CA/Required or Recommended Practices

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This page contains a set of practices for CAs wishing to have their root CA certificates included in Mozilla products. In some cases these practices are required by the Mozilla Root Store Policy and so are mandatory for a CA to have its root certificate(s) included. They are listed here because they are things CAs often get wrong. In other cases the practices are only recommended, but will help speed up a CA's application for inclusion and maximize the chances of its application being approved.

Required Practices

Publicly Available CP and CPS

CAs must supply the complete Certification Policy (CP) and Certification Practice Statement (CPS) containing sufficient information to determine whether and how the CA complies with the Mozilla policy requirements.

  • The CP/CPS must be publicly available from the CA's official web site.
  • The format of the CP/CPS document must be PDF or another suitable format for reading documents. CAs should not use Microsoft Word or other formats intended primarily for editable documents.
  • The CP/CPS must be available in an English version. The non-English version may be authoritative (as that's the working language of the CA) but the CA is responsible for ensuring that the translation is not materially different from the authoritative version of the document.
  • The CA must provide references to the CP/CPS sections (e.g., by section number and/or page number) that address the requirements of the Mozilla policy.
CP/CPS Documents will be Reviewed!

During the public discussion phase, your CP/CPS documentation will be reviewed and commented on in the mozilla.dev.security.policy forum. Here are some examples of previous reviews of CP/CPS documents:

Audit Criteria

CAs must supply evidence of their being evaluated according to one or more of the criteria accepted as suitable per the Mozilla policy.

  • The CA must indicate exactly which criteria they are being evaluated against (i.e., which of the criteria listed in the Mozilla policy).
  • All documents supplied as evidence must be publicly available.
  • Documents purporting to be from the CA's auditor (or other evaluator) should be available directly from the auditor (e.g., as documents downloadable from the auditor's web site).

Revocation of Compromised Certificates

CAs must revoke certificates with private keys that are known to be compromised, or for which verification of subscriber information is known to be invalid.

Verifying Domain Name Ownership

We rely on public documentation and audits of those documented processes to ascertain that the requirements of the Mozilla Root Store Policy are met.

Section 2.2.3 of the Mozilla Root Store Policy states: “for a certificate to be used for SSL-enabled servers, the CA takes reasonable measures to verify that the entity submitting the certificate signing request has registered the domain(s) referenced in the certificate or has been authorized by the domain registrant to act on the registrant's behalf"

The CA's public documentation needs to provide sufficient information describing the steps taken by the CA to confirm that the certificate subscriber owns/controls the domain name to be included in the certificate. For instance, if a challenge-response type of procedure is used, then there needs to be a brief description of the process. If public resources are used, then there should be a description of which public resources are used, what data is retrieved from public resources, and how that data is used to verify that the certificate subscriber owns/controls the domain name.

Baseline Requirements:

It is not sufficient to simply reference section 3.2.2.4 of the CA/Brower Forum's Baseline Requirements (BR). The BRs list several ways in which the CA may confirm that the certificate subscriber owns/controls the domain name to be included in the certificate. Simply referencing section 3.2.2.4 of the BRs does not specify which of those options the CA uses, and is insufficient for describing how the CA conforms to the BRs. Section 2 of the BRs says: "The CA SHALL develop, implement, enforce, and annually update a Certificate Policy and/or Certification Practice Statement that describes in detail how the CA implements the latest version of these Requirements."

WHOIS

WHOIS is used by some CAs as a source of information for checking ownership/control of the domain name for SSL certificate applications. WHOIS information may be subject to compromise. CAs are responsible for implementing appropriate methods to reduce the risk of compromise. For example, direct command line, HTTPS to the original registrar, or correlating multiple sources. The CA should include information in their CP/CPS about the method that they use to validate the integrity of the data.

It is not sufficient for the CP/CPS to just state that WHOIS is checked. The CP/CPS needs to have a high level description of how the WHOIS information is used. What information must match with that provided by the certificate subscriber? Is a phone call made or email sent to the technical or administrative contact field of the domain's WHOIS record? If an email is sent, does it include non-predictable information that the technical or administrative contact must use to respond?

Email Challenge-Response:

Many CAs use an email challenge-response mechanism to verify that the SSL certificate subscriber owns/controls the domain to be included in the certificate. Some CAs allow applicants to select an address from a predetermined list to be used for this verification. See Mozilla's restrictions on the set of verification addresses that may be used.

CAs using this method of domain validation must ensure that they are using the full email, and that none of their systems misinterpret the localpart of an email. If your system fails to process the exact email, it will misinterpret the '+' sign as a delimiter. Consider, for example, the WHOIS contact information for a domain of "user+word@example.com" or "user=word@example.com". Both of these email addresses conform to the rules set out in RFC 822 with respect to the 'localpart' syntax. If your system emails to "word@example.com", rather than to the full "user+word@example.com" / "user=word@example.com", it will allow an attacker (who obtains "word@example.com") to cause and obtain certificates for example.com. Please work with your engineering department to ensure your systems properly handle such cases, as well as handling productions such as quoted-string, as also detailed in RFC 822 and subsequent RFCs. This is especially important as EAI addresses (described in RFC 6530 and related) come into play.

