Download error on [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661)

I'm using Plone 5.0.3rc. I'm getting this error message every time I add addon products in buildout.cfg.

Please help.

Thank you in advance

If there is a certificate problem with then contact the maintainers of PyPI.
And ensure that your system is up-to-date enough for knowing the most decent root certs.
And why 5.0.3RC????? If 5.0.X than use the latest 5.0 release with 5.0.8 I think about not some weird version in between.

This error message tells you that the (https) certificate verification failed for "".

This is likely a local problem (at your site) as I can access "" without problems. The certificate is verified by "DigiCert Inc" and probably, your site does not yet recognize this CA (= "Certificate authority"). Ensure that this CA's certificate is installed at the appropriate place on the system you try to run your buildout.

Alternatively, it could have been a temporary problem (but I estimate this less likely). Retry your buildout to check for a temporary problem.

If you're doing this from a corporate network, chances are good that you have an appliance between you and that is doing an intentional "man in the middle attack" to "protect" the corporation. Appliances such as Websense or ForcePoint inject their certificates into the certificate chain and then cause errors with any SSL-based application. I see this error all the time and Dieter is right - it's a problem with the CA.

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I agree with @LeaderOne. I saw that many times in corporate networks. Try to access via a simple command line command (e.g. curl). If you see the cert error there as well this has nothing to do with buildout or Plone.

If you can share the appliance you are using in your network, we might be able to help here.