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IDN wildcard match

Project curl Security Advisory, May 17th 2023 - Permalink


curl supports matching of wildcard patterns when listed as "Subject Alternative Name" in TLS server certificates. curl can be built to use its own name matching function for TLS rather than one provided by a TLS library. This private wildcard matching function would match IDN (International Domain Name) hosts incorrectly and could as a result accept patterns that otherwise should mismatch.

IDN hostnames are converted to puny code before used for certificate checks. Puny coded names always start with xn-- and should not be allowed to pattern match, but the wildcard check in curl could still check for x*, which would match even though the IDN name most likely contained nothing even resembling an x.


curl's wildcard matching function is used only when curl was built to use OpenSSL, Schannel or Gskit. All other backends use the matching functions of the corresponding TLS library and are thus not vulnerable to this flaw.

This flaw is lessened somewhat by two factors:

curl does not need to be built with IDN support to be vulnerable, as a user can pass in a puny coded version of the host name directly in the URL and can then trigger this flaw.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2023-28321 to this issue.

CWE-295: Improper Certificate Validation

Severity: Low


This bug was introduced in curl when IDN support was first introduced, in curl 7.12.0 - June 2004. The wildcard function was subsequently updated for this case in 2012 (the IDN problem is mentioned in RFC 6125 in a far from obvious way) but was done wrongly, so the flaw remained.

libcurl is used by many applications, but not always advertised as such!


curl 8.1.0 completely removes the support for "partial" matches and now only supports *.. No a*, a*b or *b matches. For all host names, IDN or not.


A - Upgrade curl to version 8.1.0

B - Apply the patch to your local version


This issue was reported to the curl project on April 17 2023. We contacted distros@openwall on May 9, 2023.

curl 8.1.0 was released on May 17 2023, coordinated with the publication of this advisory.


Thanks a lot!