The XSS filter introduced in IE8 is a really powerful defence against XSS. I tested the filter for a number of years and found various bypasses one of which I would like to share with you now. You can read more about the filter and its goal in the following blog post . Scope There have been numerous public bypasses of the filter however very few within the intended scope of the filter. The filter blocks reflected XSS in HTML context, script, style and event context. It does not support attacks that use multiple parameters or same origin requests. Once you are aware of the intended scope the difficulty of bypassing the filter is very high. Function reassignment This bypass was fixed in later versions of Internet Explorer but still works in compatibility mode. You can use the vector in a penetration test by forcing the target site into compatibility mode using an iframe with an EmulateIE7 meta element as shown below. <meta http-equiv="X-UA-Compatible" content="IE=Emula