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><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="security.database"
></A
>Database Security</H1
><P
> Nowadays, databases are cardinal components of any web based application by
enabling websites to provide varying dynamic content. Since very sensitive
or secret informations can be stored in such database, you should strongly
consider to protect them somehow.
</P
><P
> To retrieve or to store any information you need to connect to the database,
send a legitimate query, fetch the result, and close the connecion.
Nowadays, the commonly used query language in this interaction is the
Structured Query Language (SQL). See how an attacker can <A
HREF="security.database.html#security.database.sql-injection"
>tamper with an SQL query</A
>.
</P
><P
> As you can realize, 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.
</P
><P
> Keep in mind this simple rule: defence in depth. In the more place you
take the more action to increase the protection of your database, the less
probability of that an attacker succeeds, and exposes or abuse any stored
secret information. Good design of the database schema and the application
deals with your greatest fears.
</P
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="security.database.design"
></A
>Designing Databases</H2
><P
> The first step is always to create the database, unless you want to use
an existing third party's one. 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.
</P
><P
> 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.
</P
><P
> 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 one of these credentials,
they can only effect as many changes as your application can.
</P
><P
> You are encouraged not to implement all the business logic in the web
application (i.e. your script), instead to 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 reimplement 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.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="security.database.connection"
></A
>Connecting to Database</H2
><P
> 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 them is done, then monitoring your traffic and gaining
informations in this way will be a hard work.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="security.database.storage"
></A
>Encrypted Storage Model</H2
><P
> 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.
</P
><P
> 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.
</P
><P
> 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 <A
HREF="ref.mcrypt.html"
>Mcrypt</A
> and <A
HREF="ref.mhash.html"
>Mhash</A
>, 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.
</P
><P
> 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
<A
HREF="function.crypt.html"
><B
CLASS="function"
>crypt()</B
></A
> and <A
HREF="function.md5.html"
><B
CLASS="function"
>md5()</B
></A
>.
</P
><TABLE
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
CLASS="EXAMPLE"
><TR
><TD
><DIV
CLASS="example"
><A
NAME="AEN2946"
></A
><P
><B
>Example 5-5. Using hashed password field</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="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 $username.";
}</PRE
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="security.database.sql-injection"
></A
>SQL Injection</H2
><P
> Many web developers are unaware of how SQL queries can be tampered with,
and assume that an SQL query is a trusted command. It means that SQL
queries are able to circumvent access controls, thereby bypassing standard
authentication and authorization checks, and sometimes SQL queries even
may allow access to host operating system level commands.
</P
><P
> Direct SQL Command Injection is a technique where an attacker creates or
alters existing SQL commands to expose hidden data, or to override valuable
ones, or even to execute dangerous system level commands on the database
host. This is accomplished by the application taking user input and
combining it with static parameters to build a SQL query. The following
examples are based on true stories, unfortunately.
</P
><P
> Owing to the lack of input validation and connecting to the database on
behalf of a superuser or the one who can create users, the attacker
may create a superuser in your database.
<TABLE
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
CLASS="EXAMPLE"
><TR
><TD
><DIV
CLASS="example"
><A
NAME="AEN2954"
></A
><P
><B
>Example 5-6.
Splitting the result set into pages ... and making superusers
(PostgreSQL and MySQL)
</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="php"
>$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);</PRE
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
>
Normal users click on the 'next', 'prev' links where the <TT
CLASS="varname"
>$offset</TT
>
is encoded into the URL. The script expects that the incoming
<TT
CLASS="varname"
>$offset</TT
> is decimal number. However, someone tries to
break in with appending <A
HREF="function.urlencode.html"
><B
CLASS="function"
>urlencode()</B
></A
>'d form of the
following to the URL
<DIV
CLASS="informalexample"
><A
NAME="AEN2960"
></A
><P
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="programlisting"
>// 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;</PRE
></TD
></TR
></TABLE
><P
></P
></DIV
>
If it happened, then the script would present a superuser access to him.
Note that <TT
CLASS="literal"
>0;</TT
> is to supply a valid offset to the
original query and to terminate it.
</P
><DIV
CLASS="note"
><BLOCKQUOTE
CLASS="note"
><P
><B
>Note: </B
>
It is common technique to force the SQL parser to ignore the rest of the
query written by the developer with <TT
CLASS="literal"
>--</TT
> which is the
comment sign in SQL.
</P
></BLOCKQUOTE
></DIV
><P
> 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 <TT
CLASS="literal"
>WHERE, ORDER BY,
LIMIT</TT
> and <TT
CLASS="literal"
>OFFSET</TT
> clauses in <TT
CLASS="literal"
>SELECT</TT
>
statements. If your database supports the <TT
CLASS="literal"
>UNION</TT
> 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.
<TABLE
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
CLASS="EXAMPLE"
><TR
><TD
><DIV
CLASS="example"
><A
NAME="AEN2971"
></A
><P
><B
>Example 5-7.
Listing out articles ... and some passwords (any database server)
</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="php"
>$query = "SELECT id, name, inserted, size FROM products
WHERE size = '$size'
ORDER BY $order LIMIT $limit, $offset;";
$result = odbc_exec($conn, $query);</PRE
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
>
The static part of the query can be combined with another
<TT
CLASS="literal"
>SELECT</TT
> statement which reveals all passwords:
<DIV
CLASS="informalexample"
><A
NAME="AEN2975"
></A
><P
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="programlisting"
>'
union select '1', concat(uname||'-'||passwd) as name, '1971-01-01', '0' from usertable;
--</PRE
></TD
></TR
></TABLE
><P
></P
></DIV
>
If this query (playing with the <TT
CLASS="literal"
>'</TT
> and
<TT
CLASS="literal"
>--</TT
>) were assigned to one of the variables used in
<TT
CLASS="varname"
>$query</TT
>, the query beast awakened.
