Why is openssl complaining that my certificate chain is self-signed?

Question

I am trying to set up a certificate chain for a lab server. I have created my own root CA, an intermediate CA and a server certificate. I supplied these certificates along with the server key to the openssl s_server command. When I run openssl s_client and connect to that server, openssl complains that there is a self-signed certificate in the chain.

When I connect to a public web server using s_client, however, not only does the server not send all of the certificates in the chain (just the intermediate parent certificate of the server certificate) but openssl doesn't complain about a self-signed certificate, let alone an incomplete certificate chain.

If I use s_server with a CA file containing just the server's parent intermediate certificate, s_client complains that it can't get the local issuer certificate. I never see this error with public web servers even though they don't send the entire certificate chain.

In none of these tests (using my own certificates or public web servers) I am using the -CApath, -CAfile or -verify options with the s_client command.

I don't know what I'm doing wrong. Why does s_client complain about my certificate chain even though I don't use -verify? Why is it complaining about my (I assume) root certificate being self-signed when all root certificates are self-signed?

For example, this is what I get with a public web server:

openssl s_client -showcerts -servername security.stackexchange.com -connect security.stackexchange.com:443
CONNECTED(00000004)
depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
verify return:1
depth=0 CN = *.stackexchange.com
verify return:1
---

But using s_server with my full certificate chain, I get this:

openssl s_client -showcerts -servername server.domain.com -connect server.domain.com:443
CONNECTED(00000004)
depth=2 C = US, ST = State, L = City, O = Company, OU = Company CA
verify error:num=19:self signed certificate in certificate chain
---

Here are my certificates. And yes, I have the constraints CA=TRUE, Digital Signature and Certificate Sign set in my root CA.

openssl x509 -in root_ca.cert.pem -noout -text
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            bc:e0:9f:2a:5d:25:6e:8f
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: C=US, ST=State, L=City, O=Company, OU=Company CA
        Validity
            Not Before: May 30 22:35:50 2019 GMT
            Not After : May 25 22:35:50 2039 GMT
        Subject: C=US, ST=State, L=City, O=Company, OU=Company CA
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
.........
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Subject Key Identifier:
                2A:0A:D6:EF:96:02:70:4F:89:7A:69:C5:3E:37:47:EE:B1:E1:92:C0
            X509v3 Authority Key Identifier:
                keyid:2A:0A:D6:EF:96:02:70:4F:89:7A:69:C5:3E:37:47:EE:B1:E1:92:C0

            X509v3 Basic Constraints: critical
                CA:TRUE
            X509v3 Key Usage: critical
                Digital Signature, Certificate Sign, CRL Sign
    Signature Algorithm: sha256WithRSAEncryption
.........

root CA:

intermediate CA:

server certificate:

s_client does verify the cert chain using default roots installed by your distro. By default it doesn't stop after verification failed because it's a test tool, but it does verify. If you specify -CAfile, does it work? The complaint about self-signed is because the domain's cert is issued by something not trusted by the trusted roots, not because the root itself is self-signed (as you noted, all roots are self-signed). — Z.T., May 30, 2019 at 23:50

Community · Accepted Answer · 2021-10-07 07:59:29Z

First, it is quite correct for a TLS/SSL server to send the intermediate aka chain cert(s) but not the root cert; see rfc 5246 sec 7.4.2 or the slightly more verbose version in rfc 8446 sec 4.4.2.

As Z.T. mostly-correctly commented, -verify is the default for s_client (you don't need to specify it) and if you don't specify -CAfile and/or -CApath by default (except for bugs in some obsolete versions) it uses a default truststore, configured at compile time, which can include either a single file with concatenated PEM certs or a directory containing separate PEM files with names or links using truncated subject hashes usually created by c_rehash (add) or, as commented, manually determined with x509 -hash; see the man page for verify(1ssl) on your system or on the web. OpenSSL itself doesn't provide the root certs that go in such a truststore; if you are using a distro or other packaged version, the builder usually configures OpenSSL to use a set of root certs provided by the distro or package. (On distros I know, this distro-provided truststore is also used for other software including NSS, GNUtls, and Java; see below.) If you (or someone) built OpenSSL by hand, you have to do this yourself. Since you are not getting a verify error on public servers, your build presumably is using a truststore that has (at least some) public CA's preinstalled; you can see where this is with openssl version -d (plus the hardcoded names cert.pem and/or certs).

