Hi,
After a bit of struggling I've been able to set up TLS client certificate authentication with Dovecot for both IMAP and Submission. Users are required to present a valid certificate, cool so far!
What I noticed however on non-StartTLS listeners IMAPS (993) and SMTPS (465), is that during the TLS handhake presenting a valid client certificate is seemingly optional and is only checked later at time of protocol-level login (external auth in my case). I'd like to change that for security purposes and also a bit of obscurity. Not having the ability to interact with Dovecot on protocol level lowers the attack vector as well as the ease of checking my network security. Also, it would prevent (anonymous) network scanners to easily detect what kind of service is running on the port as they can see the IMAP/SMTP banner without presenting a valid TLS client certificate currently.
Is it possible in any version of Dovecot to configure it to set up a TLS server listening context that requires a handshake with a valid client certificate?
I'm using Dovecot 2.3.4.1 (actual Debian Buster version 2.3.4.1-5+deb10u6), but willing to upgrade to any newer version when this is offered. Going through some newer revisions changelog this seems not the case, so I didn't spend time on upgrading yet.
Relevant config snippets for IMAP authenticating proxy that I use (something similar for Submission):
# Opens 143 on StartTLS and 993 in wrapped TLS-only mode. # Only 993 is exposed to the internet. protocols = imap passdb { driver = static args = proxy=y host=10.1.2.3 port=1143 pass=masterpass nopassword=y } auth_username_format = %n ssl = required ssl_cert =
Current behaviour from a client when deliberately not presenting a client certificate shows the IMAP banner: $ openssl s_client -connect myserver:993 CONNECTED(00000003)
Certificate chain 0 [...]
Server certificate -----BEGIN CERTIFICATE----- [...]
Acceptable client certificate CA names C = NL, O = Gert van Dijk, [...]
SSL handshake has read 1096 bytes and written 413 bytes Verification: OK
New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384 Server public key is 384 bit Secure Renegotiation IS NOT supported Compression: NONE Expansion: NONE No ALPN negotiated Early data was not sent Verify return code: 0 (ok)
- OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE LITERAL+ LOGINDISABLED AUTH=EXTERNAL] Dovecot (Debian) ready.
Expected: TLS alert during handshake, e.g. on my Apache TLS client certificate required configured instance: $ openssl s_client -connect myserver:443 139676785239360:error:14094410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure:../ssl/record/rec_layer_s3.c:1543:SSL alert number 40
IOW, I was hoping for something like this to exist: ssl_verify_client_cert = required OR service imap-login { inet_listener imaps { port = 993 ssl = client-cert-required } }
Thanks,
Gert