I have been reading about Kerberos, and I understand the messages exchanged, and that it uses symmetric encryption in order to protect some of the messages. But, how does it protect against replay attacks? For example, the first message from client to KDC is not encrypted as long as I know. Even if something is encrypted, we can still replay it later. Plus I read that the clocks in Kerberos can have at most ~5 min difference. So, I guess server also keeps in cache some of the tickets received, in order to prevent from similar attacks, right?

1 Answer 1


Kerberos uses an 'authenticator' during protocol exchanges that occur between the client and the server. It holds additional authentication data, such as the ticket lifetime, and most important, the client's timestamp.

When Server-Side Kerberos validates an authentication message, it will check the authenticator's timestamp. If the timestamp is earlier or the same as a previous authenticators received within the five minutes, it will reject the packet because it treats it as a replay attack and user authentication will fail.

Server-side Kerberos additionally compares the timestamp in the authenticator to the server time. If the timestamp in the authenticator isn't within five minutes of the time on the server, it will also reject the packet.

The five minutes is the maximum difference it can tolerate between client and server, but you can change this value in windows through group policy.

In a nutshell, the crucial part is the 'authenticator' time validation.

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    Thanks for the answer. But, as long as I remember the first message from the client to the KDC does not use any authenticator, and is not encrypted. Can't it be replayed?
    – typos
    Commented Nov 15, 2016 at 18:37
  • 2
    Yes first message is plaintext. Kerberos is designed with the notion that it's authenticating in open traffic. The first message only checks that the login id exists in the KDC. Does not validate any other credentials details. Replay attack in this part doesn't really affect anything.
    – Lester T.
    Commented Nov 16, 2016 at 0:15

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