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Disabling RC4 HMAC encryption in Windows Active Directory prevents current Kerberos attacks?

I understand that RC4 HMAC encryption is dangerous in Windows Active Directory, since it relies on the user's NT hash as the encryption key for requesting a TGT ticket.

The recommendation seems to be to upgrade from RC4 to AES. But why is this more secure?

Indeed, AES is stronger encryption than RC4, but can't the attacker still request a TGT on behalf of a specific user by obtaining the user's NT hash?

Please correct me, if some details are wrong.

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You would have to ban RC4 entirely to be protected. This is possible and it would prevent an attacker from requesting a TGT with an RC4 request. This is a global setting you apply to DCs. One thing to note is that most users do end up using AES eventually (unless it's explicitly disabled) as Windows/Active Directory will optimistically enable it as it determines that users can support it.

RC4 is weak for a number of reasons, but it's a weakness of the crypto system and not just the algorithm.

  • RC4 is just horribly broken
  • RC4 key derivation has no salt, so it's relatively trivial to brute force
  • The integrity protection is an MD5-based HMAC

AES is preferred because it uses salts during key derivation, and uses a stronger integrity protection algorithm (SHA1 HMAC).

  • but does AES prevent anything related to an attacker dumping NT hashes and start using PtH and PtT? Your answer doesn't really answer this part. In particular, is the NT hash used for AES key derivation? – Shuzheng Jan 4 at 16:21
  • Short answer: no. The AES key is derived from the raw password. – Steve Jan 4 at 16:46

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