Implications of Common Name deprecation for Dogtag and FreeIPA

Or, ERR_CERT_COMMON_NAME_INVALID, and what we are doing about it.

Google Chrome version 58, released in April 2017, removed support for the X.509 certificate Subject Common Name (CN) as a source of naming information when validating certificates. As a result, certificates that do not carry all relevant domain names in the Subject Alternative Name (SAN) extension result in validation failures.

At the time of writing this post Chrome is just the first mover, but Mozilla Firefox and other programs and libraries will follow suit. The public PKI used to secure the web and other internet communiations is largely unaffected (browsers and CAs moved a long time ago to ensure that certificates issued by publicly trusted CAs carried all DNS naming information in the SAN extension), but some enterprises running internal PKIs are feeling the pain.

In this post I will provide some historical and technical context to the situation, and explain what we are are doing in Dogtag and FreeIPA to ensure that we issue valid certificates.

Background

X.509 certificates carry subject naming information in two places: the Subject Distinguished Name (DN) field, and the Subject Alternative Name extension. There are many types of attributes available in the DN, including organisation, country, and common name. The definitions of these attribute types came from X.500 (the precursor to LDAP) and all have an ASN.1 representation.

Within the X.509 standard, the CN has no special interpretation, but when certificates first entered widespread use in the SSL protocol, it was used to carry the domain name of the subject site or service. When connecting to a web server using TLS/SSL, the client would check that the CN matches the domain name they used to reach the server. If the certificate is chained to a trusted CA, the signature checks out, and the domain name matches, then the client has confidence that all is well and continues the handshake.

But there were a few problems with using the Common Name. First, what if you want a certificate to support multiple domain names? This was especially a problem for virtual hosts in the pre-SNI days where one IP address could only have one certificate associated with it. You can have multiple CNs in a Distinguished Name, but the semantics of X.500 DNs is strictly heirarichical. It is not an appropriate use of the DN to cram multiple, possibly non-hierarchical domain names into it.

Second, the CN in X.509 has a length limit of 64 characters. DNS names can be longer. The length limit is too restrictive, especially in the world of IaaS and PaaS where hosts and services are spawned and destroyed en masse by orchestration frameworks.

Third, some types of subject names do not have a corresponding X.500 attribute, including domain names. The solution to all three of these problems was the introduction of the Subject Alternative Name X.509 extension, to allow more types of names to be used in a certificate. (The SAN extensions is itself extensible; apart from DNS names other important name types include IP addresses, email addresses, URIs and Kerberos principal names). TLS clients added support for validating SAN DNSName values in addition to the CN.

The use of the CN field to carry DNS names was never a standard. The Common Name field does not have these semantics; but using the CN in this way was an approach that worked. This interpretation was later formalised by the CA/B Forum in their Baseline Requirements for CAs, but only as a reflection of a current practice in SSL/TLS server and client implementations. Even in the Baseline Requirements the CN was a second-class citizen; they mandated that if the CN was present at all, it must reflect one of the DNSName or IP address values from the SAN extension. All public CAs had to comply with this requirement, which is why Chrome’s removal of CN support is only affecting private PKIs, not public web sites.

Why remove CN validation?

So, Common Name was not ideal for carrying DNS naming information, but given that we now have SAN, was it really necessary to deprecate it, and is it really necessary to follow through and actually stop using it, causing non-compliant certificates that were previously accepted to now be rejected?

The most important reason for deprecating CN validation is the X.509 Name Constraints extension. Name Constraints, if they appear in a CA certificate or intermediate CA certificate, constrain the valid subject names on leaf certificates. Various name types are supported including DNS names; a DNS name constraint restricts the domain of validity to the domain(s) listed and subdomains thereof. For example, if the DNS name example.com appears in a CA certificate’s Name Constraints extension, leaf certificates with a DNS name of example.com or foo.example.com could be valid, but a DNS name of foo.example.net could not be valid. Conforming X.509 implementations must enforce these constraints.

But these constraints only apply to SAN DNSName values, not to the CN. This is why accepting DNS naming information in the CN had to be deprecated – the name constraints cannot be properly enforced!

So back in May 2000 the use of Common Name for carrying a DNS name was deprecated by RFC 2818. Although it deprecated the practice this RFC required clients to fall back to the Common Name if there were no SAN DNSName values on the certificate. Then in 2011 RFC 6125 removed the requirement for clients to fall back to the common name, making this optional behaviour. Over recent years, some TLS clients began emitting warnings when they encountered certificates without SAN DNSNames, or where a DNS name in the CN did not also appear in the SAN extension. Finally, Chrome has become the first widely used client to remove support.

Despite more than 15 years notice on the deprecation of this use of Common Name, a lot of CA software and client tooling still does not have first-class support for the SAN extension. Most tools used to generate CSRs do not even ask about SAN, and require complex configuration to generate a request bearing the SAN extension. Similarly, some CA programs does not do a good job of issuing RFC-compliant certificates. Right now, this includes Dogtag and FreeIPA.

Subject Alternative Name and FreeIPA

For some years, FreeIPA (in particular, the default profile for host and service certificates, called caIPAserviceCert) has supported the SAN extension, but the client is required to submit a CSR containing the desired SAN extension data. The names in the CSR (the CN and all alternative names) get validated against the subject principal, and then the CA would issue the certificate with exactly those names. There was no way to ensure that the domain name in the CN was also present in the SAN extension.

