RFC 9905 PROPOSED STANDARD DNS · SECURITY · CRYPTOGRAPHY
Deprecating the Use of SHA-1 in DNSSEC Signature Algorithms
Declares SHA-1 cryptography unsafe for DNS security signing.
RFC EDITOR
Hardaker, W. and W. Kumari, "Deprecating the Use of SHA-1 in DNSSEC Signature Algorithms", RFC 9905, DOI 10.17487/RFC9905, November 2025, <https://www.rfc-editor.org/info/rfc9905>.
BIBTEX
@misc{rfc9905,
series = {Request for Comments},
number = 9905,
howpublished = {RFC 9905},
publisher = {RFC Editor},
doi = {10.17487/RFC9905},
url = {https://www.rfc-editor.org/info/rfc9905},
author = {W. Hardaker and W. Kumari},
title = {{Deprecating the Use of SHA-1 in DNSSEC Signature Algorithms}},
pagetotal = 5,
year = 2025,
month = nov,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) W. Hardaker
Request for Comments: 9905 USC/ISI
Updates: 4034, 5155 W. Kumari
Category: Standards Track Google
ISSN: 2070-1721 November 2025
Deprecating the Use of SHA-1 in DNSSEC Signature Algorithms
Abstract
This document deprecates the use of the RSASHA1 and
RSASHA1-NSEC3-SHA1 algorithms for the creation of DNS Public Key
(DNSKEY) and Resource Record Signature (RRSIG) records.
It updates RFCs 4034 and 5155 as it deprecates the use of these
algorithms.
Status of This Memo
This is an Internet Standards Track document.
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
Internet Standards 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/rfc9905.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
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
The security of the protection provided by the SHA-1 algorithm
[RFC3174] has been slowly diminishing over time as various forms of
attacks have weakened its cryptographic underpinning. DNSSEC
[RFC9364] (originally defined in [RFC3110]) made extensive use of
SHA-1, for example, as a cryptographic hash algorithm in Resource
Record Signature (RRSIG) and Delegation Signer (DS) records. Since
then, multiple 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
consider switching to one of the recommended algorithms listed in the
"DNS Security Algorithm Numbers" [DNSKEY-IANA] and "DNS Security
Algorithm Numbers" [DS-IANA] registries, respectively. Further,
support for validating SHA-1-based signatures has been removed from
some systems. As a result, SHA-1 as part of a signature algorithm is
no longer fully interoperable in the context of DNSSEC. As adequate
alternatives exist, the use of SHA-1 is no longer advisable.
This document thus deprecates the use of RSASHA1 and
RSASHA1-NSEC3-SHA1 for DNS Security Algorithms.
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
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 no other DS records of accepted cryptographic
algorithms are available, 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 as they are diminishing in
use but still actively in use for some domains as of this
publication. Operators of validating resolvers MUST treat DNSSEC
signing algorithms RSASHA1 and RSASHA1-NSEC3-SHA1 as unsupported,
rendering responses insecure if they cannot be validated by other
supported signing algorithms.
3. Security Considerations
This document deprecates the use of RSASHA1 and RSASHA1-NSEC3-SHA1
for DNSSEC delegation and DNSSEC signing since these algorithms are
no longer considered to be secure.
4. Operational Considerations
Zone owners currently making use of SHA-1-based algorithms should
immediately switch to algorithms with stronger cryptographic
algorithms, such as the recommended algorithms in the IANA registries
[DNSKEY-IANA] [DS-IANA].
Operators should take care when deploying software packages and
operating systems that may have already removed support for the SHA-1
algorithm. In these situations, software may need to be manually
built and deployed by an operator to continue supporting the required
levels indicated by the "Use for DNSSEC Validation" and "Implement
for DNSSEC Validation" columns, which this document is not changing.
5. IANA Considerations
IANA has updated the SHA-1 (1) entry in the "Digest Algorithms"
registry [DS-IANA] [RFC9904] as follows and has 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 updated the RSASHA1 (5) and RSASHA1-NSEC3-SHA1 (7) algorithm
entries in the "DNS Security Algorithm Numbers" registry
[DNSKEY-IANA] [RFC9904] as follows and has added this document as a
reference for the 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
The authors appreciate the comments and suggestions from the
following IETF participants in helping produce this document: 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 that
discussed this specification.
Authors' Addresses
Wes Hardaker
USC/ISI
Email: ietf@hardakers.net
Warren Kumari
Google
Email: warren@kumari.net
AI ELI5
On this page
PROBLEMSHA-1 is an old cryptographic hash function that has been weakened by attacks over decades. DNSSEC relies on SHA-1 (RSASHA1 and RSASHA1-NSEC3-SHA1) to sign DNS records, but these algorithms are no longer secure enough.
MECHANISMRFC 9905 deprecates the use of RSASHA1 and RSASHA1-NSEC3-SHA1 for creating new DNSKEY and RRSIG records. Resolvers must treat such signatures as insecure and switch to stronger algorithms.
CRYPTOGRAPHIC DEPRECATIONDNS operators are encouraged to migrate to RSASHA256, ECDSAP256SHA256 or other stronger algorithms. This prevents reliance on weakened cryptography.
§1Why SHA-1 Must Go
DNSSEC signs DNS records to prove they have not been tampered with. For decades it used SHA-1, a cryptographic hash function, to create those signatures. SHA-1 was adequate when DNSSEC was designed, but modern cryptanalysis has weakened it so much that signatures using it are no longer trustworthy.
RFC 9905 declares that RSASHA1 and RSASHA1-NSEC3-SHA1 must not be used for signing anymore. Some systems have already removed support for SHA-1, making signed zones unreadable without a workaround.
DNS used an old lock that is no longer strong enough. This says stop using it.
§2What Is Deprecated
The RFC forbids two DNSSEC algorithms: RSASHA1 (algorithm number 5) and RSASHA1-NSEC3-SHA1 (algorithm number 7). Both MUST NOT be used when creating new DNSKEY records (the zone's signing keys) or RRSIG records (the signatures on resource records). The rule also applies to DS records (delegation signer records that tie a child zone to its parent).
Resolvers validating DNS responses must treat RSASHA1 and RSASHA1-NSEC3-SHA1 signatures and DS records as insecure. If no stronger signature is available, the entire zone below the delegation point must be treated as insecure.
Zones cannot use SHA-1 signatures anymore, and validators will not trust them if they do.
§3The Transition
Zone operators currently using SHA-1 must immediately switch to stronger algorithms. The IANA registries list recommended alternatives such as RSASHA256 (algorithm 8) and ECDSAP256SHA256 (algorithm 13).
Operators must be careful with software that may have already dropped SHA-1 support. Manually building and deploying systems may be necessary to maintain backward compatibility during the transition, but new zones must use only the recommended algorithms.
DNS administrators need to switch their signatures to newer, stronger methods right away.
§4References
[RFC4034]RFC 4034Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005.March 2005[RFC5155]RFC 5155Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS Security (DNSSEC) Hashed Authenticated Denial of Existence", RFC 5155, March 2008.March 2008[RFC3110]RFC 3110Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)", RFC 3110, May 2001.May 2001