PHP 매뉴얼
이전 | 15. 장. 보안 | 다음

# Database Security

Nowadays, databases are cardinal components of any web based application by enabling websites to provide varying dynamic content. Since very sensitive or secret information can be stored in a database, you should strongly consider protecting your databases.

To retrieve or to store any information you need to connect to the database, send a legitimate query, fetch the result, and close the connection. Nowadays, the commonly used query language in this interaction is the Structured Query Language (SQL). See how an attacker can tamper with an SQL query.

As you can surmise, PHP cannot protect your database by itself. The following sections aim to be an introduction into the very basics of how to access and manipulate databases within PHP scripts.

Keep in mind this simple rule: defense in depth. The more places you take action to increase the protection of your database, the less probability of an attacker succeeding in exposing or abusing any stored information. Good design of the database schema and the application deals with your greatest fears.

## Designing Databases

The first step is always to create the database, unless you want to use one from a third party. When a database is created, it is assigned to an owner, who executed the creation statement. Usually, only the owner (or a superuser) can do anything with the objects in that database, and in order to allow other users to use it, privileges must be granted.

Applications should never connect to the database as its owner or a superuser, because these users can execute any query at will, for example, modifying the schema (e.g. dropping tables) or deleting its entire content.

You may create different database users for every aspect of your application with very limited rights to database objects. The most required privileges should be granted only, and avoid that the same user can interact with the database in different use cases. This means that if intruders gain access to your database using your applications credentials, they can only effect as many changes as your application can.

You are encouraged not to implement all the business logic in the web application (i.e. your script), instead do it in the database schema using views, triggers or rules. If the system evolves, new ports will be intended to open to the database, and you have to re-implement the logic in each separate database client. Over and above, triggers can be used to transparently and automatically handle fields, which often provides insight when debugging problems with your application or tracing back transactions.

## Connecting to Database

You may want to estabilish the connections over SSL to encrypt client/server communications for increased security, or you can use ssh to encrypt the network connection between clients and the database server. If either of these is used, then monitoring your traffic and gaining information about your database will be difficult for a would-be attacker.

## Encrypted Storage Model

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 attacker 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 itself. 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 with several extensions, such as Mcrypt and Mhash, covering a wide variety of encryption algorithms. The script encrypts the data before inserting it into the database, and decrypts it when retrieving. See the references for further examples of how encryption works.

In case of truly hidden data, if its raw representation is not needed (i.e. not be displayed), hashing may also be 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().

예 15-5. Using hashed password field

`<?php
// 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 Injection

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.

예 15-6. Splitting the result set into pages ... and making superusers (PostgreSQL and MySQL)

`<?php $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 a decimal number. However, what if someone tries to break in by appending a 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.

> 참고: 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. The only thing the attacker needs to do is to see if there are any submitted variables used in SQL statements which are 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.

예 15-7. Listing out articles ... and some passwords (any database server)

`<?php $query = "SELECT id, name, inserted, size FROM products WHERE size = '$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 UPDATE's are also susceptible to attack. These queries are also threatened 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 examining the form variable names, or just simply brute forcing. There are not so many naming conventions for fields storing passwords or usernames.

예 15-8. From resetting a password ... to gaining more privileges (any database server)

`<?php $query = "UPDATE usertable SET pwd='$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:

`<?php // $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.

예 15-9. Attacking the database hosts operating system (MSSQL Server)

`<?php $query = "SELECT * FROM products WHERE id LIKE '%$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:

`<?php $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.

> 참고: 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 similarly vulnerable in another manner.

### Avoiding techniques

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 system 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 any kind of input, especially that 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) and onwards to the Perl compatible Regular Expressions support.

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

  예 15-10. A more secure way to compose a query for paging

  `<?php 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 cracked.

- 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 logging. 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. More detail is generally better than less.

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