/bin/login in shadow 4.0.18.1 in Debian GNU/Linux, and probably other Linux distributions, allows local users in the utmp group to overwrite arbitrary files via a symlink attack on a temporary file referenced in a line (aka ut_line) field in a utmp entry.
https://www.exploit-db.com/exploits/7313
https://exchange.xforce.ibmcloud.com/vulnerabilities/47037
http://www.ubuntu.com/usn/usn-695-1
http://www.securityfocus.com/bid/32552
http://www.securityfocus.com/archive/1/498769/100/0/threaded
http://www.mandriva.com/security/advisories?name=MDVSA-2009:062
http://securityreason.com/securityalert/4695