Verifying Email Address Control

We rely on public documentation and audits of those documented processes to ascertain that the requirements of the Mozilla Root Store Policy are met.

Section 2.2.2 of the Mozilla Root Store Policy states: “for a certificate to be used for digitally signing and/or encrypting email messages, the CA takes reasonable measures to verify that the entity submitting the request controls the email account associated with the email address referenced in the certificate”

The CA's public documentation needs to provide sufficient information describing how the email address is verified to be owned/controlled by the certificate subscriber. For instance, if a challenge-response type of procedure is used, then there needs to be a brief description of the process. If public resources are used, then there should be a description of which public resources are used, what data is retrieved from public resources, and how that data is used to verify that the certificate subscriber owns/controls the email address.

The recommended way to satisfy this requirement is to perform a challenge-response type of procedure in which the CA sends email to the email address to be included in the certificate, and the applicant must respond in a way that demonstrates that they have control over that email address. For instance, the CA may send an email to the address to be included in the certificate, containing secret unpredictable information, giving the applicant a limited time to use the information within.

It is not sufficient for the CP/CPS to just say that an email is sent to the customer. The CP/CPS needs to be clear that the RA sends email to the email address to be included in the certificate. The CP/CPS needs to be clear that the email shall contain some non-predictable information that the subscriber must then use or respond with to confirm that the owner of the email address actually received the email and responded.

DNS names go in SAN

Some CAs mistakenly believe that one primary DNS name should go into the Subject Common Name and all the others into the SAN.

According to the CA/Browser Forum Baseline Requirements:

  • BR #9.2.1 (section 7.1.4.2.1 in BR version 1.3), Subject Alternative Name Extension
    • Required/Optional: Required
    • Contents: This extension MUST contain at least one entry. Each entry MUST be either a dNSName containing the Fully-Qualified Domain Name or an iPAddress containing the IP address of a server.
  • BR #9.2.2 (section 7.1.4.2.2 in BR version 1.3) , Subject Common Name Field
    • Required/Optional: Deprecated (Discouraged, but not prohibited)
    • Contents: If present, this field MUST contain a single IP address or Fully-Qualified Domain Name that is one of the values contained in the Certificate’s subjectAltName extension (see Section 9.2.1 - or section 7.1.4.2.1 in BR version 1.3).

OCSP

OCSP responders should be set up to listen on a standard port (e.g. port 80), because firewalls may block ports other than 80/443. Firefox and some other clients do not work with HTTPS OCSP responders, and many firewalls block requests that aren't over port 80, so OCSP responders must be accessible over HTTP (not only HTTPS) on port 80.

As per the CA/Browser Forum’s Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates, the OCSP URI must be provided in the certificate, except when OCSP stapling is used. BR #13.2.2 (section 4.9.10 in BR version 1.3): "The CA SHALL update information provided via an Online Certificate Status Protocol..." From Appendix B (section 7.1.2 in BR version 1.3) regarding authorityInformationAccess in Subordinate CA Certificate and Subscriber Certificate: "With the exception of stapling ... this extension MUST be present ... and it MUST contain the HTTP URL of the Issuing CA’s OCSP responder..."

As per the CA/Browser Forum’s Guidelines for EV Certs, CAs must provide an OCSP capability for end-entity certificates that are issued after Dec 31, 2010. Mozilla is considering technical ways to enforce this OCSP requirement such that if Firefox cannot obtain a valid response from the OCSP responder, then the certificate will not be given EV treatment. (bug 585122)

OCSP service for end-entity certs must be updated at least every four days, and OCSP responses must have a maximum expiration time of ten days.

RFC 2560, sections 2.2, 2.6, 3.2 and 4.2.2.2 define the requirements for the OCSP response signer's certificate and certificate chain. NSS enforces these requirements exactly.

You MUST test your OCSP service in Firefox! We expect OCSP responders to function without error in Mozilla products. To test in Firefox:

  • Go to Firefox -> Preferences... -> Advanced -> Certificates
  • Check the box for "Query OCSP responder servers to confirm the current validity of certificates"
  • Close the popup
  • You may need to clear your history cache
  • Browse to a website whose SSL certificate chains up to your root and has the corresponding OCSP URI in the AIA extension.

