利用PHP编程防范XSS跨站脚本攻击

网络整理 - 08-26

  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++) {
 $pattern = 'http://www.cuoxin.com/';
 for ($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;
}
 }
}
}