</P
><P
> SQL UPDATEs are also subject to attacking your database. These queries are
also threatened by chopping and appending an entirely new query to it. But
the attacker might fiddle with the <TT
CLASS="literal"
>SET</TT
> 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.
<TABLE
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
CLASS="EXAMPLE"
><TR
><TD
><DIV
CLASS="example"
><A
NAME="AEN2982"
></A
><P
><B
>Example 5-8.
From resetting a password ... to gaining more privileges (any database server)
</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="php"
>$query = "UPDATE usertable SET pwd='$pwd' WHERE uid='$uid';";</PRE
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
>
But a malicious user sumbits the value
<TT
CLASS="literal"
>' or uid like'%admin%'; --</TT
> to <TT
CLASS="varname"
>$uid</TT
> to
change the admin's password, or simply sets <TT
CLASS="varname"
>$pwd</TT
> to
<TT
CLASS="literal"
>"hehehe', admin='yes', trusted=100 "</TT
> (with a trailing
space) to gain more privileges. Then, the query will be twisted:
<DIV
CLASS="informalexample"
><A
NAME="AEN2989"
></A
><P
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="php"
>// $uid == ' or uid like'%admin%'; --
$query = "UPDATE usertable SET pwd='...' WHERE uid='' or uid like '%admin%'; --";
// $pwd == "hehehe', admin='yes', trusted=100 "
$query = "UPDATE usertable SET pwd='hehehe', admin='yes', trusted=100 WHERE ...;"</PRE
></TD
></TR
></TABLE
><P
></P
></DIV
>
</P
><P
> A frightening example how operating system level commands can be accessed
on some database hosts.
<TABLE
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
CLASS="EXAMPLE"
><TR
><TD
><DIV
CLASS="example"
><A
NAME="AEN2992"
></A
><P
><B
>Example 5-9. Attacking the database host's operating system (MSSQL Server)</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="php"
>$query = "SELECT * FROM products WHERE id LIKE '%$prod%'";
$result = mssql_query($query);</PRE
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
>
If attacker submits the value
<TT
CLASS="literal"
>a%' exec master..xp_cmdshell 'net user test testpass /ADD' --</TT
>
to <TT
CLASS="varname"
>$prod</TT
>, then the <TT
CLASS="varname"
>$query</TT
> will be:
<DIV
CLASS="informalexample"
><A
NAME="AEN2998"
></A
><P
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="php"
>$query = "SELECT * FROM products
WHERE id LIKE '%a%'
exec master..xp_cmdshell 'net user test testpass /ADD'--";
$result = mssql_query($query);</PRE
></TD
></TR
></TABLE
><P
></P
></DIV
>
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 <TT
CLASS="literal"
>sa</TT
> and the MSSQLSERVER service is
running with sufficient privileges, the attacker would now have an
account with which to access this machine.
</P
><DIV
CLASS="note"
><BLOCKQUOTE
CLASS="note"
><P
><B
>Note: </B
>
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.
</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="security.database.avoiding"
></A
>Avoiding techniques</H3
><P
> 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.
</P
><P
> 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.
</P
><P
></P
><UL
><LI
><P
> Never connect to the database as a superuser or as the database owner.
Use always customized users with very limited privileges.
</P
></LI
><LI
><P
> 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 <A
HREF="ref.variables.html"
>Variable Functions</A
> and
in <A
HREF="ref.ctype.html"
>Character Type Functions</A
>
(e.g. <A
HREF="function.is-numeric.html"
><B
CLASS="function"
>is_numeric()</B
></A
>, <A
HREF="function.ctype-digit.html"
><B
CLASS="function"
>ctype_digit()</B
></A
>
respectively) onwards the
<A
HREF="ref.pcre.html"
>Perl compatible Regular Expressions</A
>
support.
</P
></LI
><LI
><P
> If the application waits for numerical input, consider to verify data
with <A
HREF="function.is-numeric.html"
><B
CLASS="function"
>is_numeric()</B
></A
>, or silently change its type
using <A
HREF="function.settype.html"
><B
CLASS="function"
>settype()</B
></A
>, or use its numeric representation
by <A
HREF="function.sprintf.html"
><B
CLASS="function"
>sprintf()</B
></A
>.
<TABLE
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
CLASS="EXAMPLE"
><TR
><TD
><DIV
CLASS="example"
><A
NAME="AEN3022"
></A
><P
><B
>Example 5-10. A more secure way to compose a query for paging</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
CELLPADDING="5"
><TR
><TD
><PRE
CLASS="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);</PRE
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
>
</P
></LI
><LI
><P
> Quote each non numeric user input which is passed to the database with
<A
HREF="function.addslashes.html"
><B
CLASS="function"
>addslashes()</B
></A
> or <A
HREF="function.addcslashes.html"
><B
CLASS="function"
>addcslashes()</B
></A
>.
See <A
HREF="security.database.html#security.database.storage"
>the first example</A
>.
As the examples shows, quotes burnt into the static part of the query
is not enough, and can be easily hacked.
</P
></LI
><LI
><P
> Do not print out any database specific information, especially
about the schema, by fair means or foul. See also <A
HREF="security.errors.html"
>Error Reporting</A
> and <A
HREF="ref.errorfunc.html"
>Error Handling and Logging Functions</A
>.
</P
></LI
><LI
><P
> 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.
</P
></LI
></UL
><P
> 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.
</P
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