Hi Dwarden,
An effective defense can be replacing the original script part with the following code, which verifies that the string written to the HTML page consists of alphanumeric characters only:
<SCRIPT>
var pos=document.URL.indexOf("name=")+5;
var name=document.URL.substring(pos,document.URL.length);
if (name.match(/^[a-zA-Z0-9]$/))
{
document.write(name);
}
else
{
window.alert("Security error");
}
</SCRIPT>
In the above example, the situation is very simple, in more complex scenarios wherein the security checks are less than perfect, this may come to play.
Such functionality can (and perhaps should) be provided through a generic library for sanitation of data (i.e. a set of Javascript functions that perform input validation and/or sanitation). The downside is that the security logic is exposed to the attackers - it is embedded in the HTML code. This makes it easier to understand and to attack it,
SecFilterEngine On
SecFilterScanPOST On
SecFilter "<(.|\n)+>"
Input fn="insert-filter"
method="POST"
filter="sed-request"
sed="s/(<|%3c)/\\</gi"
sed="s/(>|%3e)/\\>/gi"
function RemoveBad(strTemp) {
strTemp = strTemp.replace(/\<|\>|\"|\'|\%|\;|\(|\)|\&|\+|\-/g,"");
return strTemp;
}
<% Session("StoredPreference") = RemoveBad(Request.Cookies("UserColor"));
var TempStr = RemoveBad(Request.QueryString("UserName"));
function RemoveBad(strTemp) {
strTemp = strTemp.replace
(';alert(String.fromCharCode(88,83,83))//\';alert(String.fromCharCode(88,83,83))//";alert(String.fromCharCode(88,83,83))//\";alert(String.fromCharCode(88,83,83))//--></SCRIPT>">'><SCRIPT>alert(String.fromCharCode(88,83,83))</SCRIPT>
'';!--"<XSS>=&{()}
<IMG SRC=JaVaScRiPt:alert('XSS')>
<IMG SRC=javascript:alert("XSS")>
<IMG SRC=`javascript:alert("RSnake says, 'XSS'")`>
<IMG """><SCRIPT>alert("XSS")</SCRIPT>">
<IMG SRC
=javascript:alert(String.fromCharCode(88,83,83))>
<IMG SRC=javascript:alert('XSS')>
<IMG SRC="jav ascript:alert('XSS');">
perl -e 'print "<IMG SRC=java\0script:alert(\"XSS\")>";' > out
<BODY onload!#$%&()*~+-_.,:;?@[/|\]^`=alert("XSS")>
</TITLE><SCRIPT>alert("XSS");</SCRIPT>
<IMG SRC='vbscript:msgbox("XSS")'>
<A HREF="http://%77%77%77%2E%67%6F%6F%67%6C%65%2E%63%6F%6D">XSS</A>)
return strTemp;
} <script type="text/javascript">
function displayName() {
var description = document.getElementById("description").value;
var display = document.getElementById("display");
description = description .replace(/</g, "<").replace(/>/g, ">");
display.innerHTML = description;
}
</script>description.replace(/[\"\'][\s]*javascript:(.*)[\"\']/g, "\"\"");
description = description.replace(/script(.*)/g, "");
description = description.replace(/eval\((.*)\)/g, "");
description = description.replaceAll("<", "<").replaceAll(">", ">");
description = description.replaceAll("eval\\((.*)\\)", "");
description = description.replaceAll("[\\\"\\\'][\\s]*javascript:(.*)[\\\"\\\']", "\"\"");
description = description.replaceAll("script", "");
function RemoveXSS($val) {
// remove all non-printable characters. CR(0a) and LF(0b) and TAB(9) are allowed
// this prevents some character re-spacing such as <java\0script>
