In computer networking, a wildcard certificate is a public key certificate which can be used with multiple subdomains of a domain. The principal use is for securing web sites with HTTPS, but there are also applications in many other fields. Compared with conventional certificates, a wildcard certificate can be cheaper and more convenient than a certificate for each domain.
Video Wildcard certificate
Example
A single wildcard certificate for *.example.com, will secure all these domains:
- payment.example.com
- contact.example.com
- login-secure.example.com
- www.example.com
Instead of getting separate certificates for sub domains, you can use a single certificate for all main domains and sub domains and save your money.
Because the wildcard only covers one level of subdomains (the asterisk doesn't match full stops), 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):
- 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
Maps Wildcard certificate
Limitations
Only a single level of subdomain matching is supported.
It is not possible to get a wildcard for an Extended Validation Certificate. A workaround could be to add every virtual host name in the Subject Alternative Name (SAN) extension, the major problem being that the certificate needs to be reissued whenever a new virtual server is added.
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 https://www.wikipedia.org as well as the completely different website name https://meta.m.wikimedia.org.
Examples drawn from RFCs and blogs and behavior of browsers: The wildcard applies only to just one label of the domain name. label.label.label.TLD *.domain.com is OK. It will match www.domain.com but not domain.com and not zzz.www.domain.com The wildcard may appear anywhere inside a label (aka "partial-wildcard") 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 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 w*.xn--caf-dma.com, is allowed
RFC 6125 argues against wildcard certificates on security grounds.
References
Relevant RFCs
- "RFC 2595 - Using TLS with IMAP, POP3 and ACAP". Internet Engineering Task Force. June 1999. p. 3.
- "RFC 2818 - HTTP Over TLS". Internet Engineering Task Force. May 2000. p. 5.
- "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. March 2011.
Source of the article : Wikipedia
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