MIT Kerberos Documentation

Account lockout

As of release 1.8, the KDC can be configured to lock out principals after a number of failed authentication attempts within a period of time. Account lockout can make it more difficult to attack a principal’s password by brute force, but also makes it easy for an attacker to deny access to a principal.

Configuring account lockout

Account lockout only works for principals with the +requires_preauth flag set. Without this flag, the KDC cannot know whether or not a client successfully decrypted the ticket it issued. It is also important to set the -allow_svr flag on a principal to protect its password from an off-line dictionary attack through a TGS request. You can set these flags on a principal with kadmin as follows:

kadmin: modprinc +requires_preauth -allow_svr PRINCNAME

Account lockout parameters are configured via policy objects. There may be an existing policy associated with user principals (such as the “default” policy), or you may need to create a new one and associate it with each user principal.

The policy parameters related to account lockout are:

Here is an example of setting these parameters on a new policy and associating it with a principal:

kadmin: addpol -maxfailure 10 -failurecountinterval 180
    -lockoutduration 60 lockout_policy
kadmin: modprinc -policy lockout_policy PRINCNAME

Testing account lockout

To test that account lockout is working, try authenticating as the principal (hopefully not one that might be in use) multiple times with the wrong password. For instance, if maxfailure is set to 2, you might see:

$ kinit user
Password for user@KRBTEST.COM:
kinit: Password incorrect while getting initial credentials
$ kinit user
Password for user@KRBTEST.COM:
kinit: Password incorrect while getting initial credentials
$ kinit user
kinit: Client's credentials have been revoked while getting initial credentials

Account lockout principal state

A principal entry keeps three pieces of state related to account lockout:

  • The time of last successful authentication
  • The time of last failed authentication
  • A counter of failed attempts

The time of last successful authentication is not actually needed for the account lockout system to function, but may be of administrative interest. These fields can be observed with the getprinc kadmin command. For example:

kadmin: getprinc user
Principal: user@KRBTEST.COM
Last successful authentication: [never]
Last failed authentication: Mon Dec 03 12:30:33 EST 2012
Failed password attempts: 2

A principal which has been locked out can be administratively unlocked with the -unlock option to the modprinc kadmin command:

kadmin: modprinc -unlock PRINCNAME

This command will reset the number of failed attempts to 0.

KDC replication and account lockout

The account lockout state of a principal is not replicated by either traditional kprop or incremental propagation. Because of this, the number of attempts an attacker can make within a time period is multiplied by the number of KDCs. For instance, if the maxfailure parameter on a policy is 10 and there are four KDCs in the environment (a master and three slaves), an attacker could make as many as 40 attempts before the principal is locked out on all four KDCs.

An administrative unlock is propagated from the master to the slave KDCs during the next propagation. Propagation of an administrative unlock will cause the counter of failed attempts on each slave to reset to 1 on the next failure.

If a KDC environment uses a replication strategy other than kprop or incremental propagation, such as the LDAP KDB module with multi-master LDAP replication, then account lockout state may be replicated between KDCs and the concerns of this section may not apply.

KDC performance and account lockout

In order to fully track account lockout state, the KDC must write to the the database on each successful and failed authentication. Writing to the database is generally more expensive than reading from it, so these writes may have a significant impact on KDC performance. As of release 1.9, it is possible to turn off account lockout state tracking in order to improve performance, by setting the disable_last_success and disable_lockout variables in the database module subsection of kdc.conf. For example:

    DB = {
        disable_last_success = true
        disable_lockout = true

Of the two variables, setting disable_last_success will usually have the largest positive impact on performance, and will still allow account lockout policies to operate. However, it will make it impossible to observe the last successful authentication time with kadmin.

KDC setup and account lockout

To update the account lockout state on principals, the KDC must be able to write to the principal database. For the DB2 module, no special setup is required. For the LDAP module, the KDC DN must be granted write access to the principal objects. If the KDC DN has only read access, account lockout will not function.