The util-linux package shipped with CentOS Linux Advanced Server contains a locally exploitable vulnerability.
The util-linux package contains a large variety of low-level system utilities that are necessary for a Linux system to function. The 'chfn' utility included in this package allows users to modify personal information stored in the system-wide password file, /etc/passwd. In order to modify this file, this application is installed setuid root. Under certain conditions, a carefully crafted attack sequence can be performed to exploit a complex file locking and modification race present in this utility allowing changes to be made to /etc/passwd. In order to successfully exploit the vulnerability and perform privilege escalation there is a need for a minimal administrator interaction. Additionally, the password file must be over 4 kilobytes, and the local attackers entry must not be in the last 4 kilobytes of the password file. The Common Vulnerabilities and Exposures project (cve.mitre.org) has assigned the name CAN-2002-0638 to this issue. An interim workaround is to remove setuid flags from /usr/bin/chfn and /usr/bin/chsh. All users of CentOS Linux should update to the errata util-linux packages which contain a patch to correct this vulnerability. Many thanks to Michal Zalewski of Bindview for alerting us to this issue.