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Web应用系统的攻击大部分是来自于外部,如Url上添加一些字段注入($_GET输入),表单的提交注入(一般为$_POST),所以在接收数据时对数据进行过滤,是必须的:
过滤方法有以下几种:
trim过滤字符串首尾空格

$test1 = trim($_POST['test1']);

strip_tags函数过滤PHP、HTML标签 :
strip_tags会将字符串中的php标签(<?php ?>)Html标签(<h1></h1><script></script>....等)移除。一定程序上阻止了恶意注入。

$_POST['name'] = "<script>alert('hehe');</script>";
var_dump($_POST['name']);//弹出信息框 'hehe'
$name = strip_tags($_POST['name']);
var_dump($name);  //string(14) "alert('hehe');"

转数据类型
接收的数据是整形或浮点形,可以直接转数据类型。

//转整形 
$number = intval($_POST['number']);
$price  = floatval($_POST['price']);

移除攻跨站脚本xss攻击
xss攻击有时会把标签转换成其他数据,strip_tags防止不了,
ThinkPHP中有个remove_xss方法,可以将大部分xss攻击阻止。./ThinkPHP/Extend/Function/extend.php中,为了方便使用,可以放到项目的common.php里或公用文件里面直接调用:

/**
 * @from extend.php
 * 过滤xss攻击
 * @param str $val
 * @return mixed
 */
function remove_xss($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-\x19])/', '', $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
        $val = preg_replace('/(&#[xX]0{0,8}'.dechex(ord($search[$i])).';?)/i', $search[$i], $val); // with a ;
        // @ @ 0{0,7} matches '0' zero to seven times
        $val = preg_replace('/(&#0{0,8}'.ord($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 .= '(&#[xX]0{0,8}([9ab]);)';
                    $pattern .= '|';
                    $pattern .= '|(&#0{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;
}

调用方法:

$name = remove_xss($_POST['name']);
也可以和strip_tags结合起来使用
$name = strip_tags(remove_xss($_POST['name']));

保存文章内容类转义:
使用kindeditor之类的内容编辑器时,因为提交到后台时是以Html形式提交的,而且需要保存到数据库,为了防止sql注入,需要在进数据库前进行特殊字符转义,这时用过滤标签的方法或各类的方法都不适合。只能对标签和特殊符号进行转义,这时使用到的方法是addslashes。
addslashes在使用前先检查一下,php是否自动开启了自动转义。用get_magic_quotes_gpc()方法判断,如果已开,则是true,否为false。

    if(!get_magic_quotes_gpc()){
     $content = addslashes($_POST['content']);
    }else{
      $content= $_POST['content'];
    }

这样就完成了转义,然而在展示页面,从数据库拿出来的内容是经过转义的html,如果直接展示,html标签等都识别不到,会直接输出转义过的字符串。这时需要用反转义来还原数据。如下

echo stripslashes($content);

PHP内置过滤器参考使用 PHP内置过滤器filter_input
其他参考:
http://www.w3school.com.cn/php/func_filter_input_array.asp
https://my.oschina.net/jiec/blog/309467

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