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I am trying to determine the impact of re-mediating a lucky 13 vulnerability; which i understand requires disabling CBC cipher modes. So far i have added custom logging to my IIS instance to capture and translate the algorithms that clients are connecting with.
Using this i am able to get the cipher, hash, and kex algorithms for each connection. However I was hoping for insight into the MODE (CBC,CTR,GCM). Am i able to derive the mode from the data i am capturing?

Some columns ommited for privacy.

  • SChannel (the library providing TLS support for IIS) was never vulnerable to Lucky 13. – Xander Aug 20 at 16:50
  • I see that would be why "potentially vulnerable" Is used. So then, am I to understand that it is totally reasonable to be running in CBC mode as long as you are not using the vulnerable libraries? Thanks @Xander – MaCuban Aug 21 at 20:36
  • They aren't ideal, which is why they're not allowed in TLS v1.3, but no, you're not going to be imminently vulnerable to an attack by allowing them. What I don't see there is logging CRYPT_PROTOCOL, which is going to be important for determining if you can turn off versions earlier than v1.2. Not only would that be the preferred option and have some real security value, but leaving CBC enabled is moot if you can't, since there are no AEAD ciphersuites supported by earlier versions. – Xander Aug 21 at 21:03

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