Wildcard certificate
 | It has been suggested that this article be merged into Public key certificate. (Discuss) Proposed since November 2023. |
 | This article may be too technical for most readers to understand. (July 2014) |
*)
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.comcontact.example.comlogin-secure.example.comwww.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
 | This section needs additional citations for verification. (January 2020) |
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.comis OK. It will matchfrog.domain.combut notfrog.super.domain.com
baz*.example.netis OK and matchesbaz1.example.net*baz.example.netis OK and matchesfoobaz.example.netb*z.example.netis OK and matchesbuzz.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.comis 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.comiscafé.comxn--caf-dma*.comis not allowedLw*.xn--caf-dma.comis allowed
References
- ^ "Wildcard Certificate Explained in Simpler Terms". 23 May 2016.
- ^
"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.
- ^
Newman, C. (June 1999). RFC 2595 - Using TLS with IMAP, POP3 and ACAP. Internet Engineering Task Force. p. 3. doi:10.17487/RFC2595. RFC 2595. Retrieved 2014-12-15.
For example, *.example.com would match a.example.com, foo.example.com, etc. but would not match example.com.
- ^ Wildcard SSL certificate limitation on QuovadisGlobal.com
- ^
"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.
- ^ x509v3_config Subject Alternative Name
- ^ The SAN option is available for EV SSL Certificates on Symantec.com
- ^ SSLTools Certificate Lookup of Wikipedia.org's wildcard ssl certificate
- ^
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. p. 31. doi:10.17487/RFC6125. RFC 6125. 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: [...]
- ^ Rescorla, E. (May 2000). "RFC 2818 - HTTP Over TLS". tools.ietf.org. doi:10.17487/RFC2818. RFC 2818. Retrieved 2019-04-20.
- ^ 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. RFC 6125. Retrieved 2019-04-20.
- ^ "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.
- ^ "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.
- ^ "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
- Newman, C. (June 1999). RFC 2595 - Using TLS with IMAP, POP3 and ACAP. Internet Engineering Task Force. p. 3. doi:10.17487/RFC2595. RFC 2595.
- RFC 2818 - HTTP Over TLS. Internet Engineering Task Force. May 2000. p. 5. doi:10.17487/RFC2818. RFC 2818.
- 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. doi:10.17487/RFC6125. RFC 6125.