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Date: Thu, 2 Feb 2023 22:38:23 +0000
From: Qualys Security Advisory <>
To: Georgi Guninski <>
CC: "" <>
Subject: Re: double-free vulnerability in OpenSSH server 9.1

Hi Georgi, all,

On Thu, Feb 02, 2023 at 09:06:19PM +0200, Georgi Guninski wrote:
> Nice find :)
> This is very complicated codepath, did a human found it "manually"
> or some analysis program found it?

Good question! Technically, we did not find the double free: we found
the underlying bug in compat_kex_proposal() (the "unintended" free of
options.kex_algorithms) during a manual code review, and reported it to
the OpenSSH developers in July 2022.

Unfortunately, back then we (Qualys) mistakenly believed that "this does
not seem to lead to a use-after-free or double-free, but the dangling
pointer in options.kex_algorithms is probably not ideal."

Then, in January 2023, Mantas Mikulenas reported a double free in sshd
to the OpenSSH bugzilla, and we immediately realized that this was a
direct consequence of the bug in compat_kex_proposal().

Thank you very much for your mail! With best regards,

the Qualys Security Advisory team

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