Re: [WEB SECURITY] Universal XSS with PDF files: highly dangerous

[Amit Klein <aksecurity@xxxxxxxxx>]

James Landis wrote:
> I never said your solution was WEAK, just that is is WEAKER. 
I think you're missing my point. An algorithm with one-use token (with 
no IP binding) is the weaker algorithm. Here's an attack against it:

Attacker sends the user a link to http://www.attacker.site/script.cgi


When the user requests http://www.attacker.site/script.cgi, the 
script.cgi requests file.pdf from vuln.site. It gets back a redirection 
URL and a session cookie/one-use-cookie. Then, it creates a Flash object 
that requests the URL with an injected Cookie header (with the session 
cookie/one-use-cookie) and serves this to the victim client. Voila.


> Yes, it is still theoretically very difficult to construct valid 
> tokens by breaking the encryption algorithm, 
You mean, it's as difficult as, say, breaking the streaming encryption 
of SSL... (if we implement with say AES based algo) the de-facto 
standard, considered pretty secure.
> but it is certainly weaker to expose reconstructable values to the 
> client than to expose only a random value from the same keyspace. Also 
> note the class of insider attacks which involve gaining the key and 
> algorithm without reversing it.
An insider can probably manipulate the server in this case, which is 
end-game. He can also probably steal the SSL private key, etc.
> In addition, if the server is using a published algorithm, the 
> attacker can also mount chosen-key attacks as well.
Huh? cipher algorithms are usually public. And I don't get the "chosen 
key attack"? the whole idea is that the key is secret, maintained on the 
server. If the attacker chooses this key, then the game is over...
> In practice, for both methods it is probably easier to attack the 
> client by forcing them to use a valid generated token via one of the 
> attack schemes already discussed.
What attack methods do you refer to, which are applicable to my algorithm?

-Amit

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