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I ask this question, because Comodo are telling me that a wildcard certificate for *.example.com will also secure the root domain example.com. So with a single certificate, both my.example.com and example.com are secured without warning from a browser.

However, this is not the case with the certificate I've been provided. My sub-domains are secured fine and do not give an error, but the root domain throws up an error in the browser, saying the identify can't be verified.

When I compare this certificate to other similar scenarios, I see that in the scenarios that work without error, the Subject Alternative Name (SAN) lists both *.example.com and example.com, whereas the recent certificate from Comodo only lists *.example.com as the Common Name and NOT example.com as the Subject Alternative Name.

Can anyone confirm/clarify that the root domain should be listed in SAN details if it is also to be secured correctly?

When I read this: http://www.digicert.com/subject-alternative-name.htm It seems that the SAN must list both in order to work as I need it to. What's your experience?

Thanks very much.

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2 Answers 2

up vote 36 down vote accepted

There's some inconsistency between SSL implementations on how they match wildcards, however you'll need the root as an alternate name for that to work with most clients.

For a *.example.com cert,

  • a.example.com should pass
  • www.example.com should pass
  • example.com shouldn't pass
  • a.b.example.com may pass depending on implementation (but probably not).

Essentially, the standards say that the * should match 1 or more non-dot characters, but some implementations allow a dot.

The canonical answer should be in RFC 2818 (HTTP Over TLS):

Matching is performed using the matching rules specified by [RFC2459]. If more than one identity of a given type is present in the certificate (e.g., more than one dNSName name, a match in any one of the set is considered acceptable.) Names may contain the wildcard character * which is considered to match any single domain name component or component fragment. E.g., *.a.com matches foo.a.com but not bar.foo.a.com. f*.com matches foo.com but not bar.com.

RFC 2459 says:

  • A "*" wildcard character MAY be used as the left-most name component in the certificate. For example, *.example.com would match a.example.com, foo.example.com, etc. but would not match example.com.

If you need a cert to work for example.com, www.example.com and foo.example.com, you need a certificate with subjectAltNames so that you have "example.com" and "*.example.com" (or example.com and all the other names you might need to match).

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You're correct, the root domain needs to be an alternate name for it to validate.

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