Errors that CAs sometimes encounter when testing OCSP in Firefox:

  • Error code: sec_error_ocsp_unauthorized_response
    • Please read section 4.2.2.2 "Authorized Responders" on pages 10-11 of RFC 2560. CAs that emit certificates for the general public must use a configuration that conforms to either rule 2 or 3. NSS also supports rule 1, but it requires manually configuring Firefox to set the trusted OCSP responder. This makes this choice relevant only when the Firefox installation is part of a centralized deployment where a local OCSP responder has been setup to send back OCSP responses for all the CAs that are locally trusted. The IETF pkix group that authored RFC 2560 has confirmed that rule 1 is intended to cover similar situations and not public deployments.
  • Error code: sec_error_ocsp_bad_http_response
    • the http response from the OCSP responder had some result code other than 200.
    • The http 200 response from the OCSP responder could not be decoded.
  • Error code: sec_error_ocsp_invalid_signing_cert
    • OCSP response signer's certificate was issued by the CA that issued the certificate whose status is being checked, but the response signer's certificate does not bear an ExtendedKeyUsage extension with the OCSP Responder OID, or
    • OCSP response signer's certificate chain does not validate (e.g. expired, or bad signature, etc.)
    • Trusted OCSP Responder Signing cert has not been imported. Mozilla users should not have to find and install the OCSP responder's certificate. See Potentially Problematic Practices.
  • Error code: sec_error_bad_database
    • the OCSP response gives a cert subject name to identify its signer's certificate, but no certificate by that name can be found -- not in the response, not in the database, and not in the cert chain of the certificate whose status is being checked. See this bugzilla bug for more details.

Network Security Controls

CAs must maintain current best practices for network security, and have qualified network security audits performed on a regular basis. The CA/Browser Forum has published a document called Network and Certificate System Security Requirements which should be used as guidance for protecting network and supporting systems.

It is expected that CAs do the following on a regular basis:

  • Maintain network security controls that at minimum meet the Network and Certificate System Security Requirements.
  • Check for mis-issuance of certificates, especially for high-profile domains.
  • Review network infrastructure, monitoring, passwords, etc. for signs of intrusion or weakness.
  • Ensure Intrusion Detection System and other monitoring software is up-to-date.
  • Confirm the ability to shut down certificate issuance quickly if alerted of intrusion.

Recommended Practices

CA Hierarchy

A hierarchical structure of a single root with intermediate certs (subroots) is preferred. The single top-level root's public certificate is supplied for Mozilla's root list; the subroots are not. See CA:Recommendations_for_Roots.

CAs that issue certificates under multiple subordinate CAs (i.e., under a root CA whose CA certificate is being requested for inclusion) or under multiple CA hierarchies (i.e., rooted at multiple root CAs, one or more of whose certificates is being requested for inclusion) should provide additional information as noted:

  • The CA should provide a graphical or textual description of the CA hierarchy or hierarchies, including which subordinates are under which root CAs
  • The CA should indicate the general types of certificates (i.e., for SSL/TLS servers, email signing/encryption, and code signing) issued by each subordinate CA under each root.
  • Where a CP/CPS applies to multiple subordinate CAs and/or multiple CA hierarchies, the CA should indicate whether particular sections of the CP/CPS apply to different subordinates and/or hierarchies and, if so, what the differences are.

We also recommend that CAs consider using separate root CA certificates with separate public keys (or separate intermediate CA certificates with separate public keys under a single root) when issuing certificates according to different Certificate Policies, so that we or others may selectively enable or disable acceptance of certificates issued according to a particular policy, or may otherwise treat such certificates differently (e.g., in our products' user interfaces).

Document Handling of IDNs in CP/CPS

If a CA allows the use of internationalized domain names (IDNs) in certificates (e.g., as issued for SSL/TLS-enabled servers), the CA should address the issue of homographic spoofing of IDNs in their CP/CPS, even if primary responsibility for dealing with this issue falls on domain registries. (This doesn't mean that the CAs must prevent such spoofing. It merely means that a CA should describe how it handles the issue of spoofing when authenticating the owner of a domain.)

Usage of Appropriate Constraints

Root certificates in Mozilla's program can have either the SSL trust bit set, or the Email trust bit, or both. If only one of those bits is set, relying software will trust certificates in that hierarchy for only that purpose. Nevertheless, it is a best practice for the immediately-issuing intermediate certificate to also contain EKU constraints of either id-kp-serverAuth or id-kp-emailProtection as appropriate, such that if the trust status of the root ever changes (or is not enforced for some reason), the end-entity certificates are still appropriately constrained.

Pre-Issuance Linting

Recently, several tools have been developed (certlint/cablint, x509lint, zlint) which can check a tbsCertificate (To Be Signed Certificate - the certificate complete except for the signature) for a large number of standards violations (BRs, RFCs etc.). It is strongly recommended that CAs integrate such tools into their issuance pipelines such that issuance is, minimally, held up for manual review if an error or warning is found. Because BR or RFC violations are generally considered by Mozilla to be misissuance, such integration will reduce the number of misissuance events a CA experiences, if earlier parts of their pipeline fail in their job of keeping certificates compliant.

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