For doing business with a company I have to browse to https://www.prioenergie.de/nc/mein-prioenergie.html (German website).

Firefox says the certificate is invalid, because it is not signed by a trusted root CA.

This is the certificate data:

Created for
CN: www.prioenergie.de
Serial: 1A:AE:76:6C:D0:09:24:AC:41:07:2D:27:7E:42:4A:FB

Created by
CN: thawte DV SSL CA - G2
O: thawte, Inc.
OU: Domain Validated SSL

Valid from: 06.11.2014 01:00:00
valid through: 12.09.2017 01:59:59

SHA256 fingerprint: 45:8B:50:A6:21:F1:17:7F:AE:63:24:EE:ED:DA:CE:6A:53:38:C4:AC:F6:48:0D:0A:52:49:F8:B3:07:4E:1B:25

What I did so far:

  • I already googled for the fingerprint, but didn't find anything useful.
  • I wrote an email to the support hotline and told them to investigate in this issue
  • I have submitted the URL at SSLLabs and it got a grade C, which is certainly not the best, but also not very meaningful for me.

What would be the next step to find out whether I can or cannot trust the certificate?


If you run the following command, you will see that when the person managing TLS for the server replaced the leaf certificate (the certificate for the server himself), he did not replace the chain (the certificates of the Certifying Authorities that build the chain of trust).

echo |openssl s_client -connect www.prioenergie.de:443 -showcerts 2>&1 |grep -E '^ [0-9 ] (s|i):'

Specifically, the leaf certificate was issued by CN=thawte DV SSL CA - G2, but the first CA certificate in the chain handed back by the server is CN=Thawte DV SSL CA.

Chances are, the person managing the server tested it with Internet Explorer, which tries to be permissive with what it accepts by walking the AIA if the chain does not match, but which does not follow the TLS spec and which in uncompatible with the way most other TLS clients (other browsers like Firefox, OpenSSL, NSS, the Java TLS libraries, etc) behave.

I would recommend to contact the company and let them know they need to install the new chain provided by the Certifying Authority (Thawte).

At this point, you need to make the business decision. Do you lower your security posture to be able to do business with this company, based on the presumption that this is a configuration mistake? Or do you decide this is not an acceptable risk and only deal with the company via known acceptable channels, such as the phone/fax number printed in the phone book?


This is a purely technical error on the side of the website administrators. This is not a security issue.

They simply exchanged the end entity certificate but forgot to exchange the chain. This makes it seem invalid to some browsers.

It is required by the standards that the webserver must present this this chain. But this is a courtesy to the client and a not a security feature. (All the members of the chain are public anyway.)

Some browsers however can fix these broken chains ("AIA chasing") some don't. These can't verify the trust and display a warning.

The interesting bit in the SSL-Labs report is this line:


Chain issues Incomplete, Extra certs

Now SSL-Labs can do AIA chasing. That is how it was able to get the cert labeled "extra download".

The administrators made a mistake. It's ugly and it should be fixed. But it's not a security issue.


The certificate is not valid. By definition you cannot trust it. That's the whole point of having a certificate chain. In all evidence, it has simply been incorrectly configured; you can find the same problem - from the point of view of the webmaster - here.

You might query Thawte... but I wouldn't hold my breath for an answer. They'll probably feel, quite correctly at that, that it's not their problem.

You can trust the provider "on faith" and temporarily accept the certificate for the time necessary to download your invoice. It's rather unlikely that it's a forged site; but just in case, you can wait a little time to see whether they fix the issue, or at the very least acknowledge it.

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