数据库安全基础入门知识简介

来源：ccidnet    作者：wenzhai

现在，为了使得网站能够提供各种各样动态的内容，数据库已经成为所有基于 WEB 应用程序最重要的组件。由于一些十分敏感或者保密的信息可能会存储在这样的数据库中，因此，您需要非常慎重的考虑如何保护它们。

为了能够存储或者检索信息，您需要连接到数据库，发送一个合法的查询命令，得到结果，然后关闭连接。目前，在这个交互过程中最常用的查询语言是“结构化查询语言（Structured Query Language, SQL）”。请参阅以下内容以了解黑客如何“利用 SQL 查询攻击”。

正如您所知道的，PHP 自身并无法保护您的数据库。以下章节的内容旨在介绍有关如何利用 PHP 脚本访问和操作数据库的最基础的知识。

请记住这个简单的法则：尽可能深层次的防御。为了保护您的数

If attacker submits the value a%' exec master..xp_cmdshell 'net user test testpass /ADD' -- to $prod, then the $query will be:

据库所采取得措施和场所越多，攻击者暴露、滥用数据库中存储的保密信息的可能性就越小。请尽可能更好的设计数据库结构和应用程序。

设计数据库

一般来说第一步总是建立数据库，除非您希望使用已经存在的第三方数据库。当一个数据库被建立后，它将被指定给一个所有者，即运行建立数据库语句的用户。通常，只有所有者（或者超级用户）才能对该数据库中的对象进行任何操作，为了能让其它用户使用该数据库，需要进行权限设置。

应用程序决不能用所有者或者超级用户的账号来连接到数据库，因为这些用户可以执行任何查询，例如，修改数据结构（如删除表格）或者删除所有的内容。

您可以为应用程序不同的部分建立不同的数据库账号，使得它们职能对数据库对象行使非常有限的权限。对这些账号应该只赋予最需要的权限，同时应该防止相同的用户能够在不同的使用情况与数据库进行交流。这也就是说，如果某一个入侵者利用这些账号中的某一个获得了访问您数据库的权限，他们也仅仅能够影响到您的应用程序力所能及的范围。

您最好不要把所有的事物过程都放在 WEB 应用程序（即您的脚本）中来实施，而充分的利用数据库结构，使用视图（view）、触发器（trigger）或者规则（rule）。如果需要对系统进行升级，则需要对数据库开辟新的端口，因此您需要对每一个独立的数据库客户重新设置权限。总之，触发器可以被用来透明地和自动地处理字段，该特性可以在调试应用程序地问题或者追踪后台事物时提供便利信息。

连接数据库

您可能希望通过 SSL 建立连接来加密客户端和服务端之间的通讯以增加安全性，或者您可以使用 ssh 来加密客户端和数据库服务器之间的网络连接。如果您实施了这些措施，则监视您的网络流量以及以这种方式获取信息将变成十分复杂的工作。

加密存储模型

SSL/SSH protects data travelling from the client to the server, SSL/SSH does not protect the persistent data stored in a database. SSL is an on-the-wire protocol. Once an ttacker gains access to your database directly (bypassing the webserver), the stored sensitive data may be exposed or misused, unless the information is protected by the database tself. Encrypting the data is a good way to mitigate this threat, but very few databases offer this type of data encryption. The easiest way to work around this problem is to first create your own encryption package, and then use it from within your PHP scripts. PHP can assist you in this case with its several extensions, such as Mcrypt and Mhash, covering a wide variety of encryption algorithms. The script encrypts the data be stored first, and decrypts it when retrieving. See the references for further examples how encryption works. In case of truly hidden data, if its raw representation is not needed (i.e. not be displayed), hashing may be also taken into consideration. The well-known example for the hashing is storing the MD5 hash of a password in a database, instead of the password itself. See also crypt() and md5().

例子 15-5. 使用经过哈希运算的密码字段

// storing password hash
$query  = sprintf("INSERT INTO users(name,pwd) VALUES('%s','%s');",
            addslashes($username), md5($password));
$result = pg_exec($connection, $query);

// querying if user submitted the right password
$query = sprintf("SELECT 1 FROM users WHERE name='%s' AND pwd='%s';",
            addslashes($username), md5($password));
$result = pg_exec($connection, $query);

if (pg_numrows($result) > 0) {
    echo "Welcome, $username!";
}
else {
    echo "Authentication failed for $username.";
}

SQL 攻击

Many web developers are unaware of how SQL queries can be tampered with, and assume that an SQL query is a trusted command. It means that SQL queries are able to circumvent access controls, thereby bypassing standard authentication and authorization checks,

and sometimes SQL queries even may allow access to host operating system level commands.Direct SQL Command Injection is a technique where an attacker creates or alters existing SQL commands to expose hidden data, or to override valuable ones, or even to execute dangerous system level commands on the database host. This is accomplished by the

application taking user input and combining it with static parameters to build a SQL query. The following examples are based on true stories, unfortunately. Owing to the lack of input validation and connecting to the database on behalf of a superuser or the one who can create users, the attacker may create a superuser in your database.

