Gentoo mirrorselect contains a flaw that may allow a malicious user to overwrite arbitrary files using symlink attacks. The issue is triggered when mirrorselect is executed and it overwritten the file with the user running mirror select permissions. It is possible that the flaw may allow arbitrary files being overwritten resulting in a loss of integrity.
Upgrade to version 0.89 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.