How would dialback-over-TLS be "more vulnerable to MITM" though? I think that claim was what led to the confusion, I don't see how TLS-with-client-EKU is more secure then TLS-with-dialback
Firstly, nobody is actually calling for authentication using client certificates. We use "normal" server certificates and validate the usual way, the only difference is that such a certificate may be presented on the "client" side of a connection when the connection is between two servers.
The statement that dialback is generally more susceptible to MITM is based on the premise that it is easier to MITM a single victim XMPP server (e.g. hijack its DNS queries or install an intercepting proxy somewhere on the path between the two servers) than it is to do the same attack to Let's Encrypt, which has various additional protections such as performing verification from multiple vantage points, always using DNSSEC, etc.
If an attacker gets a misissued cert not through BGP or DNS hijacks, but by exploiting a domain validation flaw in a CA (e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=2011713) then it's trivial for them to use it as a client certificate, even if you're requiring the serverAuth EKU. On the other hand, dialback over TLS would require the attacker to also MitM the connection between XMPP servers, which is a higher bar.
The good news is that since Prosody requires the serverAuth EKU, the misissued cert would be in-scope of Mozilla's root program, so if it's discovered, Mozilla would require an incident report and potentially distrust the CA. But that's reactive, not proactive.
Firstly, nobody is actually calling for authentication using client certificates. We use "normal" server certificates and validate the usual way
I'm not sure I understand this point. You authenticate the data you receive using the client's certificate. How is that "nobody is calling for authentication using client certificates"? Maybe there's some nuance I'm missing here but if you're authenticating the data you're receiving based on the client's certificate, then how is that "validating the usual way"?
MattJ100|20 days ago
The statement that dialback is generally more susceptible to MITM is based on the premise that it is easier to MITM a single victim XMPP server (e.g. hijack its DNS queries or install an intercepting proxy somewhere on the path between the two servers) than it is to do the same attack to Let's Encrypt, which has various additional protections such as performing verification from multiple vantage points, always using DNSSEC, etc.
agwa|19 days ago
The good news is that since Prosody requires the serverAuth EKU, the misissued cert would be in-scope of Mozilla's root program, so if it's discovered, Mozilla would require an incident report and potentially distrust the CA. But that's reactive, not proactive.
nightpool|19 days ago