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Date: Wed, 11 Aug 2021 18:02:35 +0200
From: Hanno Böck <>
Subject: Re: STARTTLS vulnerabilities

On Wed, 11 Aug 2021 10:31:58 -0500
Eric Blake <> wrote:

> Not mentioned in that list was ndb, but as far as I can tell, that
> project has already documented the ramifications of opportunistic
> encryption as being a security risk, and all known implementations
> (both servers and clients) with TLS support have a mode of execution
> that ensures the connection is dropped if a downgrade attack is
> attempted:

I should point out that our research is not on simple downgrade attacks.
These are kinda obvious by the design of STARTTLS if you implement it
in an opportunistic way.

The buffering vulnerabilities we found are in STARTTLS implementations
that have the expectation to enforce a secure connection, but suffer
from various vulnerabilities in the implementation.

Hanno Böck

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