XF 2.1 Javascript in node description broke post new thread function

[Danloona]

Hello,

I have little trouble with xenForo 2.1 I have code to add to dynamically generate math number in url to avoid cache.

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here is how problem looks like.

Here is code injected in description.

<script type="text/javascript">
document.write('<img src="https://beta.hectorsky.net/bannery/share/voice_small.png?'+Math.random()+'">');
</script>

Thanks for help