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I have the following X509 certificate:

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

When I open it up in Windows I see a path from the cert to the CA with one intermediate cert:

enter image description here

So the CA is pre-loaded on the system but how is it able to get the intermediate cert?

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  • $\begingroup$ Well obviously it must also be present on the system. There is a store for intermediate certificates and it would not hurt to cache those kind of certs either. A lot of protocols also allow the sending of additional certs together with the leaf certificate. $\endgroup$ – Maarten Bodewes Feb 26 '18 at 19:25
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An X.509 certificate may contain extensions, one of them called Authority Information Access, which contains "pointers" to the certificate for the (intermediate) CA that issued it. In your case, the AIA extension is present and contains the following URL:

http://cacerts.digicert.com/DigiCertSHA2SecureServerCA.crt

Windows simply sends an HTTP GET request to that URL, and tries to interpret what it obtains as an encoded certificate. Which is the case. Windows can then verify that the intermediate certificate is indeed a valid issuer for your certificate (the signature is valid, the subject/issuer names match, the validity dates are correct, and so on...). Once it has obtained the intermediate CA cert, it repeats the process, until it hits one of the preloaded root CA.

The certificate viewer of Windows can show you such URL (see the "details" pane). Alternatively, you can decode the certificate structure to see for yourself the gory details (personally, I use my own tool called DDer).

Amusingly, this feature can be used to make, in some circumstances, a remote machine issue HTTP GET requests at URL of your choosing (but only GET, not POST, and only HTTP, not HTTPS; and you don't get to see what is obtained). For instance, if there is a Windows-based SSL server that asks for client certificates, then a malicious client may submit a phony certificate that makes the server issue on HTTP GET at a URL that will be interpreted by the server itself (and thus may contain "internal" IP addresses, or even "localhost").

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