本文原载于公众号：猪猪谈安全
作者：随风kali
什么是SQL注入
SQL注入即是指web应用程序对用户输入数据的合法性没有判断或过滤不严，攻击者可以在web应用程序中事先定义好的查询语句的结尾上添加额外的SQL语句，在管理员不知情的情况下实现非法操作，以此来实现欺骗数据库服务器执行非授权的任意查询，从而进一步得到相应的数据信息。
这里以一个PHP的SQL语句代码为例
$SQL = “select * from ‘某字段’ where id = $id”;
由于这里的参数id可以控制，且这个id被带进了数据库查询，所以一些意图不轨的人可以通过拼接SQL语句来进行攻击。
产生SQL注入需要的条件
SQL注入的产生需要两个条件
1.我们传递给后端的参数是可以控制的
2.参数内容会被带入到数据库查询
验证是否存在SQL注入
还是以这个代码为例 $SQL = “select * from ‘某字段’ where id = $id”;
在这里我们可以控制的参数是id这个参数，所以我们输入1‘的时候。这时查询语句执行的内容就为
select * from ‘某字段’ where id = 1’
由于后面有一个单引号，这样的语句不符合数据库语法的规范，所以会报错，从而判断出该处是否存在SQL注入。
SQL注入的分类
SQL注入分为很多种，有联合注入、布尔注入、报错注入、时间注入、堆叠注入、二次注入、宽字节注入、cookie注入等等等。当然这些注入所产生的原理都是一样，如上文所讲。在接下来的文章呢，笔者也会对这些注入结合实例进行全部写出来。
SQL注入的防御
方案一
采用预编译技术
INSERT INTO MyGuests (firstname, lastname, email) VALUES(?, ?, ?)
使用预编译的SQL语句，SQL语句的语义不会是不会发生改变的。预编译语句在创建的时候就已经将指定的SQL语句发送给了DBMS，完成了解析，检查，编译等工作，所以攻击者无法改变SQL语句的结构，只是把值赋给？，然后将？这个变量传给SQL语句。当然还有一些通过预编译绕过某些安全防护的操作，大家感兴趣可以去搜索一下。
方案二
严格控制数据类型
在java、c等强类型语言中一般是不存在数字型注入的，因为在接受到用户输入id时，代码一般会做一个int id 的数据类型转换，假如我们输入的是字符串的话，那么这种情况下，程序就会报错。但是在PHP、ASP这些没有强调处理数据类型的语言，一般我们看到的接收id的代码都是如下等代码。
$id = $_GET[‘id’];
$SQL = “select * from ‘某字段’ where id = $id;”;
这样的代码攻击者就可以通过构造id参数运用联合查询等手法进行SQL注入，假如这里我们加入一个检查数字类型函数is_numeric()这样就可以防止数字型注入了。
方案三
对特殊的字符进行转义
数字型注入可以通过检查数据类型防止，但是字符型不可以，那么怎么办呢，最好的办法就是对特殊的字符进行转义了。比如在MySQL中我们可以对” ‘ “进行转义，这样就防止了一些恶意攻击者来闭合语句。当然我们也可以通过一些安全函数来转义特殊字符。如addslashes（）等，但是这些函数并非一劳永逸，攻击者还可以通过一些特殊的方式绕过。
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