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I trying to perform POODLE attack on a hosted server that I've configured to support only SSLv3. The scenario is I logged on my server and it sends the cookie in response that will be stored by the client. I need not be in middle since I'm the attacker and the victim. So to get the decrypted cookie I have to craft a POST request and replay it to my server. The crafted request will be

POST Cookie: name="cookie value" "\r\n\r\n"body "MAC" padding bytes

So, how to get length of the cookie value since it is encrypted and it is required to add the padding bytes ?

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The Poodle attack model is one where the attacker triggers the requests (normally with some Javascript in the client -- note that, in that case, requests will be GET, not POST, since that "evil Javascript" will be served as part of an unprotected Web page load from another site, and the same-origin policy will prevent arbitrary POST) and manipulates the resulting on-the-wire packets. The two parts of the attacker (the insider and the outsider) dynamically talk to each other.

Therefore, you can organize some initial discovery procedure in which the insider first triggers some requests with paths of increasing lengths, and the outsider observes at which point the encrypted request length increases by a full block (8 bytes for 3DES or RC2, 16 bytes for AES).

Of course, in practice, the attacker knows the length of the cookie of the target site, because he registered his own account to see what to expect. In many cases, the length for an authentication cookie is constant. Such a preparation step is needed for the attacker to reliably know at which point the cookie starts in the requests, anyway.

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  • Correct ! The request will be GET when you will use JavaScript, but I'm acting as a both both attacker and server.In this case I'm manually sending the request to server using POST.
    – ifexploit
    Oct 21, 2014 at 3:27

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