On Tue, 24 Apr 2007 11:10:27 +0200
Stanislaw Klekot <dozzie@dynamit.im.pwr.wroc.pl> wrote:
On Sat, Apr 21, 2007 at 02:27:17AM +0200, rembrandt wrote:
As you can see clearly OpenSSH discloses the existence of system accounts.
A possible solution for this problem would be to print a fake S/Key-Request
even for non existing users as well as it`s done with the
Passwordauthentication.
This issue is known not only for S/Key, but for OPIE (PAM version) as
well, although it's a bit different for the latter.
Look closer to challenge message. There's salt and key number included.
Consider now three logins: first isn't valid account on the target
system, second is valid but without OTP set, and third with OTP set.
First two are indistinguishable for attacker as in these cases system
presents random challenge, but for third account system will present the
same challenge over and over again.
How about that?
--
Stanislaw Klekot
Dear Stanislaw,
I know that the issue is not related to S/Key only but I had reasons to
just write about S/KEY so far.
I did not played with PAM because I`ve no OS wich supports it.
But your example should also allow to determine existing user accounts
because it`s pretty familiar with the S/KEY issue.
There`s propably only just one solution to solve such issues.
The OS has to present the uniq Challenges even for non existing user.
By this I mean the OS has to fake realy everything and also has to
decrese the specific values (like a user logged in).
During a specific amount of time (lets take 2 weeks as default) the OS
could fake the Challange requests this way:
opt-md5 97 some12345
(f.e. 2 hours later)
opt-md5 96 some12345
(f.e. 13 days later)
opt-md5 2 some12345
I hope you get the idea.
The OS does not have to store any Hashs anywhere.
In fact those things could get faked:
opt-&ALGO <- just choose some, it just has to be the same for a while
Count <- Well just decrese it during a specific period so that it isn`t
always the same nor does jump from f.e. 100 to 44 if a
attacker checks for this account twice a day.
Seed <- Could get handled like the &ALGO, so it just has to be the same
for a specific time.
I`m pretty sure these things can get done and also that it should be
familiar for PAM/OPIE or any other Challange-Response system.
I hope I answered to everything you wanted to know. :-)
Kind regards,
Rembrandt
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