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 . When a client attempts to use PKINIT to obtain credentials from the KDC, the client can specify, using an issuer and serial number, which of the KDC"s possibly-many certificates the client has in its possession, as a hint to the KDC that it should use the corresponding key to sign its response. If that specification was malformed, the KDC could attempt to dereference a NULL pointer and crash. When a client attempts to use PKINIT to obtain credentials from the KDC, the client will typically format its request to conform to the specification published in RFC 4556. For interoperability reasons, clients and servers also provide support for an older, draft version of that specification. If a client formatted its request to conform to this older version of the specification, with a non-default key agreement option, it could cause the KDC to attempt to dereference a NULL pointer and crash. All krb5 users should upgrade to these updated packages, which contain backported patches to correct these issues. After installing the updated packages, the krb5kdc daemon will be restarted automatically.