Re: Solution for man-in-the-browser

["Wall, Kevin" <Kevin.Wall () qwest com>]

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.

<...deleted...>

Jim Manico responded:
> 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 is absolutely correct. You are better off spending time removing
all the malware and securing your machines properly, trying to
educate your users, etc. You may also want to add AV scanning
during the web browsing sessions if you don't already support that.

Besides, once your browser is trojaned, there is no shared "secret", or more
accurately, you would also be "sharing" your secret with the malware
which obviously would not do you any good. Once the browser endpoint
is compromised, NOTHING sent from it can be trusted any longer. For
instance, since TLS provides only point-to-point encryption, malware
running in the browser can read plaintext and insert data at will.

Bottom line, don't waste your development $$ on a problem that cannot
be fixed in this manner.

-kevin
--
Kevin W. Wall   614.215.4788            Application Security Team / Qwest IT
"The most likely way for the world to be destroyed, most experts agree,
is by accident. That's where we come in; we're computer professionals.
We cause accidents."        -- Nathaniel Borenstein, co-creator of MIME

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