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From Stefan Eissing <>
Subject Re: APLN negotiation and TLS cross-protocols attack
Date Thu, 10 Jun 2021 11:35:44 GMT

> Am 10.06.2021 um 13:33 schrieb Eric Covener <>:
> On Thu, Jun 10, 2021 at 7:28 AM Stefan Eissing
> <> wrote:
>> In short: there is the possibility of a middle-man tricking a client into accepting
the response from another TLS server, if it uses the same certificate. This seems to be in
the open, so we can talk about it here.
>> People think about how to prevent this and enforce stricter ALPN negotiation. But
it is hairy since ALPN has been deployed for over 5 years. Breakage may ensue.
>> Our server is also 'relaxed' about this. If you look at ssl_engine_kernel.c#2760,
if no protocol overlap was found, we continue the handshake as if no ALPN was supplied at
>> I guess we can always add a "SSLStrictALPN on", but let's see where this discussion
> Strict HTTP parsing goes a long way too. Hard to respond in an
> exploitable way to a misdirected client who is speaking a different
> protocol.

Yes. However if your SMTP server uses the same TLS cert as your https: server, you can feed
our error pages to a SMTP client as well, as I understand it.

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