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<?xml version="1.0" encoding="utf-8"?> | |
<!-- $Revision$ --> | |
<chapter xml:id="security.database" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink"> | |
<title>Database Security</title> | |
<simpara> | |
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. | |
</simpara> | |
<simpara> | |
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 <link | |
linkend="security.database.sql-injection">tamper with an SQL query</link>. | |
</simpara> | |
<simpara> | |
As you can surmise, <acronym>PHP</acronym> 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 <acronym>PHP</acronym> scripts. | |
</simpara> | |
<simpara> | |
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. | |
</simpara> | |
<sect1 xml:id="security.database.design"> | |
<title>Designing Databases</title> | |
<simpara> | |
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. | |
</simpara> | |
<simpara> | |
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. | |
</simpara> | |
<simpara> | |
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. | |
</simpara> | |
</sect1> | |
<sect1 xml:id="security.database.connection"> | |
<title>Connecting to Database</title> | |
<simpara> | |
You may want to establish 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. | |
</simpara> | |
<!--simpara> | |
If your database server has native SSL support, consider using <link | |
linkend="ref.openssl">OpenSSL functions</link> in communication between | |
<acronym>PHP</acronym> and database via SSL. | |
</simpara--> | |
</sect1> | |
<sect1 xml:id="security.database.storage"> | |
<title>Encrypted Storage Model</title> | |
<simpara> | |
SSL/SSH protects data travelling from the client to the server: SSL/SSH | |
does not protect persistent data stored in a database. SSL is an | |
on-the-wire protocol. | |
</simpara> | |
<simpara> | |
Once an attacker gains access to your database directly (bypassing the | |
webserver), 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. | |
</simpara> | |
<simpara> | |
The easiest way to work around this problem is to first create your own | |
encryption package, and then use it from within your <acronym>PHP</acronym> scripts. <acronym>PHP</acronym> | |
can assist you in this with several extensions, such as <link | |
linkend="book.openssl">OpenSSL</link> and <link | |
linkend="book.sodium">Sodium</link>, 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. | |
</simpara> | |
<sect2 xml:id="security.database.storage.hashing"> | |
<title>Hashing</title> | |
<simpara> | |
In the case of truly hidden data, if its raw representation is not needed | |
(i.e. will not be displayed), hashing should be taken into consideration. | |
The well-known example for hashing is storing the cryptographic hash of a | |
password in a database, instead of the password itself. | |
</simpara> | |
<simpara> | |
The <link linkend="ref.password">password</link> functions | |
provide a convenient way to hash sensitive data and work with these hashes. | |
</simpara> | |
<simpara> | |
<function>password_hash</function> is used to hash a given string using the | |
strongest algorithm currently available and <function>password_verify</function> | |
checks whether the given password matches the hash stored in database. | |
</simpara> | |
<example> | |
<title>Hashing password field</title> | |
<programlisting role="php"> | |
<![CDATA[ | |
<?php | |
// storing password hash | |
$query = sprintf("INSERT INTO users(name,pwd) VALUES('%s','%s');", | |
pg_escape_string($username), | |
password_hash($password, PASSWORD_DEFAULT)); | |
$result = pg_query($connection, $query); | |
// querying if user submitted the right password | |
$query = sprintf("SELECT pwd FROM users WHERE name='%s';", | |
pg_escape_string($username)); | |
$row = pg_fetch_assoc(pg_query($connection, $query)); | |
if ($row && password_verify($password, $row['pwd'])) { | |
echo 'Welcome, ' . htmlspecialchars($username) . '!'; | |
} else { | |
echo 'Authentication failed for ' . htmlspecialchars($username) . '.'; | |
} | |
?> | |
]]> | |
</programlisting> | |
</example> | |
</sect2> | |
</sect1> | |
<sect1 xml:id="security.database.sql-injection"> | |
<title>SQL Injection</title> | |
<simpara> | |
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. | |
</simpara> | |
<simpara> | |
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 an SQL query. The following | |
examples are based on true stories, unfortunately. | |
</simpara> | |
<para> | |
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. | |
<example> | |
<title> | |
