Openssl different intermediates for client and server certs - ssl

Question

I am trying to generate a certificate-(authority)-chain, where some intermediates have specific tasks.
There will be a root certificate (skipped for testing) that signs the actual CA, this CA on the one hand, signs a CA that should only be able to sign client-certificates, and on the other hand, signs one/multiple CA's that can only perform server-certificates-signing. The idea is, that this "server-ca" will be deployed into an embedded system in order to create a new server certificate with different sans if nescessary, but should not be able to sign client-certificates. (see my fine drawing)
I did not find much about constraining a ca. Here it says it would be possible, so I tried to create a test setup where I defined an extendedKeyUsage for the device/server-CA. The X509v3 extensions in the certs I created look like the following
Server/Device-CA:
X509v3 extensions:
X509v3 Basic Constraints: critical
CA:TRUE <-(yeah, forgot 'bout the path-length - hope this is not the problem)
X509v3 Subject Key Identifier:
51:0F:8C:55:88:4D:3E:25:DC:EC:6D:73:39:E4:7D:27:2E:AF:E4:2D
X509v3 Authority Key Identifier:
keyid:B3:6A:53:D9:9B:CF:74:69:B5:64:73:91:D7:18:92:30:E3:A7:7A:A6
X509v3 Key Usage: critical
Digital Signature, Certificate Sign, CRL Sign
X509v3 Extended Key Usage: critical
TLS Web Server Authentication <--
Server/Device-CA -> Server-Cert
X509v3 extensions:
X509v3 Basic Constraints:
CA:FALSE
Netscape Cert Type:
SSL Server
Netscape Comment:
OpenSSL Generated Server Certificate
X509v3 Subject Key Identifier:
AD:BC:E0:73:50:AB:7F:BE:3C:43:71:4F:07:06:D8:3F:1A:38:81:4C
X509v3 Authority Key Identifier:
keyid:51:0F:8C:55:88:4D:3E:25:DC:EC:6D:73:39:E4:7D:27:2E:AF:E4:2D
DirName:/C=XXX/ST=XXX/L=XX/O=XXX/OU=XXX/CN=XXX/emailAddress=XXX
serial:10:00
X509v3 Key Usage: critical
Digital Signature, Key Encipherment
X509v3 Extended Key Usage:
TLS Web Server Authentication <--
Server/Device-CA -> Client-Cert
X509v3 extensions:
X509v3 Basic Constraints:
CA:FALSE
Netscape Cert Type:
SSL Client, S/MIME
Netscape Comment:
OpenSSL Generated Client Certificate
X509v3 Subject Key Identifier:
AD:BC:E0:73:50:AB:7F:BE:3C:43:71:4F:07:06:D8:3F:1A:38:81:4C
X509v3 Authority Key Identifier:
keyid:51:0F:8C:55:88:4D:3E:25:DC:EC:6D:73:39:E4:7D:27:2E:AF:E4:2D
X509v3 Key Usage: critical
Digital Signature, Non Repudiation, Key Encipherment
X509v3 Extended Key Usage:
TLS Web Client Authentication, E-mail Protection <--
Anyway when I use openssl verify it says both certificates were valid. I hope you can help me.
Thank you

Answer 1

A simple openssl verify does not know that the leaf certificate should be used as a client certificate. That's why it will not take the purpose of the certificates into account when validating the trust chain:
$ openssl verify -CAfile root.pem -untrusted middle.pem client.pem
client.pem: OK
With an explicitly given purpose of sslclient it fails instead:
$ openssl verify -purpose sslclient -CAfile root.pem -untrusted middle.pem client.pem
CN = middle
error 26 at 1 depth lookup: unsupported certificate purpose
error client.pem: verification failed
Having a different intermediate certificate which has sslclient as purpose to it instead succeeds:
$ openssl verify -purpose sslclient -CAfile root.pem -untrusted alt-middle.pem client.pem
client.pem: OK