例子 15-6. 将结果集分离到页面中，然后创造超级用户(PostgreSQL and MySQL)

$offset = argv[0]; // beware, no input validation!
$query  = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;";
// with PostgreSQL 
$result = pg_exec($conn, $query);
// with MySQL
$result = mysql_query($query);  
 
Normal users click on the 'next', 'prev' links where the $offset is encoded into the 
URL. The script expects that the incoming $offset is decimal number. However, someone
 tries to break in with appending urlencode()'d form of the following to the URL 


// in case of PostgreSQL
0;
insert into pg_shadow(usename,usesysid,usesuper,usecatupd,passwd)
    select 'crack', usesysid, 't','t','crack'
    from pg_shadow where usename='postgres';
--

// in case of MySQL
0;
UPDATE user SET Password=PASSWORD('crack') WHERE user='root';
FLUSH PRIVILEGES;

If it happened, then the script would present a superuser access to him. Note that 0; is to supply a valid offset to the original query and to terminate it.

注: It is common technique to force the SQL parser to ignore the rest of the query written by the developer with -- which is the comment sign in SQL. A feasible way to gain passwords is to circumvent your search result pages. What the attacker needs only is to try if there is any submitted variable used in SQL statement which is not handled properly. These filters can be set commonly in a preceding form to customize WHERE, ORDER BY, LIMIT and OFFSET clauses in SELECT statements. If your database supports the UNION construct, the attacker may try to append an entire query to the original one to list passwords from an arbitrary table. Using encrypted password fields is strongly encouraged.

例子 15-7. 列出文章，以及一些密码（任何数据库服务器）

$query  = "SELECT id, name, inserted, size FROM products
                  WHERE size = '$size'
                  ORDER BY $order LIMIT $limit, $offset;";
$result = odbc_exec($conn, $query);   
The static part of the query can be combined with another SELECT statement which 
reveals all passwords: 
'
union select '1', concat(uname||'-'||passwd) as name, '1971-01-01', '0' from usertable;
--

If this query (playing with the ' and --) were assigned to one of the variables used in $query, the query beast awakened.

SQL UPDATEs are also subject to attacking your database. These queries are also hreatened by chopping and appending an entirely new query to it. But the attacker might fiddle with the SET clause. In this case some schema information must be possessed to manipulate the query successfully. This can be acquired by examing the form variable names, or just simply brute forcing. There are not so many naming convention for fields storing passwords or usernames.

例子 15-8. 利用重设密码来获取更多的权限（任何数据库服务器）

$query = "UPDATE usertable SET pwd='$pwd' WHERE uid='$uid';"; 
But a malicious user sumbits the value ' or uid like'%admin%'; -- to $uid to change the 
admin's password, or simply sets $pwd to "hehehe', admin='yes', trusted=100 " (with a 
trailing space) to gain more privileges. Then, the query will be twisted: 
// $uid == ' or uid like'%admin%'; --
$query = "UPDATE usertable SET pwd='...' WHERE uid='' or uid like '%admin%'; --";
// $pwd == "hehehe', admin='yes', trusted=100 "
$query = "UPDATE usertable SET pwd='hehehe', admin='yes', trusted=100 WHERE ...;"  

A frightening example how operating system level commands can be accessed on some 
database hosts. 例子 15-9. 攻击数据库主机的操作系统 (MSSQL Server)

$query  = "SELECT * FROM products WHERE id LIKE '%$prod%'";
$result = mssql_query($query);  
 
If attacker submits the value a%' exec master..xp_cmdshell 'net user test testpass 
/ADD' -- to $prod, then the $query will be: 


$query  = "SELECT * FROM products
                    WHERE id LIKE '%a%'
                    exec master..xp_cmdshell 'net user test testpass /ADD'--";
$result = mssql_query($query);

MSSQL Server executes the SQL statements in the batch including a command to add a new user to the local accounts database. If this application were running as sa and the MSSQLSERVER service is running with sufficient privileges, the attacker would now have an account with which to access this machine.

注: Some of the examples above is tied to a specific database server. This does not mean that a similar attack is impossible against other products. Your database server may be so vulnerable in other manner.

预防的技巧

You may plead that the attacker must possess a piece of information about the database schema in most examples. You are right, but you never know when and how it can be taken out, and if it happens, your database may be exposed. If you are using an open source, or publicly available database handling package, which may belong to a content management ystem or forum, the intruders easily produce a copy of a piece of your code. It may be also a security risk if it is a poorly designed one.

These attacks are mainly based on exploiting the code not being written with security in mind. Never trust on any kind of input, especially which comes from the client side, even though it comes from a select box, a hidden input field or a cookie. The first example shows that such a blameless query can cause disasters.

Never connect to the database as a superuser or as the database owner. Use always customized users with very limited privileges.

Check if the given input has the expected data type. PHP has a wide range of input validating functions, from the simplest ones found in Variable Functions and in Character Type Functions (e.g. is_numeric(), ctype_digit() respectively) onwards the Perl compatible Regular Expressions support.

If the application waits for numerical input, consider to verify data with is_numeric(), or silently change its type using settype(), or use its numeric representation by sprintf().

例子 15-10. 更加安全的分页查询语句

settype($offset, 'integer');
$query = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;";

// please note %d in the format string, using %s would be meaningless
$query = sprintf("SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET %d;",
                 $offset);

Quote each non numeric user input which is passed to the database with addslashes() or addcslashes(). See the first example. As the examples shows, quotes burnt into the static part of the query is not enough, and can be easily hacked.

Do not print out any database specific information, especially about the schema, by fair means or foul. See also Error Reporting and Error Handling and Logging Functions.

You may use stored procedures and previously defined cursors to abstract data access so that users do not directly access tables or views, but this solution has another impacts.Besides these, you benefit from logging queries either within your script or by the database itself, if it supports. Obviously, the logging is unable to prevent any harmful attempt, but it can be helpful to trace back which application has been circumvented. The log is not useful by itself, but through the information it contains. The more detail is generally better.