4.1 Environmental Assumptions
Kerberos imposes a few assumptions on the environment in which it can
properly function:
- "Denial of service" attacks are not solved with Kerberos. There
are places in the protocols where an intruder can prevent an
application from participating in the proper authentication steps.
Detection and solution of such attacks (some of which can appear
to be not-uncommon "normal" failure modes for the system) is
usually best left to the human administrators and users.
- Principals MUST keep their secret keys secret. If an intruder
somehow steals a principal’s key, it will be able to masquerade as
that principal or impersonate any server to the legitimate
principal.
- "Password guessing" attacks are not solved by Kerberos. If a user
chooses a poor password, it is possible for an attacker to
successfully mount an offline dictionary attack by repeatedly
attempting to decrypt, with successive entries from a dictionary,
messages obtained which are encrypted under a key derived from the
user’s password.
- Each host on the network MUST have a clock which is "loosely
synchronized" to the time of the other hosts; this synchronization
is used to reduce the bookkeeping needs of application servers
when they do replay detection. The degree of "looseness" can be
configured on a per-server basis, but is typically on the order of
5 minutes. If the clocks are synchronized over the network, the
clock synchronization protocol MUST itself be secured from network
attackers.
- Principal identifiers are not recycled on a short-term basis. A
typical mode of access control will use access control lists
(ACLs) to grant permissions to particular principals. If a stale
ACL entry remains for a deleted principal and the principal
identifier is reused, the new principal will inherit rights
specified in the stale ACL entry. By not re-using principal
identifiers, the danger of inadvertent access is removed.