Likelihood of brute force attacks against web apps

[Stephen de Vries <stephen () corsaire com>]

Hi list,

We frequently warn clients of the risks of brute force or automated 
attacks against their sites and recommend the use of CAPTCHA 
(www.captcha.net) systems, or "secret questions" to mitigate this risk. 
 For example:

- The registration process does not use captcha like systems and could 
allow attackers to use an automated script to generate thousands of 
fictitious users.  These users can then be used to perform transactions 
that could lead to financial loss (e.g. costs due to rejected credit 
card), or waste resources through database access (and wasted storage).

- The password reminder mechanism requires only the user's email 
address which is used to send them an email for resetting their 
password.  But since it requires no further auth from the user (such as 
an answer to a secret question), an attacker could enumerate all the 
valid email addresses registered to the site by writing a brute force 
script and using a database of email addresses (this may be 
particularly useful to a competitor).  Of course, if the email address 
is used as the username, this problem becomes more serious.

Although these risks are real - and I don't doubt they will be used in 
the future - I'm not aware of any attacks of this sort being  conducted 
in the past.  Is anyone aware of these types of attacks in real-world 
scenarios?  Do you think these pose a serious threat?

regards,
Stephen


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