curl, or an application that uses libcurl, may have a problem with an SSL certificate that works fine when using a web browser to access the same URL. Typical error output from curl looks like this:

$ curl -v https://my-subdomain.mysecuresite.com

 Trying xxx.xxx.xxx.xxx:443…
 TCP_NODELAY set
 Connected to my-subdomain.mysecuresite.com (xxx.xxx.xxx.xxx) port 443 (#0)
 ALPN, offering h2
 ALPN, offering http/1.1
 successfully set certificate verify locations:
 CAfile: /etc/ssl/certs/ca-certificates.crt
 CApath: none
 TLSv1.3 (OUT), TLS handshake, Client hello (1):
 TLSv1.3 (IN), TLS handshake, Server hello (2):
 TLSv1.2 (IN), TLS handshake, Certificate (11):
 TLSv1.2 (OUT), TLS alert, unknown CA (560):
 SSL certificate problem: unable to get local issuer certificate
 Closing connection 0
 curl: (60) SSL certificate problem: unable to get local issuer certificate
 More details here: https://curl.haxx.se/docs/sslcerts.html 
 curl failed to verify the legitimacy of the server and therefore could not
 establish a secure connection to it. To learn more about this situation and
 how to fix it, please visit the web page mentioned above.

Troubleshooting Strategy

1. A good starting point for any SSL error on a public-facing URL is to analyze the URL at SSL Labs.
2. Does the error happen for all certificates issued by a specific Certificate Authority (CA)? If so, the system running curl may need to have a root certificate for that CA added or updated in its certificate repository. That’s a relatively rare problem, but might occur if the system running curl is very old. There’s a comprehensive thread about this issue on Stack Overflow.
3. If the error only happens for one specific site, it’s likely that the site is missing an intermediate certificate. The command to diagnose this issue is also found in that Stack Overflow thread:

openssl s_client -connect myhost.com:443 -servername myhost.com -showcerts

The output should show a series of certificates, starting with the site certificate, and ending with the root certificate for the Certification Authority. If this chain only shows the site certificate, that’s the problem.