You can add your root or other anchor (see below) to this default truststore, or use -CAfile and/or -CApath to specify a custom one containing your own root(s) or anchor(s). On the distros I know, and probably others, you should NOT hand-modify the default files because they are automatically generated by a process that also sets the truststores used by other software such as NSS, GNUtls, and Java; these need to contain the same data (certs) but in different formats. Instead see e.g. man update-ca-trust for RedHat family or man update-ca-certificates for Debian family.

OpenSSL will use an intermediate (aka chain) cert or certs in the truststore to build the cert chain if needed, i.e. if not sent by the server (in violation of the RFC, but many do that), but historically it will only accept a chain -- either fully received from the server or (partly) built from the local truststore -- if it ends at a root that is in the local truststore. For recent versions, namely 1.0.2 up but only documented since 1.1.0, there is an option -partial_chain which does accept a chain ending in an intermediate (non-root) that is in the local truststore.

Thanks! I've figured out part of my confusion. Although my version of OpenSSL does not include a default truststore, at some point last year I must have done something that caused it to get copies of a large number of root-level certificates. They were all stored in /var/ssl/certs, but that directory does not exist by default on AIX. I renamed the directory and the openssl command then behaved consistently for anything I connected to--"verify error:num=20:unable to get local issuer certificate". Now I just need to figure out why it considers my certificate chain to be self-signed. — Steve Talmage, Jun 2, 2019 at 3:43
It considers your root cert to be self-signed because it is; that's the definition of a root cert. If you configure the server to not send the root cert s_client will report verify error 20 instead of verify error 19, if you think that is somehow significant, but in either case the cert cannot be verified. What matters about a root is not that it is self-signed, which it always is, but whether it is trusted. — dave_thompson_085, Jun 2, 2019 at 6:12
I am aware that all root certificates are self-signed. I was just trying to figure out why s_client was complaining about mine and no one else's. After I discovered that a truststore actually existed on my system, I added my root certificate to it, used x509 -hash to get the hash value, created a symbolic link from the hash value to my root certificate, and s_client stopped complaining. Now I fully understand s_client's criteria for determining if a root certificate is to be trusted. Thanks for your help! — Steve Talmage, Jun 3, 2019 at 21:12
As I said, the difference is that if the server sends a chain including the root, and the root is not trusted, it gives error 19. If the server sends a chain excluding the root, and the root is not trusted, it gives error 20. That difference is not in the cert, but in the server. If the root is in the truststore -- and being in the default dir with a hash name is one way, another is to put it in the default file -- verification succeeds, whether or not the server sends it. — dave_thompson_085, Jun 4, 2019 at 7:57
Right. I understood that part after you initially explained it. I just didn't realize that I had a truststore in the first place. — Steve Talmage, Jun 5, 2019 at 16:45

safesploit · Accepted Answer · 2019-05-30 23:48:01Z

Regarding the fullchain I generated mine for use with Apache2 using:

cat intermediateCA.crt rootCA.crt > fullchain.crt

Without testing your exact circumstances I cannot suggest more than the order the web server is reading the certificate chain.

For OpenSSL stating your rootCA being self-signed. This could be due to your rootCA not being installed into your certificate store on the OS (or even web browser). Installing a .crt file across operating systems can vary, Adding trusted root certificates to the server will explain how. Bear in mind that Mozilla Firefox uses its own certificate store.

When I first started being my own certificate authority, for similar purposes, I began issuing my own rootCA using a tutorial by Jaime Linux which may be more appropriate given you are issuing for a lab server, as the tutorial explains creating a flat database to track signed certificates.

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