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On my site, if I were to block <script> from the url, could this prevent some cases of XSS? Any way someone could bypass that?

closed as unclear what you're asking by RoraΖ, schroeder, TildalWave, Eric G, Stephane May 12 '15 at 7:45

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    I think you have some misunderstandings here. I assume you mean to filter out the string <script> from the URL parameter strings, like a query parameter. Have you done any research into how to prevent XSS? You will find that blacklisting strings in a query parameter is not the recommended way to address this. – schroeder May 11 '15 at 19:39
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    possible duplicate of Can anybody explain XSS? – Anonymous May 11 '15 at 21:08
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Yes, it can be bypassed. First, many filters that attempt to remove <script> tags do so in a way that is easily defeated. For example, they may improperly handle input like <scr<script>ipt>.

But even when implemented "properly", that is not sufficient, because <script> tags are not required to execute Javascript on a page: event handlers, script URIs, and data URIs can be used as well (note that I am unsure about browser support for the latter two). See OWASP's XSS page for more information.

And in DOM-based XSS, you may not be worried about HTML at all, instead stuffing Javascript into an execution sink.

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