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Message-ID: <661603D4.4020908@gmail.com>
Date: Tue, 09 Apr 2024 22:13:24 -0500
From: Jacob Bachmeyer <jcb62281@...il.com>
To: oss-security@...ts.openwall.com
Subject: Re: xz backdoor prevention using hosts.deny?
Andres Freund wrote:
> Hi,
>
> On 2024-04-08 23:00:48 -0500, Jacob Bachmeyer wrote:
>
>> I am not so sure about this. The original discovery of this backdoor
>> observed a slowdown in refusing a session for a nonexistent account using
>> only SSH publickey auth, *not* SSH certificate auth. Reports have also
>> suggested that testing began after common botnet scans were observed to be
>> causing sshd to use an inordinate amount of CPU time. I doubt botnets are
>> presenting certificates either.
>>
>> I am unsure how sshd would call RSA_public_decrypt in those situations,
>> which suggests that the backdoor blob is more complex than we currently
>> think.
>>
>
> The slow part of the backdoor is *not* when actually calling
> RSA_public_decrypt(). It's the "initialization" of the backdoor, which happens
> very early during sshd startup, well before reaching main(). Note how, in the
> initial report, even "sshd -h" is slow if called in the right environment.
>
Yes, but the initial report also gave timings for "ssh
nonexistent@...alhost", see the quote below.
> https://www.openwall.com/lists/oss-security/2024/03/29/4 :
>
>> In fact, openssh does not need to be started as a server to observe the
>> slowdown:
>>
>> slow:
>> env -i LANG=C /usr/sbin/sshd -h
>>
Also from <URL:https://www.openwall.com/lists/oss-security/2024/03/29/4>:
> == Observing Impact on openssh server ==
>
> With the backdoored liblzma installed, logins via ssh become a lot slower.
>
> time ssh nonexistant@...alhost
>
> before:
> nonexistant@...alhost: Permission denied (publickey).
>
> before:
> real 0m0.299s
> user 0m0.202s
> sys 0m0.006s
>
> after:
> nonexistant@...alhost: Permission denied (publickey).
>
> real 0m0.807s
> user 0m0.202s
> sys 0m0.006s
This seems to indicate that sshd takes more than twice as long to reject
a login (using SSH publickey auth, /not/ a certificate) to a
non-existent account with the backdoor active.
>> In fact, I would expect sshd to reject the connection without ever
>> attempting to verify a signature if the requested account does not exist,
>> yet a significant delay in that rejection led to the discovery of the
>> backdoor.
>>
>
> See https://www.openwall.com/lists/oss-security/2024/03/30/37 for the path
> leading to certification validation before certificate validity, users, etc
> are checked.
>
Yes, but that is for logins where the client presents a certificate,
which I understand that your client did not, only requesting "publickey"
auth. How does the backdoor have such a large effect in that scenario?
-- Jacob
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