Getting SSL certificate error on valid certificate when accessing via Curl

You didn't provide the actual address of the website, nor the SSL provider's name, nor any other information about the certificate, and basically want us to guess at various possible causes.

My guess is that your certificate chain ends with "AddTrust External Root" as the topmost CA, and that root certificate just expired several hours ago.

Sectigo, the certificate's owner, has a diagram of the chain – due to old CA certificates cross-signing new ones, "AddTrust External Root" was basically a higher tier above what would normally be the company's actual root CA. New operating systems would see Sectigo/Comodo's certificates as self-signed root CAs, while old ones would see them as intermediate CAs below AddTrust Root.

So on old operating systems which only recognize Sectigo via cross-signing by the AddTrust Root CA certificate, as soon as the latter expires, the whole chain is invalid. To fix this, you have to switch to another CA which those systems recognize as a root (check /etc/ssl/certs).

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It could be that those systems do recognize Sectigo as a root CA in its own right. In theory, is completely legal for a certificate to validate through multiple chains, e.g. a CA can simultaneously be a root and be cross-signed by another CA.

But in practice, many TLS client libraries are really bad at building alternate chains and assume there's always exactly one way to validate a certificate. (Windows is a bit better, and the big web browsers also include code to handle this extra complexity on top of what the TLS library would probide.)

For example, older versions of OpenSSL were very strict about which certificates could act as trust roots. If the server sent a chain "OldCA–NewCA–Intermediate–MyServer", OpenSSL 1.0 would only accept if you had "OldCA" as a trusted certificate – but it wouldn't be smart enough to accept it if you had "NewCA" instead.

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So if Ubuntu 14.04 already recognizes Sectigo as a certificate authority, then you need to fix the certificate chain that is being sent by your webserver – I suspect removing the certificate which says "Issuer: AddTrust" should be enough.

Let's say the old certificate chain looks like this:

[AddTrust External Root]                               [included in OS]
+-- USERTrust RSA Certification Authority (Sectigo)    sent by your server
    +-- Gandi Standard SSL CA 2                        sent by your server
        +-- git.kernel.org                             sent by your server

You need to edit it so that your server stops sending the cross-signed USERTrust certificate:

[USERTrust RSA Certification Authority (Sectigo)]      [included in OS]
+-- Gandi Standard SSL CA 2                            sent by your server
    +-- git.kernel.org                                 sent by your server

You can also fix this on the client side, by removing the AddTrust root CA.

We had the same issues today with our 14.04s failing and the 18s working fine. It looks like today is the day when a popular root support stopped working for older Ubuntu boxes. Fortunately, we just decommissioned the 14s but if you can't there are some instructions in there to try to get it to work. Also, our DevOps team noted this would also probably require some heavy lifting with upgrading OpenSSL and making it compadible with the changes described here. https://support.sectigo.com/articles/Knowledge/Sectigo-AddTrust-External-CA-Root-Expiring-May-30-2020