Regarding Pubkey Enumeration

[Markus Friedl]

after twelve years, i still think that always sending
  SSH2_MSG_USERAUTH_PK_OK
for users that don't have an account is the only useful option.
back then i decided against this, because it adds complexity
and wastes CPU cycles.

sending PK_OK for uses with an account is evil as it
would force them typing the passphrase for public
keys that the server does not accept at all.

-m


On Fri, Jan 20, 2012 at 10:18 AM, Dan Kaminsky <dan at doxpara.com> wrote:
> HD Moore from MetaSploit has noted that, given a pubkey (and not the
> corresponding private key, as might be found in authorized_keys), he can
> determine if he'd be able to log into an account.
>
> It's a small thing, but he's using it for very interesting
> recon/deanonymization.  He'll be releasing a paper shortly, not overplaying
> the characteristic, but certainly showing it can be used to do cute things.
>
> I expect this is easily fixable -- simply provide the challenge for a
> pubkey whether or not it'd actually be able to log in successfully.  But
> it's worth exploring this space -- perhaps some clients behave badly.
>
> --Dan
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