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Re: OpenSSH & S/Key information leakage
From: flaps () dgp toronto edu (Alan J Rosenthal)
Date: Thu, 15 Nov 2001 17:12:26 -0500

These observations of information leakage are cute, and while some of them
might be fixable, I'm not sure that most of them are, which is dismaying.
If OPIE didn't tell you the password number, for example, it would be quite
hard to use.

Anyway, I was expecting to see the following in that list.  OPIE will tell
you whether or not a given account exists.  This is a disclosure we often
take pains to avoid on the internet these days; we expect the same error
message for 'no such account' and 'bad password'.

A login prompt for a non-account looks like this:

        login: flomp
        otp-md5 175 at2078 ext
        Response: 

So far, so good.  But press return once or twice to get "Login incorrect"
(or make a new conection), and then do

        login: flomp
        otp-md5 220 at0624 ext
        Response: 

Either the user just set a new passphrase in this one-second interval, or
"flomp" does not exist.

Compare:

        login: flaps
        otp-md5 796 qz1234 ext
        Response: 
        Response: 
        Login incorrect
        login: flaps
        otp-md5 796 qz1234 ext
        Response: 
        Response:

Always 796 qz1234 (until I next log in using OPIE and/or re-set my passphrase).

Joel Maslak <jmaslak () antelope net> writes:
- If S/Key passwords are used at all, "fake" challenge strings should be
printed whenever a real challenge string is not available.  OPIE does this
right.

I claim that what OPIE does is inadequate, but I don't have a solution
(thus this is not a criticism of OPIE).


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