Splitting the result set into pages ... and making superusers | |
(PostgreSQL) | |
</title> | |
<programlisting role="php"> | |
<![CDATA[ | |
<?php | |
$offset = $argv[0]; // beware, no input validation! | |
$query = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;"; | |
$result = pg_query($conn, $query); | |
?> | |
]]> | |
</programlisting> | |
</example> | |
Normal users click on the 'next', 'prev' links where the <varname>$offset</varname> | |
is encoded into the <acronym>URL</acronym>. The script expects that the incoming | |
<varname>$offset</varname> is a decimal number. However, what if someone tries to | |
break in by appending a <function>urlencode</function>'d form of the | |
following to the <acronym>URL</acronym> | |
<informalexample> | |
<programlisting role="sql"> | |
<![CDATA[ | |
0; | |
insert into pg_shadow(usename,usesysid,usesuper,usecatupd,passwd) | |
select 'crack', usesysid, 't','t','crack' | |
from pg_shadow where usename='postgres'; | |
-- | |
]]> | |
</programlisting> | |
</informalexample> | |
If it happened, then the script would present a superuser access to him. | |
Note that <literal>0;</literal> is to supply a valid offset to the | |
original query and to terminate it. | |
</para> | |
<note> | |
<para> | |
It is common technique to force the SQL parser to ignore the rest of the | |
query written by the developer with <literal>--</literal> which is the | |
comment sign in SQL. | |
</para> | |
</note> | |
<para> | |
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 <literal>WHERE, ORDER BY, | |
LIMIT</literal> and <literal>OFFSET</literal> clauses in <literal>SELECT</literal> | |
statements. If your database supports the <literal>UNION</literal> 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. | |
<example> | |
<title> | |
Listing out articles ... and some passwords (any database server) | |
</title> | |
<programlisting role="php"> | |
<![CDATA[ | |
<?php | |
$query = "SELECT id, name, inserted, size FROM products | |
WHERE size = '$size'"; | |
$result = odbc_exec($conn, $query); | |
?> | |
]]> | |
</programlisting> | |
</example> | |
The static part of the query can be combined with another | |
<literal>SELECT</literal> statement which reveals all passwords: | |
<informalexample> | |
<programlisting role="sql"> | |
<![CDATA[ | |
' | |
union select '1', concat(uname||'-'||passwd) as name, '1971-01-01', '0' from usertable; | |
-- | |
]]> | |
</programlisting> | |
</informalexample> | |
If this query (playing with the <literal>'</literal> and | |
<literal>--</literal>) were assigned to one of the variables used in | |
<varname>$query</varname>, the query beast awakened. | |
</para> | |
<para> | |
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 <literal>SET</literal> 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. | |
<example> | |
<title> | |
From resetting a password ... to gaining more privileges (any database server) | |
</title> | |
<programlisting role="php"> | |
<![CDATA[ | |
<?php | |
$query = "UPDATE usertable SET pwd='$pwd' WHERE uid='$uid';"; | |
?> | |
]]> | |
</programlisting> | |
</example> | |
But if a malicious user submits the value | |
<literal>' or uid like'%admin%</literal> to <varname>$uid</varname> to | |
change the admin's password, or simply sets <varname>$pwd</varname> to | |
<literal>hehehe', trusted=100, admin='yes</literal> to gain more | |
privileges, then, the query will be twisted: | |
<informalexample> | |
<programlisting role="php"> | |
<![CDATA[ | |
<?php | |
// $uid: ' or uid like '%admin% | |
$query = "UPDATE usertable SET pwd='...' WHERE uid='' or uid like '%admin%';"; | |
// $pwd: hehehe', trusted=100, admin='yes | |
$query = "UPDATE usertable SET pwd='hehehe', trusted=100, admin='yes' WHERE | |
...;"; | |
?> | |
]]> | |
</programlisting> | |
</informalexample> | |
</para> | |
<para> | |
A frightening example of how operating system level commands can be accessed | |
on some database hosts. | |
<example> | |
<title>Attacking the database hosts operating system (MSSQL Server)</title> | |
<programlisting role="php"> | |
<![CDATA[ | |
<?php | |
$query = "SELECT * FROM products WHERE id LIKE '%$prod%'"; | |
$result = mssql_query($query); | |
?> | |
]]> | |
</programlisting> | |
</example> | |
If attacker submits the value | |
<literal>a%' exec master..xp_cmdshell 'net user test testpass /ADD' --</literal> | |
to <varname>$prod</varname>, then the <varname>$query</varname> will be: | |
<informalexample> | |
<programlisting role="php"> | |
<![CDATA[ | |
<?php | |
$query = "SELECT * FROM products | |
WHERE id LIKE '%a%' | |
exec master..xp_cmdshell 'net user test testpass /ADD' --%'"; | |
