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Re: Google Accounts Security Vulnerability
From: "Michael J. Gray" <mgray () emitcode com>
Date: Wed, 16 May 2012 17:12:43 -0700

The point of my article is to specifically show that Google has a system in place which gives the perception of a 
particular type of security; that is if their password happens to be compromised, that the attack will be limited 
unless the attacker has very specific knowledge about the user and their account. This being circumvented renders the 
system useless, especially if it's able to be bypassed on an individual basis as I had described in my article.

Google's idea seems to be that users shouldn't have to keep track of many passwords. But, how does this risk assessment 
system really help? The way I see it, Google requires you to simply provide other information in addition to a 
password, which could be seen as passwords themselves. If an account requires two passwords, this just increases the 
burden on the user. The best solution would be to use two factor authentication. You would want to combine something 
you know with something you have instead of something you know with something many people probably know. If someone has 
my password, they probably have my phone number and other information or can manually stop their attack and find the 
information or compile a list of challenge questions and their appropriate responses.

As far as it being a solution to naïve attacks, sure it probably works fine. However, it's not a difficult task for any 
serious attacker with a botnet doing the automation.

And for your comment about analyzing it as if it were a replacement... well, it kind of is what Google has done with 
it. It prompts you for a second password which is based on a variable context. It adds no security but gives the user 
peace of mind and helps them believe there is added security when there really isn't. That alone could be more harmful 
than the attackers having to do a simple upgrade.

A more reasonable way to prevent botnet driven attackers with lots of valid credentials would be to use two factor 
authentication. 

-----Original Message-----
From: Mike Hearn [mailto:hearn () google com] 
Sent: Wednesday, May 16, 2012 1:38 PM
To: full-disclosure () lists grok org uk
Cc: mgray () emitcode com
Subject: Re: Google Accounts Security Vulnerability

Hi there full-disclosure,

I wanted to respond to the recent post covering the Google real time anti-hijacking system and explain a bit more about 
what this system is and how it works. For background I am the tech lead of the relevant team, and Daniel Margolis works 
on it with me.

Firstly, I'd like to note that despite what Michael may have observed with his account, performing a programmatic login 
does not whitelist for web access. Most of the time if you would be challenged via the web then logging in via POP or 
IMAP would also be denied, and result in a notification email about the blocked login. See here for what this looks 
like:

  http://blog.plaxo.com/2012/05/google-account-%E2%80%9Csuspicious-activity%E2%80%9D-next-steps/

There are a small number of edge cases that can cause this rule to break. Unfortunately although Daniel asked for it, 
Michael has not provided the name of the account in question so we cannot check which one it was. To understand why 
this is not a problem it's important to understand the design parameters of this security system.

The real-time antihijack system was created to solve a specific problem, namely, spammers/scammers turning up at our 
front door with large numbers of valid passwords. I gave a public talk at the RIPE64 conference last month which 
provides some background:


   https://ripe64.ripe.net/archives/video/25/
   https://ripe64.ripe.net/presentations/48-AbuseAtScale.pdf  (slides)

Executive summary: it is no longer unexpected for individual attackers to own on the order of a million valid 
passwords. These passwords are taken from compromised websites and the hashes reversed using GPUs. We have in the past 
seen known attackers correctly authenticate to over
30 accounts per second and this problem is structural - it's isn't going to go away any time soon.

For this reason we now perform a risk analysis of every login and if we suspect it may not be the real owner of the 
account, redirect it to identity verification. This is what Michael saw.

The primary design principle of the system is to move all our users into the post-password age as gently as possible. 
The threat model covers attacks that operate at scale and who do not care about the specific accounts they work with. 
We provide things like 2-step verification, which authenticates you via a device or phone, for handling the stronger 
threat model of a highly motivated adversary against a specific highly motivated defender.

One outcome of this threat model is that if we can protect 95% of accounts from an attacker, that's good enough because 
it renders their attack uneconomic and they go away. See this paper from Microsoft
Research:

  http://research.microsoft.com/pubs/149885/wheredoalltheattacksgo.pdf

For this reason the system will usually fail open if there is a problem of some kind. An example of what can cause the 
type of behavior Michael saw:  if there the risk analysis subsystem misses its deadline the login processing servers 
will proceed without it.
Timeouts are rare but can occasionally happen. There are other cases involving specific types of account history and IP 
address combinations that could cause what Michael observed. Or there could be a bug :-)

It's best to view the risk analysis / id verification system as more like a spam filter than a hard-guarantee security 
system. It relies heavily on security through obscurity and exploiting weaknesses of very specific opponents, against 
which it has proven very effective.
Analyzing it as if it were a complete replacement for password security will lead only to disappointment.

thanks
/mike

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