php sql注入与防注入经典案例分析
对于sql注入与防注入其实就是一个攻与防的,今天我们要告诉大家最基本的注入和防止方法,原理都是利用了php或mysql的一些特性而我们没注意所造成的.
一个简单的SQL注入攻击案例
假如我们有一个公司网站,在网站的后台数据库中保存了所有的客户数据等重要信息,假如网站登录页面的代码中有这样一条命令来读取用户信息,代码如下:
<?php $q = "SELECT `id` FROM `users` WHERE `username`= ' " . $_GET['username'] . " ' AND `password`= ' " . $_GET['password'] . " ' "; ?>
现在有一个黑客想攻击你的数据库,他会尝试在此登录页面的用户名的输入框中输入以下代码:;SHOW TABLES;点击登陆键,这个页面就会显示出数据库中的所有表,如果他现在使用下面这行命令:;DROP TABLE [table name];这样他就把一张表删除了.
当然,这只是一个很简单的例子,实际的SQL注入方法比这个要复杂得多,黑客也愿意花大量的时间来不断尝试来攻击你的代码,有一些程序软件也可以自动地来不断尝试SQL注入攻击,了解了SQL注入的攻击原理后,我们来看一下如何防范SQL注入攻击.
magic_quotes_gpc = On 时的注入攻击
当 magic_quotes_gpc = On 时,攻击者无法对字符型的字段进行 SQL 注入,这并不代表这就安全了,这时,可以通过数值型的字段进行SQL注入.
在最新版的 MYSQL 5.x 中,已经严格了数据类型的输入,已默认关闭自动类型转换。数值型的字段,不能是引号标记的字符型。也就是说,假设 uid 是数值型的,在以前的 mysql 版本中,这样的语句是合法的,代码如下:
INSERT INTO tbl_user SET uid="1";SELECT * FROM tbl_user WHERE uid="1";
在最新的 MYSQL 5.x 中,上面的语句不是合法的,必须写成这样,代码如下:
INSERT INTO tbl_user SET uid=1;SELECT * FROM tbl_user WHERE uid=1;
这样我认为是正确的,因为作为开发者,向数据库提交正确的符合规则的数据类型,这是最基本的要求.
那么攻击者在 magic_quotes_gpc = On 时,他们怎么攻击呢？很简单,就是对数值型的字段进行 SQL 注入,以下列的 php 脚本为例,代码如下:
<?php
if (isset($_POST["f_login"])) {
// 连接数据库...
// ...代码略...
// 检查用户是否存在
$t_strUid = $_POST["f_uid"];
$t_strPwd = $_POST["f_pwd"];
$t_strSQL = "SELECT * FROM tbl_users WHERE uid=$t_strUid AND password = '$t_strPwd' LIMIT 0,1";
if ($t_hRes = mysql_query($t_strSQL)) {
// 成功查询之后的处理. 略...
}
}
?>
<html><head><title>sample test</title></head>
<body>
<form method=post action="">
User ID: <input type="text" name="f_uid" size=30><br>
Password: <input type=text name="f_pwd" size=30><br>
<input type="submit" name="f_login" value="登录">
</form>
</body>上面这段脚本要求用户输入 userid 和 password 登入,一个正常的语句,用户输入 1001和abc123,提交的 sql 语句如下:
SELECT * FROM tbl_users WHERE userid=1001 AND password = 'abc123' LIMIT 0,1
如果攻击者在 userid 处，输入：1001 OR 1 =1 #，则注入的sql语句如下：
SELECT * FROM tbl_users WHERE userid=1001 OR 1 =1 # AND password = 'abc123' LIMIT 0,1,攻击者达到了目的.
防范SQL注入 - 使用mysql_real_escape_string()函数,在数据库操作的代码中用这个函数mysql_real_escape_string()可以将代码中特殊字符过滤掉,如引号等,如下例:
<?php $q = "SELECT `id` FROM `users` WHERE `username`= ' " . mysql_real_escape_string($_GET['username']) . " ' AND `password`= ' " . mysql_real_escape_string($_GET['password']) . " ' "; ?>
防范SQL注入 - 使用mysql_query()函数
mysql_query()的特别是它将只执行SQL代码的第一条,而后面的并不会执行,回想在最前面的例子中,黑客通过代码来例后台执行了多条SQL命令,显示出了所有表的名称,所以mysql_query()函数可以取到进一步保护的作用,我们进一步演化刚才的代码就得到了下面的代码:
<?php
//connection
$database = mysql_connect("localhost", "username", "password");
//db selection
mysql_select_db("database", $database);
$q = mysql_query("SELECT `id` FROM `users` WHERE `username`= ' " . mysql_real_escape_string($_GET['username']) . " ' AND `password`= ' " . mysql_real_escape_string($_GET['password']) . " ' ", $database);
?>除此之外,我们还可以在PHP代码中判断输入值的长度,或者专门用一个函数来检查输入的值,所以在接受用户输入值的地方一定要做好输入内容的过滤和检查,当然学习和了解最新的SQL注入方式也非常重要,这样才能做到有目的的防范,如果使用的是平台式的网站系统如Wordpress,要注意及时打上官方的补丁或升级到新的版本,如果有讲得不对的地方或不理解的请在评论区留言.
php.ini 中的 display_errors 选项，应该设为 display_errors = off,这样 php 脚本出错之后,不会在 web 页面输出错误,以免让攻击者分析出有作的信息.
调用 mysql_query 等 mysql 函数时,前面应该加上 @，即 @mysql_query(...),这样 mysql 错误不会被输出,同理以免让攻击者分析出有用的信息,另外,有些程序员在做开发时,当 mysql_query出错时,习惯输出错误以及 sql 语句,代码如下:
<?php
$t_strSQL = "SELECT a from b....";
if (mysql_query($t_strSQL)) {
// 正确的处理
} else {
echo "错误! SQL 语句：$t_strSQL \r\n错误信息" . mysql_query();
exit;
}
?>这种做法是相当危险和愚蠢的,如果一定要这么做,最好在网站的配置文件中,设一个全局变量或定义一个宏,设一下 debug 标志.
全局配置文件中,代码如下:
define("DEBUG_MODE",0); // 1: DEBUG MODE; 0: RELEASE MODE
调用脚本中,代码如下:
<?php
$t_strSQL = "SELECT a from b....";
if (mysql_query($t_strSQL)) {
// 正确的处理
} else {
if (DEBUG_MODE) echo "错误! SQL 语句：$t_strSQL \r\n错误信息" . mysql_query();
exit;
}
?>永久链接：
转载随意！带上文章地址吧。
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