A series of incorrectly issued certificates from 2001 onwards damaged trust in publicly trusted certificate authorities, 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. The first draft of CAA was written by Phillip Hallam-Baker and Rob Stradling, and submitted as an IETF Internet Draft in October 2010. This was progressively improved by the PKIX Working Group, and approved by the IESG as, a Proposed Standard, in January 2013. CA/Browser Forum discussion began shortly afterward, and in March 2017 they voted in favor of making CAA implementation mandatory for all certificate authorities by September 2017. At least one certificate authority, Comodo, failed to implement CAA before the deadline. A 2017 study by the Technical University of Munich found many instances where certificate authorities failed to correctly implement some part of the standard. In September 2017, Jacob Hoffman-Andrews submitted an Internet Draft intended to simplify the CAA standard. This was improved by the LAMPS Working Group, and approved as, a Proposed Standard, in November 2019. , Qualys reports that still, only 6.8% of the 150,000 most popular TLS-supporting websites use CAA records. A bug in adherence to CAA led to Let's Encrypt being forced to rescind millions of certificates on March 3, 2020.
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. Each CAA resource record consists of the following components: ; flag : A flags byte which implements an extensible signaling system for future use., only the issuer critical flag has been defined, which instructs certificate authorities that they must understand the corresponding property tag before issuing a certificate. This flag allows the protocol to be extended in the future with mandatory extensions, similar to critical extensions in X.509 certificates. ; tag :One of the following property: ; 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. 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 must not use CAA as part of their certificate validation process.
Extensions
A draft of the first extension to the CAA standard was published on October 26, 2016, proposing a new account-uri token to the end of the issue property, which ties a domain to a specific Automated Certificate Management Environment account. This was amended on August 30, 2017, to also include a new validation-methods token, which ties a domain to a specific validation method, and then further amended on June 21, 2018 to remove the hyphen in account-uri and validation-methods making them instead accounturi and validationmethods.
Examples
To indicate that only the certificate authority identified by ca.example.net is authorized to issue certificates for example.com and all subdomains, one may use this CAA record: 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"
