

It updates RFCs 4034 and 5155 because it deprecates these algorithms. 💅

Status of This Memo ✨

This is an Internet Standards Track document.

It's a product of the Internet Engineering Task Force (IETF), so it represents the consensus of the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). For more on Internet Standards, check Section 2 of RFC 7841.

For the current status of this document, any errata, and how to send feedback, go to https://www.rfc-editor.org/info/rfc9905.

Copyright Notice 🎀

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as in effect on the date of publication. Please read these carefully, because they describe your rights and restrictions for 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 they're provided without warranty as described in the Revised BSD License.

Table of Contents 🧚

1.  Introduction
  1.1.  Requirements Notation
2.  Deprecating SHA-1 from DNSSEC Signatures and Delegation RRs
3.  Security Considerations
4.  Operational Considerations
5.  IANA Considerations
6.  Normative References
Acknowledgments
Authors' Addresses

1. Introduction 🌈

okay so the SHA-1 algorithm [RFC3174] has been getting weaker over time as different attacks chip away at its cryptographic foundations 💀 DNSSEC [RFC9364] (first defined in [RFC3110]) leaned on SHA-1 a LOT. For example, it was the hash in Resource Record Signature (RRSIG) and Delegation Signer (DS) records. Since then, a bunch of other algorithms with stronger cryptographic strength have become widely available for DS records and for RRSIG and DNS Public Key (DNSKEY) records [RFC4034]. Operators are encouraged to switch to one of the recommended algorithms in the "DNS Security Algorithm Numbers" [DNSKEY-IANA] and "DNS Security Algorithm Numbers" [DS-IANA] registries, respectively.

It's also giving "not supported anymore" because some systems have already removed support for validating SHA-1-based signatures. So SHA-1 as part of a signature algorithm is no longer fully interoperable in DNSSEC. Since good alternatives exist, using SHA-1 is no longer advisable 🙈

So this document deprecates RSASHA1 and RSASHA1-NSEC3-SHA1 for DNS Security Algorithms. Bye, SHA-1 👋

1.1. Requirements Notation 🦋

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.

2. Deprecating SHA-1 from DNSSEC Signatures and Delegation RRs 💅

This is the main event, so pay attention 👑

The RSASHA1 [RFC4034] and RSASHA1-NSEC3-SHA1 [RFC5155] algorithms MUST NOT be used when creating DS records. Operators of validating resolvers MUST treat RSASHA1 and RSASHA1-NSEC3-SHA1 DS records as insecure. If there are no other DS records using accepted cryptographic algorithms, the DNS records below the delegation point MUST be treated as insecure.

The RSASHA1 [RFC4034] and RSASHA1-NSEC3-SHA1 [RFC5155] algorithms MUST NOT be used when creating DNSKEY and RRSIG records. Validating resolver implementations ([RFC9499], Section 10) MUST continue to support validation using these algorithms, because they're diminishing in use but still actively used for some domains as of this publication 🫶 Operators of validating resolvers MUST treat the DNSSEC signing algorithms RSASHA1 and RSASHA1-NSEC3-SHA1 as unsupported. That means responses get marked insecure if they can't be validated by other supported signing algorithms.

3. Security Considerations 🔥

This doc deprecates RSASHA1 and RSASHA1-NSEC3-SHA1 for DNSSEC delegation and DNSSEC signing, since these algorithms are no longer considered secure. This is the whole reason we're doing it, so take it seriously 😭

4. Operational Considerations 🍓

Zone owners who are still using SHA-1-based algorithms should immediately switch to algorithms with stronger cryptography, like the recommended ones in the IANA registries [DNSKEY-IANA] [DS-IANA]. No cap, do it now.

Operators should be careful when deploying software packages and operating systems that may already have removed SHA-1 support. In those situations, software may need to be built and deployed manually by an operator to keep supporting the required levels shown in the "Use for DNSSEC Validation" and "Implement for DNSSEC Validation" columns. This document doesn't change those columns 🧚

5. IANA Considerations 🌸

IANA has updated the SHA-1 (1) entry in the "Digest Algorithms" registry [DS-IANA] [RFC9904] like this, and it added this document as a reference for the entry:

Value:  1
Description:  SHA-1
Use for DNSSEC Delegation:  MUST NOT
Use for DNSSEC Validation:  RECOMMENDED
Implement for DNSSEC Delegation:  MUST NOT
Implement for DNSSEC Validation:  MUST

IANA has also updated the RSASHA1 (5) and RSASHA1-NSEC3-SHA1 (7) algorithm entries in the "DNS Security Algorithm Numbers" registry [DNSKEY-IANA] [RFC9904] like this, and it added this document as a reference for both entries:

Number:  5
Description:  RSA/SHA-1
Mnemonic:  RSASHA1
Zone Signing:  Y
Trans.  Sec.:  Y
Use for DNSSEC Signing:  MUST NOT
Use for DNSSEC Validation:  RECOMMENDED
Implement for DNSSEC Signing:  NOT RECOMMENDED
Implement for DNSSEC Validation:  MUST

Number:  7
Description:  RSASHA1-NSEC3-SHA1
Mnemonic:  RSASHA1-NSEC3-SHA1
Zone Signing:  Y
Trans.  Sec.:  Y
Use for DNSSEC Signing:  MUST NOT
Use for DNSSEC Validation:  RECOMMENDED
Implement for DNSSEC Signing:  NOT RECOMMENDED
Implement for DNSSEC Validation:  MUST

6. Normative References 🦋

[DNSKEY-IANA]
           IANA, "DNS Security Algorithm Numbers",
           <https://www.iana.org/assignments/dns-sec-alg-numbers>.

[DS-IANA]  IANA, "Digest Algorithms",
           <http://www.iana.org/assignments/ds-rr-types>.

[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>.

[RFC3110]  Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
           Domain Name System (DNS)", RFC 3110, DOI 10.17487/RFC3110,
           May 2001, <https://www.rfc-editor.org/info/rfc3110>.

[RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
           (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
           <https://www.rfc-editor.org/info/rfc3174>.

[RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
           Rose, "Resource Records for the DNS Security Extensions",
           RFC 4034, DOI 10.17487/RFC4034, March 2005,
           <https://www.rfc-editor.org/info/rfc4034>.

[RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
           Security (DNSSEC) Hashed Authenticated Denial of
           Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
           <https://www.rfc-editor.org/info/rfc5155>.

[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>.

[RFC9364]  Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
           RFC 9364, DOI 10.17487/RFC9364, February 2023,
           <https://www.rfc-editor.org/info/rfc9364>.

[RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
           RFC 9499, DOI 10.17487/RFC9499, March 2024,
           <https://www.rfc-editor.org/info/rfc9499>.

[RFC9904]  Hardaker, W. and W. Kumari, "DNSSEC Cryptographic
           Algorithm Recommendation Update Process", RFC 9904,
           DOI 10.17487/RFC9904, November 2025,
           <https://www.rfc-editor.org/info/rfc9904>.

Acknowledgments 🫶

Big thank-yous to the IETF folks who helped make this doc happen with comments and suggestions: Mark Andrews, Steve Crocker, Peter Dickson, Thomas Graf, Paul Hoffman, Russ Housley, Shumon Huque, Barry Leiba, S. Moonesamy, Yoav Nir, Florian Obser, Peter Thomassen, Stefan Ubbink, Paul Wouters, Tim Wicinski, and the many members of the DNSOP Working Group who discussed this spec 💜

Authors' Addresses 🧚

Wes Hardaker USC/ISI Email: ietf@hardakers.net

Warren Kumari Google Email: warren@kumari.net

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