Re: Solution for man-in-the-browser

[James Manico <jim () manico net>]

I do not think this will work. Once your browser is trojaned, it's
game over. The Trojan has the capability to just sit in your browser
and wait for the user to log in. (Trojans do not need to steal
credentials to cause harm). Once the user has logged on, the Trojan
can simulate any user activity such as requesting and submitting
forms, circumventing CSRF tokens and other web app defenses.

-Jim Manico
http://manico.net

On Sep 10, 2010, at 5:34 PM, smurray1 <smurray1 () nycap rr com> wrote:

> Hello,
>
> I have been discussing an issue with an organization that is having an issue with malware on it's customer's clients 
> that is intercepting user credentials and using them to create fraudulent transactions.  (man-in-the-browser type 
> attacks similar to what Zeus and other trojans are capable of).  I told them that they should be using a shared 
> secret between the user and the system to use as an input to HMAC to create a MAC for the form for the critical 
> transaction.
> I see it working like this.  The form that is used for the critical transaction would have either a java object or 
> javascript that, after the user fills the field and the presses the "submit" button:
> 1) Accepts  a single use shared secret from the user.
> 2) Uses that as the "key" input for an HMAC that is run over the concatenation of all the values  for each field in 
> the form (that would include a time stamp or nonce).
> 3) Uses the result of the HMAC to fill a hidden field on the form. 4) Submits the form
>
> The server receives the form, uses the shared secret as the "key" input for the HMAC and validates the result is the 
> same as the hidden field.  If they do not match the transaction is rejected.
> I understand the there is issues of the HMAC strength due to the limited size of the shared secret, and  that there 
> is the ugly problem of distributing the single-use shared secret to the user.
>
> The question is will this work?  If an attacker steals the credentials, they still could not submit a transaction 
> without knowing the shared secret, which never goes over the wire.  If course it is not a perfect solution, as the 
> malware could use a key logger to grab the shared secret when the user inputs it and block the users submittal and 
> then use the stolen shared secret to create a MAC for  a transaction of it's own.  But that would require custom 
> written malware and/or the attacker waiting for input from the malware.
> Any input and/or suggestions would be appreciated.
> Thanks in advance,
>
> Sean T Murray
>
>
>
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