(RHSA-2010:0343) Important: krb5 security and bug fix update

Kerberos is a network authentication system which allows clients and
servers to authenticate to each other using symmetric encryption and a
trusted third party, the Key Distribution Center (KDC).

A use-after-free flaw was discovered in the MIT Kerberos administration
daemon, kadmind. A remote, authenticated attacker could use this flaw to
crash the kadmind daemon. Administrative privileges are not required to
trigger this flaw, as any realm user can request information about their
own principal from kadmind. (CVE-2010-0629)

This update also fixes the following bug:

• when a Kerberos client seeks tickets for use with a service, it must
  contact the Key Distribution Center (KDC) to obtain them. The client must
  also determine which realm the service belongs to and it typically does
  this with a combination of client configuration detail, DNS information and
  guesswork.

If the service belongs to a realm other than the client’s, cross-realm
authentication is required. Using a combination of client configuration and
guesswork, the client determines the trust relationship sequence which
forms the trusted path between the client’s realm and the service’s realm.
This may include one or more intermediate realms.

Anticipating the KDC has better knowledge of extant trust relationships,
the client then requests a ticket from the service’s KDC, indicating it
will accept guidance from the service’s KDC by setting a special flag in
the request. A KDC which recognizes the flag can, at its option, return a
ticket-granting ticket for the next realm along the trust path the client
should be following.

If the ticket-granting ticket returned by the service’s KDC is for use with
a realm the client has already determined was in the trusted path, the
client accepts this as an optimization and continues. If, however, the
ticket is for use in a realm the client is not expecting, the client
responds incorrectly: it treats the case as an error rather than continuing
along the path suggested by the service’s KDC.

For this update, the krb5 1.7 modifications which allow the client to trust
such KDCs to send them along the correct path, resulting in the client
obtaining the tickets it originally desired, were backported to krb 1.6.1
(the version shipped with Red Hat Enterprise Linux 5.5). (BZ#578540)

All krb5 users should upgrade to these updated packages, which contain
backported patches to correct these issues. All running KDC services must
be restarted for the update to take effect.