While I was on vacation someone replaced a certificate which had expired on the F5 Big-IP load balancer. Maybe they were not quite as careful as I would like to hope I would have been. In any case, shortly afterwards our SiteScope monitoring reported there was an untrusted server certificate chain. It took me quite some digging to get to the bottom of it.
Well, the web site came up just fine in my browser. I checked it with SSLlabs and its grade was capped at B because of problems with the server certificate chain. I also independently confirmed usnig openssl that no intermediate certificate was being presented by this virtual server. To see what that looks like with an exampkle of this problem knidly privided by badssl.com, do:
$ openssl s_client ‐showcerts ‐connect incomplete-chain.badssl.com:443
CONNECTED(00000003) depth=0 /C=US/ST=California/L=San Francisco/O=BadSSL Fallback. Unknown subdomain or no SNI./CN=badssl-fallback-unknown-subdomain-or-no-sni verify error:num=20:unable to get local issuer certificate verify return:1 depth=0 /C=US/ST=California/L=San Francisco/O=BadSSL Fallback. Unknown subdomain or no SNI./CN=badssl-fallback-unknown-subdomain-or-no-sni verify error:num=27:certificate not trusted verify return:1 depth=0 /C=US/ST=California/L=San Francisco/O=BadSSL Fallback. Unknown subdomain or no SNI./CN=badssl-fallback-unknown-subdomain-or-no-sni verify error:num=21:unable to verify the first certificate verify return:1 --- Certificate chain 0 s:/C=US/ST=California/L=San Francisco/O=BadSSL Fallback. Unknown subdomain or no SNI./CN=badssl-fallback-unknown-subdomain-or-no-sni i:/C=US/ST=California/L=San Francisco/O=BadSSL/CN=BadSSL Intermediate Certificate Authority -----BEGIN CERTIFICATE----- MIIE8DCCAtigAwIBAgIJAM28Wkrsl2exMA0GCSqGSIb3DQEBCwUAMH8xCzAJBgNV BAYTAlVTMRMwEQYDVQQIDApDYWxpZm9ybmlhMRYwFAYDVQQHDA1TYW4gRnJhbmNp ... HJKvc9OYjJD0ZuvZw9gBrY7qKyBX8g+sglEGFNhruH8/OhqrV8pBXX/EWY0fUZTh iywmc6GTT7X94Ze2F7iB45jh7WQ= -----END CERTIFICATE----- --- Server certificate subject=/C=US/ST=California/L=San Francisco/O=BadSSL Fallback. Unknown subdomain or no SNI./CN=badssl-fallback-unknown-subdomain-or-no-sni issuer=/C=US/ST=California/L=San Francisco/O=BadSSL/CN=BadSSL Intermediate Certificate Authority ... Verify return code: 21 (unable to verify the first certificate)
So you get that message about benig unable to verify the first certificate.
Here’s the weird thing, the certificate in question was issued by Globalsign, and we have used them for years so we had the intermediate certificate configured already in the SSL client profile. The so-called chain certificate was GlobalsignIntermediate. But it wasn’t being presented. What the heck? Then I checked someone else’s Globalsign certificate and found the same issue.
Then I began to get suspicious about the certificate. I checked the issuer more carefully and found that it wasn’t from the intermediate we had been using all these past years. Globalsign changed their intermediate certificate! The new one dates frmo November 2018 and expires in 2028.
And, to compound matters, F5 “helpfully” does not complain and simply does not send the wrong intermediate certificate we had specified in the SSL client profile. It just sends no intermediate certificate at all to accompany the server certificate.
The case of the missing intermediate certificate was resolved. It is not the end of the world to miss an intermediate certificate, but on the other hand it is not professional either. Sooner or later it will get you into trouble.