DNS Certification Authority Authorization

DNS Certification Authority Authorization (CAA) is an Internet security policy mechanism which allows domain name holders to indicate to certificate authorities whether they are authorized to issue digital certificates for a particular domain name. It does this by means of a new "CAA" Domain Name System (DNS) resource record.

It was drafted by computer scientists Phillip Hallam-Baker and Rob Stradling in response to increasing concerns about the security of publicly trusted certificate authorities. It is an Internet Engineering Task Force (IETF) proposed standard.

Background

A series of incorrectly issued certificates from 2001 onwards[1][2] damaged trust in publicly trusted certificate authorities,[3] and accelerated work on various security mechanisms, including Certificate Transparency to track mis-issuance, HTTP Public Key Pinning and DANE to block mis-issued certificates on the client-side, and CAA to block mis-issuance on the certificate authority side.[4]

The first draft of CAA was written by Phillip Hallam-Baker and Rob Stradling, and submitted as an IETF Internet Draft in October 2010.[5] This was progressively improved by the PKIX Working Group,[6] and submitted to the IESG as RFC 6844, a Proposed Standard, in January 2013.[7] CA/Browser Forum discussion began shortly afterwards,[4] and in March 2017 they voted in favor of making CAA implementation mandatory for all certificate authorities by September 2017.[8][9] At least one certificate authority, Comodo, failed to implement CAA before the deadline.[10] A 2017 study by the Technical University of Munich found many instances where certificate authorities failed to correctly implement some part of the standard.[4]

As of June 2018, Qualys reports that still only 3.4% of the 150,000 most popular TLS-supporting websites use CAA records.[11]

Record

Certificate authorities implementing CAA perform a DNS lookup for CAA resource records, and if any are found, ensure that they are listed as an authorized party before issuing a digital certificate.[7] Each CAA resource record consists of the following components, separated by whitespace:[7]

flag 
A flags byte which implements an extensible signaling system for future use. As of 2018, only the issuer critical flag has been defined, which instructs certificate authorities that they must understand the corresponding property tag before issuing a certificate.[7] This flag allows the protocol to be extended in the future with mandatory extensions,[4] similar to critical extensions in X.509 certificates.
tag 
One of the following property:
issue 
This property authorizes the holder of the domain specified in associated property value to issue certificates for the domain for which the property is published.
issuewild 
This property acts like issue but only authorizes issuance of wildcard certificates, and takes precedence over the issue property for wildcard certificate requests.
iodef 
This property specifies a method for certificate authorities to report invalid certificate requests to the domain name holder using the Incident Object Description Exchange Format. As of 2018, not all certificate authorities support this tag, so there is no guarantee that all certificate issuances will be reported.
value
The value associated with the chosen property tag.

The lack of any CAA records authorizes normal unrestricted issuance, and the presence of a single blank issue tag disallows all issuance.[7][9][12]

Third parties monitoring certificate authority behavior might check newly issued certificates against the domain's CAA records, but must be aware that a domain's CAA records may have changed between the time the certificate was issued and the time the third-party checks them. Clients should not use CAA as part of their certificate validation process.[7]

Extensions

A draft of the first extension to the CAA standard was published on October 26, 2016, adding a new account-uri token to the end of the issue property, which ties a domain to a specific Automated Certificate Management Environment account.[13] This was amended on August 30, 2017, to also include a new validation-methods token, which ties a domain to a specific validation method.[14]

Examples

To indicate that only the certificate authority identified by ca.example.net is authorised to issue certificates for example.com and all subdomains, one may use this CAA record:[7]

example.com.  IN  CAA 0 issue "ca.example.net"

To disallow any certificate issuance, one may allow issuance only to an empty issuer list:

example.com.  IN  CAA  0 issue ";"

To indicate that certificate authorities should report invalid certificate requests to an email address and a Real-time Inter-network Defense endpoint:

example.com.  IN  CAA 0 iodef "mailto:security@example.com"
example.com.  IN  CAA 0 iodef "http://iodef.example.com/"

To use a future extension of the protocol, for example one which defines a new future property, which needs to be understood by the certificate authority before they can safely proceed, one may set the issuer critical flag:

example.com.  IN  CAA  0 issue "ca.example.net"
example.com.  IN  CAA  128 future "value"

See also

References

  1. ↑ Ristić, Ivan. "SSL/TLS and PKI History". Feisty Duck. Retrieved June 8, 2018.
  2. ↑ Bright, Peter (August 30, 2011). "Another fraudulent certificate raises the same old questions about certificate authorities". Ars Technica. Retrieved February 10, 2018.
  3. ↑ Ruohonen, Jukka (April 20, 2018). "An Empirical Survey on the Early Adoption of DNS Certification Authority Authorization". arXiv:1804.07604 [cs.CR].
  4. 1 2 3 4 Scheitle, Quirin; Chung, Taejoong; et al. (April 2018). "A First Look at Certification Authority Authorization (CAA)" (PDF). ACM SIGCOMM Computer Communication Review. 48 (2): 10–23. doi:10.1145/3213232.3213235. ISSN 0146-4833.
  5. ↑ Hallam-Baker, Phillip; Stradling, Rob (October 18, 2010). DNS Certification Authority Authorization (CAA) Resource Record. IETF. I-D draft-hallambaker-donotissue-00. https://tools.ietf.org/html/draft-hallambaker-donotissue-00.
  6. ↑ Hallam-Baker, Phillip; Stradling, Rob; Ben, Laurie (June 2, 2011). DNS Certification Authority Authorization (CAA) Resource Record. IETF. I-D draft-ietf-pkix-caa-00. https://tools.ietf.org/html/draft-ietf-pkix-caa-00.
  7. 1 2 3 4 5 6 7 Hallam-Baker, Phillip; Stradling, Rob (January 2013). DNS Certification Authority Authorization (CAA) Resource Record. IETF. doi:10.17487/RFC6844. ISSN 2070-1721. RFC 6844. https://tools.ietf.org/html/rfc6844.
  8. ↑ Hall, Kirk (March 8, 2017). "Results on Ballot 187 - Make CAA Checking Mandatory". CA/Browser Forum. Retrieved January 7, 2018.
  9. 1 2 Beattie, Doug (August 22, 2017). "What is CAA (Certificate Authority Authorization)?". GlobalSign. Retrieved February 2, 2018.
  10. ↑ Cimpanu, Catalin (September 11, 2017). "Comodo Caught Breaking New CAA Standard One Day After It Went Into Effect". Bleeping Computer. Retrieved January 8, 2018.
  11. ↑ "SSL Pulse". SSL Labs. Qualys. June 3, 2018. Retrieved June 9, 2018.
  12. ↑ "What is Certificate Authority Authorization (CAA)?". Symantec. Retrieved January 8, 2018.
  13. ↑ Landau, Hugo (October 26, 2016). CAA Record Extensions for Account URI and ACME Method Binding. IETF. I-D draft-ietf-acme-caa-00. https://tools.ietf.org/html/draft-ietf-acme-caa-00.
  14. ↑ Landau, Hugo (August 30, 2017). CAA Record Extensions for Account URI and ACME Method Binding. IETF. I-D draft-ietf-acme-caa-04. https://tools.ietf.org/html/draft-ietf-acme-caa-04.
  • RFC 6844
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