full-disclosure - Re: Security Problem with Google’s 2-Step Authentication

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Message-Id: <46201AB5-4214-4B1C-B04D-1B090F3A63B6@gmail.com>
Date: Mon, 30 Jul 2012 09:46:17 -0700
From: andfarm <andfarm@...il.com>
To: Pablo Ximenes <pablo@...en.es>
Cc: full-disclosure <full-disclosure@...ts.grok.org.uk>
Subject: Re: 
 Security Problem with Google’s 2-Step Authentication

On 2012-07-30, at 07:41, Pablo Ximenes <pablo@...en.es> wrote:
> I'd like to share with you one of my findings that failed to get
> Google's Security Reward. Although Google doesn't consider it a
> security problem, some might find it at least amusing if not
> interesting.

>>From the linked article, http://ximen.es/?p=653 -
> I found out they have a time window of 10 minutes in which any of the 20 OTP passwords are valid. [...] I have suggested invalidating all the time window (all the 20 OTPs) [when a user uses an OTP...]

Invalidating the entire window would make you unable to authenticate using OTP more than once every 10 minutes. In any case, I'm having a hard time imagining what sort of threat model which make this necessary -- if you can somehow predict a user's OTP code for some point in the future, you could go ahead and predict one that's even further in the future (outside the window of invalidated keys), and use it when that time arrives.

> or at least they could synchronize accounts.google.com’s watch with the user’s at some point, like some banks do.

Current versions of Google Authenticator have an option to do exactly this. The 10-minute window seems kind of wide; I'd imagine that it was introduced before the time sync option was available, for compatibility with devices that are on cell networks with bad time servers.
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