An example of a wildcard certificate on comifuro.net (note the asterisk: *)
An example of a Subject Alternative Name (SAN) field

A Public key certificate which uses an asterisk * (the wildcard) in its domain name fragment is called a Wildcard certificate. Through the use of *, a single certificate may be used for multiple sub-domains. It is commonly used for transport layer security in computer networking.

Example

A single wildcard certificate for https://*.example.com will secure all these subdomains on the https://*.example.com domain:

  • payment.example.com
  • contact.example.com
  • login-secure.example.com
  • www.example.com

Instead of getting separate certificates for subdomains, you can use a single certificate for all main domains and subdomains and reduce cost.[1]

Because the wildcard only covers one level of subdomains (the asterisk doesn't match full stops),[2] these domains would not be valid for the certificate:

  • test.login.example.com

The "naked" domain is valid when added separately as a Subject Alternative Name (SubjectAltName):[3]

  • example.com

Note possible exceptions by CAs, for example wildcard-plus cert by DigiCert contains an automatic "Plus" property for the naked domain example.com.

Limitations

Only a single level of subdomain matching is supported in accordance with RFC 2818.[4]

It is not possible to get a wildcard for an Extended Validation Certificate.[5] A workaround could be to add every virtual host name in the Subject Alternative Name (SAN) extension,[6][7] the major problem being that the certificate needs to be reissued whenever a new virtual server is added. (See Transport Layer Security § Support for name-based virtual servers for more information.)

Wildcards can be added as domains in multi-domain certificates or Unified Communications Certificates (UCC). In addition, wildcards themselves can have subjectAltName extensions, including other wildcards. For example, the wildcard certificate *.wikipedia.org has *.m.wikimedia.org as a Subject Alternative Name. Thus it secures www.wikipedia.org as well as the completely different website name meta.m.wikimedia.org.[8]

RFC 6125 argues against wildcard certificates on security grounds, in particular "partial wildcards".[9]

Examples

The wildcard applies only to one level of the domain name. *.example.com matches sub1.example.com but not example.com and not sub2.sub1.domain.com

The wildcard may appear anywhere inside a label as a "partial wildcard" according to early specifications[10]

f*.domain.com is OK. It will match frog.domain.com but not frog.super.domain.com
baz*.example.net is OK and matches baz1.example.net
*baz.example.net is OK and matches foobaz.example.net
b*z.example.net is OK and matches buzz.example.net

However, use of "partial-wildcard" certs is not recommended. As of 2011, partial wildcard support is optional, and is explicitly disallowed in SubjectAltName headers that are required for multi-name certificates.[11] All major browsers have deliberately removed support for partial-wildcard certificates;[12][13] they will result in a "SSL_ERROR_BAD_CERT_DOMAIN" error. Similarly, it is typical for standard libraries in programming languages to not support "partial-wildcard" certificates. For example, any "partial-wildcard" certificate will not work with the latest versions of both Python[14] and Go. Thus,

Do not allow a label that consists entirely of just a wildcard unless it is the left-most label

sub1.*.domain.com is not allowed.

A cert with multiple wildcards in a name is not allowed.

*.*.domain.com

A cert with * plus a top-level domain is not allowed.

*.com

Too general and should not be allowed.

*

International domain names encoded in ASCII (A-label) are labels that are ASCII-encoded and begin with xn--.

Do not allow wildcards in an international label.

xn--caf-dma.com is café.com
xn--caf-dma*.com is not allowed
Lw*.xn--caf-dma.com is allowed

References

  1. ↑ "Wildcard Certificate Explained in Simpler Terms". 23 May 2016.
  2. ↑ "RFC 2818 - HTTP Over TLS". Internet Engineering Task Force. May 2000. p. 5. Retrieved 2014-12-15. [...] *.a.com matches foo.a.com but not bar.foo.a.com.
  3. ↑ Newman, C. (June 1999). "RFC 2595 - Using TLS with IMAP, POP3 and ACAP". Internet Engineering Task Force: 3. doi:10.17487/RFC2595. Retrieved 2014-12-15. For example, *.example.com would match a.example.com, foo.example.com, etc. but would not match example.com. {{cite journal}}: Cite journal requires |journal= (help)
  4. ↑ Wildcard SSL certificate limitation on QuovadisGlobal.com
  5. ↑ "Guidelines For The Issuance And Management Of Extended Validation Certificates, Version 1.5.2" (PDF). CA/Browser Forum. 2014-10-16. p. 10. Retrieved 2014-12-15. Wildcard certificates are not allowed for EV Certificates.
  6. ↑ x509v3_config Subject Alternative Name
  7. ↑ The SAN option is available for EV SSL Certificates on Symantec.com
  8. ↑ SSLTools Certificate Lookup of Wikipedia.org's wildcard ssl certificate
  9. ↑ Saint-Andre, P.; Hodges, J. (March 2011). "RFC 6125 - Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)". Internet Engineering Task Force: 31. doi:10.17487/RFC6125. Retrieved 2014-12-10. This document states that the wildcard character '*' SHOULD NOT be included in presented identifiers but MAY be checked by application clients (mainly for the sake of backward compatibility with deployed infrastructure). [...] Several security considerations justify tightening the rules: [...] {{cite journal}}: Cite journal requires |journal= (help)
  10. ↑ Rescorla, E. (May 2000). "RFC 2818 - HTTP Over TLS". tools.ietf.org. doi:10.17487/RFC2818. Retrieved 2019-04-20.
  11. ↑ Saint-Andre, P.; Hodges, J. (March 2011). "RFC 6125 - Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)". tools.ietf.org. doi:10.17487/RFC6125. Retrieved 2019-04-20.
  12. ↑ "Disallow support for a*.example.net, *a.example.net, and a*b.example.net in certificate wildcard handling". The Chromium Projects, Google Inc. 3 December 2014. Retrieved 21 October 2020.
  13. ↑ "Limit wildcard DNS ID support to names of the form *.example.com (not foo*.example.com)". The Mozilla Foundation. 10 December 2014. Retrieved 21 October 2020.
  14. ↑ "Disallow support for a*.example.net, *a.example.net, and a*b.example.net in certificate wildcard handling". The Python Software Foundation. 26 November 2017. Retrieved 21 October 2020.

Relevant RFCs

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