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I am using windows server 2008 R2 and I have lot of customers who use windows XP,7 and 8 with some of them still using IE6. So now I want to disable SSL3 and enable TLS 1.1 as a security measure for POODLE vulnerability. But I have some doubts on this.

1)Should I enable TLS 1.1 in IE or using registry as mentioned here

2)If i disable SSL 3 and enable TLS 1.1 then does my all customers who visit website do the same setting?

3)If I disable SSL 3 and enable TLS 1.1 then does customers using browsers IE6 or any browser (IE7,8 etc) with TLS 1.1 not enabled will be able to access my website? Will they get any problem? Because i have lot of users so i want to know what will happen due to these settings?

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  • Why you are talking about TLS1.1 and TLS1.2 only? Where is TLS1.0 (which is not affected by POODLE)? IE/XP will not support TLS1.1 but will support TLS1.0. – Steffen Ullrich Jan 10 '15 at 8:04
  • @SteffenUllrich In most of the article it was mentioned to use TLS 1.1 and TLS 1.2 isc.sans.edu/forums/diary/… – IT researcher Jan 10 '15 at 9:32
  • I think you should read the articles in more detail (or maybe with better understanding of what they are talking about). It talks about moving to TLS1.1+ in connection to the BEAST attack, which is much less practical then POODLE. And it talks about protecting against POODLE by disabling SSL3.0 which means to leave TLS1.0+. – Steffen Ullrich Jan 10 '15 at 9:52
  • @SteffenUllrich Ok. So your advice is to enable TLS 1.0 and above and disable SSL 3? – IT researcher Jan 10 '15 at 10:33
  • Exactly. This is the same advice as given in the article you referenced. – Steffen Ullrich Jan 10 '15 at 10:41
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Disabling SSL3.0 is one of the ways of mitigating POODLE attack. The other way is to apply the TLS_FALLBACK_SCSV patch. There are downsides for TLS_FALLBACK_SCSV as well:

  1. Microsoft doesn't support it yet. A bug has been raised in this regard: https://connect.microsoft.com/IE/feedback/details/1002874/internet-explorer-should-send-tls-fallback-scsv
  2. Both the SSL peers are required to support this cipher suite. i.e., even if your server supports it and the clients coming in dont, we're still vulnerable to POODLE attack.

Now since Microsoft doesnt support TLS_FALLBACK_SCSV yet, the only option for you is to disable SSL3.0. Lets come to the implications of doing this:

  1. The IE6 browser which doesnt support SSL versions greater than SSL3.0 will NOT be able to establish a SSL communication with your server.
  2. All other browsers will support TLS1.0 at least. Since you allow TLS1.0 in your server, they will be able to communicate using that.
  3. For clients advertising versions > TLS1.0, there's no problem at all(since you allow those versions as well)

The reason why SSL3.0 is vulnerable to POODLE is that when it uses CBC mode ciphers, the SSL design doesn't mention that after decrypting the blocks, the padding block needs to be validated. (why is this a problem?: https://www.imperialviolet.org/2014/10/14/poodle.html).

The reason why TLS is not vulnerable is because, it explicitly states that the padding block needs to be checked for a specific pattern. (OpenSSL and most other implementations of TLS fills the padding block with |length of padding block|).

Since this vulnerability exists only if the server and client agree upon a CBC mode cipher, another way to mitigate POODLE attack is to use non-CBC mode ciphers like RC4. But this is NOT recommended as RC-4 ciphers have their own list of weaknesses.

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