// note that you have to handle splits with \n, \r, and \t later since they *are* allowed in some inputs
$val = preg_replace('/([\x00-\x08][\x0b-\x0c][\x0e-\x20])/', '', $val);
// straight replacements, the user should never need these since they're normal characters
// this prevents like <IMG SRC=@avascript:alert('XSS')>
$search = 'abcdefghijklmnopqrstuvwxyz';
$search .= 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
$search .= '1234567890!@#$%^&*()';
$search .= '~`";:?+/={}[]-_|\'\\';
for ($i = 0; $i < strlen($search); $i++) {
// ;? matches the ;, which is optional
// 0{0,7} matches any padded zeros, which are optional and go up to 8 chars
// @ @ search for the hex values
$val = preg_replace('/(&#[x|X]0{0,8}'.dechex(ord($search[$i])).';?)/i', $search[$i], $val); // with a ;
// @ @ 0{0,7} matches '0' zero to seven times
$val = preg_replace('/(�{0,8}'.ord($search[$i]).';?)/', $search[$i], $val); // with a ;
}
// now the only remaining whitespace attacks are \t, \n, and \r
$ra1 = Array('javascript', 'vbscript', 'expression', 'applet', 'meta', 'xml', 'blink', 'link', 'style', 'script', 'embed', 'object', 'iframe', 'frame', 'frameset', 'ilayer', 'layer', 'bgsound', 'title', 'base');
$ra2 = Array('onabort', 'onactivate', 'onafterprint', 'onafterupdate', 'onbeforeactivate', 'onbeforecopy', 'onbeforecut', 'onbeforedeactivate', 'onbeforeeditfocus', 'onbeforepaste', 'onbeforeprint', 'onbeforeunload', 'onbeforeupdate', 'onblur', 'onbounce', 'oncellchange', 'onchange', 'onclick', 'oncontextmenu', 'oncontrolselect', 'oncopy', 'oncut', 'ondataavailable', 'ondatasetchanged', 'ondatasetcomplete', 'ondblclick', 'ondeactivate', 'ondrag', 'ondragend', 'ondragenter', 'ondragleave', 'ondragover', 'ondragstart', 'ondrop', 'onerror', 'onerrorupdate', 'onfilterchange', 'onfinish', 'onfocus', 'onfocusin', 'onfocusout', 'onhelp', 'onkeydown', 'onkeypress', 'onkeyup', 'onlayoutcomplete', 'onload', 'onlosecapture', 'onmousedown', 'onmouseenter', 'onmouseleave', 'onmousemove', 'onmouseout', 'onmouseover', 'onmouseup', 'onmousewheel', 'onmove', 'onmoveend', 'onmovestart', 'onpaste', 'onpropertychange', 'onreadystatechange', 'onreset', 'onresize', 'onresizeend', 'onresizestart', 'onrowenter', 'onrowexit', 'onrowsdelete', 'onrowsinserted', 'onscroll', 'onselect', 'onselectionchange', 'onselectstart', 'onstart', 'onstop', 'onsubmit', 'onunload');
$ra = array_merge($ra1, $ra2);
$found = true; // keep replacing as long as the previous round replaced something
while ($found == true) {
$val_before = $val;
for ($i = 0; $i < sizeof($ra); $i++) {
$pattern = '/';
for ($j = 0; $j < strlen($ra[$i]); $j++) {
if ($j > 0) {
$pattern .= '(';
$pattern .= '(&#[x|X]0{0,8}([9][a][b]);?)?';
$pattern .= '|(�{0,8}([9][10][13]);?)?';
$pattern .= ')?';
}
$pattern .= $ra[$i][$j];
}
$pattern .= '/i';
$replacement = substr($ra[$i], 0, 2).'<x>'.substr($ra[$i], 2); // add in <> to nerf the tag
$val = preg_replace($pattern, $replacement, $val); // filter out the hex tags
if ($val_before == $val) {
// no replacements were made, so exit the loop
$found = false;
}
}
}
return $val;
}
Code courtesy of http://sla.ckers.org/forum/
polonus