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).
Multiple integer underflow flaws, leading to heap-based corruption, were found in the way the MIT Kerberos Key Distribution Center (KDC) decrypted ciphertexts encrypted with the Advanced Encryption Standard (AES) and ARCFOUR (RC4) encryption algorithms. If a remote KDC client were able to provide a specially-crafted AES- or RC4-encrypted ciphertext or texts, it could potentially lead to either a denial of service of the central KDC (KDC crash or abort upon processing the crafted ciphertext), or arbitrary code execution with the privileges of the KDC (i.e., root privileges). (CVE-2009-4212)
All krb5 users should upgrade to these updated packages, which contain a backported patch to correct these issues. All running services using the MIT Kerberos libraries must be restarted for the update to take effect.