${?}_{{?}_{{?}_{?}}{?}_{{?}_{?}}{?}_{{?}_{?}}{?}_{u}se\text{s}\text{c}\text{a}\text{n}\text{n}\text{i}\text{n}\text{g}\text{s}\text{o}\text{f}\text{t}\text{w}\text{a}\text{r}\text{e}to\text{l}\text{o}\text{o}\text{k}\text{f}\text{o}\text{r}\text{k}\text{n}\text{o}\text{w}\text{n}\text{p}\text{r}\text{o}\text{b}\text{l}\text{e}\text{m}\text{s}\text{s}\text{u}\text{c}\text{h}as\text{b}\text{a}\text{d}}$

passwords, the removal $of$ important files, security attacks $in$ progress, and

system administration errors.

$a.$ Stateful inspections

$b.$ Intrusion detection systems

$c.$ Application proxy filtering technologies

$d.$ Packet filtering technologies