nagios.upgrade_to_v3.sh, as distributed by Red Hat and possibly others for Nagios Core 3.4.4, 3.5.1, and earlier, allows local users to overwrite arbitrary files via a symlink attack on a temporary nagioscfg file with a predictable name in /tmp/.
Improper Link Resolution Before File Access The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.