Problem with javascript - security

Hello,

if I tape this in CKeditor:
<script>alert("hello")</script>

and if I look the source, i have this :
&lt;script&gt;alert(&quot;hello&quot;)&lt;/script&gt;

It's good. If I save, I see in my page <script>alert("hello")</script> like a snippet.

BUT, if I return to CKeditor, snippet disappeared. If I save and reload my page, I have a javascript message "Hello". It's dangerous.

How to prevent CKEditor doesn't replace &lt; by "<" when it charge the page on the editor?

Excuse my very bad english

Thanks