国内不少论坛都存在跨站脚本漏洞，国外(也很)多这样(的)例子，甚至Google(也)出现过，不过在12月初时修正了。(编者注：关于跨站脚本漏洞攻击，读者可参阅《详解XSS跨站脚本攻击》)。跨站攻击(很)容易(就)可以构造，而且非常隐蔽，不易被查觉（通常盗取信息后马上跳转回原页面）。
如何攻击，在此不作说明（(也)不要问我），主要谈谈如何防范。首先，跨站脚本攻击都(是)由于对用户(的)输入没有进行严格(的)过滤造成(的)，所以我们必须在所有数据进入我们(的)网站和数据库之前把可能(的)危险拦截。针对非法(的)HTML代码包括单双引号等，可以使用htmlentities() 。
＜?php
$str = "A 'quote' is ＜b＞bold＜/b＞";
// Outputs: A 'quote' is <b>bold</b>
echo htmlentities($str);
// Outputs: A 'quote' is <b>bold</b>
echo htmlentities($str, ENT_QUOTES);
?＞
这样可以使非法(的)脚本失效。
但(是)要注意一点，htmlentities()默认编码为 ISO-8859-1，如果你(的)非法脚本编码为其它，那么可能无法过滤掉，同时浏览器却可以识别和执行。这个问题我先找几个站点测试后再说。
这里提供一个过滤非法脚本(的)函数：
function RemoveXSS($val) {
// remove all non-printable characters. CR(0a) and LF(0b) and TAB(9) are allowed
// this prevents some character re-spacing such as ＜javascript＞
// note that you have to handle splits with
,
, and 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:a&
_#X6Cert('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 ,
, and
$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;
}
}
}
}