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.
Example 27-2.
Splitting the result set into pages ... and making superusers
(PostgreSQL)
<?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);
?>
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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
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.
Note:
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.
Example 27-3.
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);
?>
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The static part of the query can be combined with another
SELECT statement which reveals all passwords:
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.
Example 27-4.
From resetting a password ... to gaining more privileges (any database server)
<?php
$query = "UPDATE usertable SET pwd='$pwd' WHERE uid='$uid';";
?>
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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:
A frightening example how operating system level commands can be accessed
on some database hosts.
Example 27-5. Attacking the database hosts operating system (MSSQL Server)
<?php
$query = "SELECT * FROM products WHERE id LIKE '%$prod%'";
$result = mssql_query($query);
?>
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If attacker submits the value
a%' exec master..xp_cmdshell 'net user test testpass /ADD' --
to
$prod, then the
$query will be:
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.
Note:
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.
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().
Example 27-6. 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);
?>
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Quote each non numeric user supplied value that is passed to the
database with the database-specific string escape function (e.g.
mysql_escape_string(),
sql_escape_string(), etc.). If a database-specific
string escape mechanism is not available, the
addslashes() and str_replace()
functions may be useful (depending on database type).
See the first example.
As the example shows, adding quotes to the static part of the query
is not enough, making this query easily crackable.
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.