weird certificate issue on libera.chat

[c-cube]

Hi, I've had issues connecting some IRC bots recently from ocaml-irc-client. Trying the stelnet example on it is interesting:

$ dune exec examples/stelnet.exe -- irc.libera.chat -p 6697
SSL connection ok.   
Certificate issuer:  /C=US/O=Let's Encrypt/CN=R3
subject: /CN=strontium.libera.chat
Cipher: TLS_AES_256_GCM_SHA384 (TLSv1.3)
TLS_AES_256_GCM_SHA384  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(256) Mac=AEAD

Type 'exit' to quit.

:strontium.libera.chat NOTICE * :*** Checking Ident
:strontium.libera.chat NOTICE * :*** Looking up your hostname...

but with paranoid:

$ dune exec examples/stelnet.exe -- irc.libera.chat -p 6697 -w
Certificate[2] subject=/C=US/O=Internet Security Research Group/CN=ISRG Root X1
Certificate[2] issuer =/O=Digital Signature Trust Co./CN=DST Root CA X3
SSL: rejecting connection - error=20
Fatal error: exception SSL connection() error: error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed

[craff]

I confirm I can reproduce.

May be this is related to Let's encrypt certificate not considered secure enough ?

[Firgeis]

This is the same output from ssl (openssl s_client -connect irc.libera.chat:6697):

CONNECTED(00000003)
depth=1 C = US, O = Let's Encrypt, CN = R3
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 CN = mercury.libera.chat
verify return:1
---
Certificate chain
 0 s:CN = mercury.libera.chat
   i:C = US, O = Let's Encrypt, CN = R3
   a:PKEY: rsaEncryption, 4096 (bit); sigalg: RSA-SHA256
   v:NotBefore: Jul 24 06:40:56 2023 GMT; NotAfter: Oct 22 06:40:55 2023 GMT
 1 s:C = US, O = Let's Encrypt, CN = R3
   i:C = US, O = Internet Security Research Group, CN = ISRG Root X1
   a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA256
   v:NotBefore: Sep  4 00:00:00 2020 GMT; NotAfter: Sep 15 16:00:00 2025 GMT

I believe this is because of the cross-signed certificate it is using which is not entirely supported by OpenSSL, see:
https://security.stackexchange.com/questions/133221/how-to-properly-create-and-use-cross-signed-cas-and-certificates

One solution off the top of my head would be to eliminate one of the root certs from your trust store so it can build the certificate chain properly.