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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