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mod_tls: Enable Prosody's certificate checking for incoming s2s connections (fixes #1916) (thanks Damian, Zash) Various options in Prosody allow control over the behaviour of the certificate verification process For example, some deployments choose to allow falling back to traditional "dialback" authentication (XEP-0220), while others verify via DANE, hard-coded fingerprints, or other custom plugins. Implementing this flexibility requires us to override OpenSSL's default certificate verification, to allow Prosody to verify the certificate itself, apply custom policies and make decisions based on the outcome. To enable our custom logic, we have to suppress OpenSSL's default behaviour of aborting the connection with a TLS alert message. With LuaSec, this can be achieved by using the verifyext "lsec_continue" flag. We also need to use the lsec_ignore_purpose flag, because XMPP s2s uses server certificates as "client" certificates (for mutual TLS verification in outgoing s2s connections). Commit 99d2100d2918 moved these settings out of the defaults and into mod_s2s, because we only really need these changes for s2s, and they should be opt-in, rather than automatically applied to all TLS services we offer. That commit was incomplete, because it only added the flags for incoming direct TLS connections. StartTLS connections are handled by mod_tls, which was not applying the lsec_* flags. It previously worked because they were already in the defaults. This resulted in incoming s2s connections with "invalid" certificates being aborted early by OpenSSL, even if settings such as `s2s_secure_auth = false` or DANE were present in the config. Outgoing s2s connections inherit verify "none" from the defaults, which means OpenSSL will receive the cert but will not terminate the connection when it is deemed invalid. This means we don't need lsec_continue there, and we also don't need lsec_ignore_purpose (because the remote peer is a "server"). Wondering why we can't just use verify "none" for incoming s2s? It's because in that mode, OpenSSL won't request a certificate from the peer for incoming connections. Setting verify "peer" is how you ask OpenSSL to request a certificate from the client, but also what triggers its built-in verification.

module:set_global();

local cert_verify_identity = require "prosody.util.x509".verify_identity;
local log = module._log;

local measure_cert_statuses = module:metric("counter", "checked", "", "Certificate validation results",
	{ "chain"; "identity" })

module:hook("s2s-check-certificate", function(event)
	local session, host, cert = event.session, event.host, event.cert;
	local conn = session.conn;
	local log = session.log or log;

	local secure_hostname = conn.extra and conn.extra.secure_hostname;

	if not cert then
		log("warn", "No certificate provided by %s", host or "unknown host");
		return;
	end

	local chain_valid, errors = conn:ssl_peerverification();
	-- Is there any interest in printing out all/the number of errors here?
	if not chain_valid then
		log("debug", "certificate chain validation result: invalid");
		if type(errors) == "table" then
			for depth, t in pairs(errors) do
				log("debug", "certificate error(s) at depth %d: %s", depth-1, table.concat(t, ", "));
			end
		else
			log("debug", "certificate error: %s", errors);
		end
		session.cert_chain_status = "invalid";
		session.cert_chain_errors = errors;
	else
		log("debug", "certificate chain validation result: valid");
		session.cert_chain_status = "valid";

		-- We'll go ahead and verify the asserted identity if the
		-- connecting server specified one.
		if host then
			if cert_verify_identity(host, "xmpp-server", cert) then
				session.cert_identity_status = "valid"
			else
				session.cert_identity_status = "invalid"
			end
			log("debug", "certificate identity validation result: %s", session.cert_identity_status);
		end

		-- Check for DNSSEC-signed SRV hostname
		if secure_hostname and session.cert_identity_status ~= "valid" then
			if cert_verify_identity(secure_hostname, "xmpp-server", cert) then
				module:log("info", "Secure SRV name delegation %q -> %q", secure_hostname, host);
				session.cert_identity_status = "valid"
			end
		end
	end
	measure_cert_statuses:with_labels(session.cert_chain_status or "unknown", session.cert_identity_status or "unknown"):add(1);
end, 509);