The real fix for this is to ensure that your server presents all certificates in the chain and not just the end-entity (server) certificate.
Point your server administrator to RFC 5246 Section 7.4.2 which clearly states that This message conveys the server's certificate chain to the client.
If your admin refuses/can't do this for some reason, your alternative option is to try and get
curl to work with the malformed handshake.
According to a message on the Curl mailing list:
Can someone confirm if cURL supports (or not) intermediate certificate?
Yes it does.
All ca certificates have a certificate chain going up to the root. The ca
bundle you use with curl needs to consist of the certs for the entire chain.
You should be able to add the Root CA and all intermediates certificates to a bundle and point
curl to it using the
--cacert <file> option.
As your browsers work, you can access the correct CA certificates from there. On the certificates tab (different for each browser, but I'm sure you'll figure that one out), view the certificate chain. Double-click the Root CA first Globalsign Root CA - G1 and on the Details tab, click on Copy to file.... Save it as
root.cer. Do the same with the AlphaSSL CA - SHA256 - G2 and save it as
issuing.cer. Join the two together in a single file (e.g.
chain.cer) and use that as the argument to
As kindly pointed out by @A.B. the missing certificate can also be found here.
Your browsers work because they cache CA certificates. If you've navigated to a correctly configured website at some point in the past, whose certificate was issued by the same CA as your server's certificate, it will be cached by the browser. When you subsequently visit your incorrectly configured site, your browser will use the CA certificates in its cache to build the chain. To you, it seems like everything is fine, although behind the scenes, the server is mis-configured.
Note that on Windows, IE/Edge and Chrome share the same cache, while Firefox uses its own.
In addition to the above, IE/Edge and Chrome (as they share the same crypto stack) will use an extension within certificates called the AuthorityInformationAccess. This has a caIssuer option which provides a URL from which the end-entity certificate's CA certificate can be downloaded. Therefore, even if one of these browsers hasn't cached the missing certificates from previous browsing, it can fetch it if required. Note that Firefox doesn't do this, which is why sometimes Firefox can show certificate errors when IE/Edge and Chrome seem to work.