We could add this requirement to FreeIPA’s CSR validation routine, but this imposes an unreasonable burden on the user to "get it right". Tools like OpenSSL have poor usability and complex configuration. Certmonger supports generating a CSR with the SAN extension but it must be explicitly requested. For FreeIPA’s own certificates, we have (in recent major releases) ensured that they have contained the SAN extension, but this is not the default behaviour and that is a problem.

FreeIPA 4.5 brought with it a CSR autogeneration feature that, for a given certificate profile, lets the administrator specify how to construct a CSR appropriate for that profile. This reduces the burden on the end user, but they must still opt in to this process.

Subject Alternative Name and Dogtag

Until Dogtag 10.4, there were two ways to produce a certificate with the SAN extension. One was the SubjectAltNameExtDefault profile component, which, for a given profile, supports a fixed number of names, either hard coded or based on particular request attributes (e.g. the CN, the email address of the authenticated user, etc). The other was the UserExtensionDefault which copies a given extension from the CSR to the final certificate verbatim (no validation of the data occurs). We use UserExtensionDefault in FreeIPA’s certificate profile (all names are validated by the FreeIPA framework before the request is submitted to Dogtag).

Unfortunately, SubjectAltNameExtDefault and UserExtensionDefault are not compatible with each other. If a profile uses both and the CSR contains the SAN extension, issuance will fail with an error because Dogtag tried to add two SAN extensions to the certificate.

In Dogtag 10.4 we introduced a new profile component that improves the situation, especially for dealing with the removal of client CN validation. The CommonNameToSANDefault will cause any profile that uses it to examine the Common Name, and if it looks like a DNS name, it will add it to the SAN extension (creating the extension if necessary).

Ultimately, what is needed is a way to define a certificate profile that just makes the right certificate, without placing an undue burden on the client (be it a human user or a software agent). The complexity and burden should rest with Dogtag, for the sake of all users. We are gradually making steps toward this, but it is still a long way off. I have discussed this utopian vision in a previous post.

Configuring CommonNameToSANDefault

If you have Dogtag 10.4, here is how to configure a profile to use the CommonNameToSANDefault. Add the following policy directives (the policyset and serverCertSet and index 12 are indicative only, but the index must not collide with other profile components):

policyset.serverCertSet.12.constraint.class_id=noConstraintImpl
policyset.serverCertSet.12.constraint.name=No Constraint
policyset.serverCertSet.12.default.class_id=commonNameToSANDefaultImpl
policyset.serverCertSet.12.default.name=Copy Common Name to Subject

Add the index to the list of profile policies:

policyset.serverCertSet.list=1,2,3,4,5,6,7,8,9,10,11,12

Then import the modified profile configuration, and you are good to go. There are a few minor caveats to be aware of:

  • Names containing wildcards are not recognised as DNS names. The rationale is twofold; wildcard DNS names, although currently recognised by most programs, are technically a violation of the X.509 specification (RFC 5280), and they are discouraged by RFC 6125. Therefore if the CN contains a wildcard DNS name, CommonNameToSANDefault will not copy it to the SAN extension.
  • Single-label DNS names are not copied. It is unlikely that people will use Dogtag to issue certificates for top-level domains. If CommonNameToSANDefault encounters a single-label DNS name, it will assume it is actually not a DNS name at all, and will not copy it to the SAN extension.
  • The CommonNameToSANDefault policy index must come after UserExtensionDefault, SubjectAltNameExtDefault, or any other component that adds the SAN extension, otherwise an error may occur because the older components do not gracefully handle the situation where the SAN extension is already present.

What we are doing in FreeIPA

Updating FreeIPA profiles to use CommonNameToSANDefault is trickier – FreeIPA configures Dogtag to use LDAP-based profile storage, and mixed-version topologies are possible, so updating a profile to use the new component could break certificate requests on other CA replicas if they are not all at the new versions. We do not want this situation to occur.

The long-term fix is to develop a general, version-aware profile update mechanism that will import the best version of a profile supported by all CA replicas in the topology. I will be starting this effort soon. When it is in place we will be able to safely update the FreeIPA-defined profiles in existing deployments.

In the meantime, we will bump the Dogtag dependency and update the default profile for new installations only in the 4.5.3 point release. This will be safe to do because you can only install replicas at the same or newer versions of FreeIPA, and it will avoid the CN validation problems for all new installations.

Conclusion

In this post we looked at the technical reasons for deprecating and removing support for CN domain validation in X.509 certificates, and discussed the implications of this finally happening, namely: none for the public CA world, but big problems for some private PKIs and programs including FreeIPA and Dogtag. We looked at the new CommonNameToSANDefault component in Dogtag that makes it easier to produce compliant certs even when the tools to generate the CSR don’t help you much, and discussed upcoming and proposed changes in FreeIPA to improve the situation there.

One big takeaway from this is to be more proactive in dealing with deprecated features in standards, APIs or programs. It is easy to punt on the work, saying "well yes it is deprecated but all the programs still support it…" The thing is, tomorrow they may not support it anymore, and when it was deprecated for good reasons you really cannot lay the blame at Google (or whoever). On the FreeIPA team we (and especially me as PKI wonk in residence) were aware of these issues but kept putting off the work. Then one day users and customers start having problems accessing their internal services in Chrome! 15 years should have been enough time to deal with it… but we (I) did not.

Lesson learned.

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