$result = mssql_query($query); | |
?> | |
]]> | |
</programlisting> | |
</informalexample> | |
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 <literal>sa</literal> and the MSSQLSERVER service is | |
running with sufficient privileges, the attacker would now have an | |
account with which to access this machine. | |
</para> | |
<note> | |
<para> | |
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. | |
</para> | |
</note> | |
<para> | |
<mediaobject> | |
<alt>A worked example of the issues regarding SQL Injection</alt> | |
<imageobject> | |
<imagedata fileref="en/security/figures/xkcd-bobby-tables.png" format="PNG"/> | |
</imageobject> | |
</mediaobject> | |
Image courtesy of <link xlink:href="&url.xkcd;327">xkcd</link> | |
</para> | |
<sect2 xml:id="security.database.avoiding"> | |
<title>Avoidance Techniques</title> | |
<simpara> | |
While it remains obvious that an attacker must possess at least some | |
knowledge of the database architecture in order to conduct a successful | |
attack, obtaining this information is often very simple. For example, | |
if the database is part of an open source or other publicly-available | |
software package with a default installation, this information is | |
completely open and available. This information may also be divulged | |
by closed-source code - even if it's encoded, obfuscated, or compiled - | |
and even by your very own code through the display of error messages. | |
Other methods include the user of common table and column names. For | |
example, a login form that uses a 'users' table with column names | |
'id', 'username', and 'password'. | |
</simpara> | |
<simpara> | |
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. | |
</simpara> | |
<itemizedlist> | |
<listitem> | |
<simpara> | |
Never connect to the database as a superuser or as the database owner. | |
Use always customized users with very limited privileges. | |
</simpara> | |
</listitem> | |
<listitem> | |
<simpara> | |
Use prepared statements with bound variables. They are provided | |
<link linkend="pdo.prepared-statements">by PDO</link>, | |
<link linkend="mysqli.quickstart.prepared-statements">by MySQLi</link> | |
and by other libraries. | |
</simpara> | |
</listitem> | |
<listitem> | |
<simpara> | |
Check if the given input has the expected data type. <acronym>PHP</acronym> has | |
a wide range of input validating functions, from the simplest ones | |
found in <link linkend="ref.var">Variable Functions</link> and | |
in <link linkend="ref.ctype">Character Type Functions</link> | |
(e.g. <function>is_numeric</function>, <function>ctype_digit</function> | |
respectively) and onwards to the | |
<link linkend="ref.pcre">Perl compatible Regular Expressions</link> | |
support. | |
</simpara> | |
</listitem> | |
<listitem> | |
<para> | |
If the application waits for numerical input, consider verifying data | |
with <function>ctype_digit</function>, or silently change its type | |
using <function>settype</function>, or use its numeric representation | |
by <function>sprintf</function>. | |
<example> | |
<title>A more secure way to compose a query for paging</title> | |
<programlisting role="php"> | |
<![CDATA[ | |
<?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); | |
?> | |
]]> | |
</programlisting> | |
</example> | |
</para> | |
</listitem> | |
<listitem> | |
<simpara> | |
If the database layer doesn't support binding variables then | |
quote each non numeric user supplied value that is passed to the | |
database with the database-specific string escape function (e.g. | |
<function>mysql_real_escape_string</function>, | |
<function>sqlite_escape_string</function>, etc.). | |
Generic functions like <function>addslashes</function> are useful only | |
in a very specific environment (e.g. MySQL in a single-byte character | |
set with disabled <varname>NO_BACKSLASH_ESCAPES</varname>) so it is | |
better to avoid them. | |
</simpara> | |
</listitem> | |
<listitem> | |
<simpara> | |
Do not print out any database specific information, especially | |
about the schema, by fair means or foul. See also <link | |
linkend="security.errors">Error Reporting</link> and <link | |
linkend="ref.errorfunc">Error Handling and Logging Functions</link>. | |
</simpara> | |
</listitem> | |
<listitem> | |
<simpara> | |
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. | |
</simpara> | |
</listitem> | |
</itemizedlist> | |
<simpara> | |
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. | |
</simpara> | |
</sect2> | |
</sect1> | |
</chapter> | |
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