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In http://www.freebsd.org/doc/handbook/kerberos5.html section 15.7.8.3 “The KDC is a Single Point of Failure” you can read:

By design, the KDC must be as secure as the master password database is contained on it. The KDC should have absolutely no other services running on it and should be physically secured. The danger is high because Kerberos stores all passwords encrypted with the same key (the “master” key), which in turn is stored as a file on the KDC.

Doesn't that mean that this is an Achilles' heel of Kerberos since if the KDC is compromised, the attacker will know all users' plaintext passwords (as opposed to getting /etc/shadow of a Linux system which will only provide the attacker with the hashed and salted passwords)?

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up vote 5 down vote accepted

The short answer is: no.

It knows a secret key that may be derived from the user password. The specification RFC 1510 says in the introduction of section 6:

It is desirable for the string to key function to be one-way, and for the mapping to be different in different realms. This is important because users who are registered in more than one realm will often use the same password in each, and it is desirable that an attacker compromising the Kerberos server in one realm not obtain or derive the user's key in another.

Note that there is no random salt unique for every account, just the realm name which is shared among all users of the same realm.

The updated version RFC 4120 allows other way to derive the key:

The user key might be stored on a smart-card, or otherwise obtained independently of a password.

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Thank you for the clarification. If the password database is stolen is it then possible to login using the stored user keys (e.g. by using a modified version of kinit that bypasses the generation of the key from the password and the realm)? –  mgd Jun 9 '12 at 18:31
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