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Date: Wed, 11 Aug 2021 09:51:04 +0200
From: Hanno Böck <>
Subject: Re: STARTTLS vulnerabilities

On Wed, 11 Aug 2021 08:09:57 +0100
Matthew Wild <> wrote:

> XMPP has some additional protections against this in its design. It is
> required, after TLS negotiation, for both parties to discard the
> pre-TLS XML stream and negotiate a new one after TLS has been
> established[1].

This is actually not much different from how STARTTLS works in SMTP or
IMAP. You are basically advised to throw away all state from pre-TLS.
But yet here we are with > 40 vulnerabilities.

The buffering issue is really subtle if you look at how such code is
written. It's basically "if you implement this with C API file
descriptors you will very likely create this bug *unless* you're aware
of it and actively avoid it". And I don't see how XMPP would be any
different here.

Hanno Böck

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