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.
0;
insert into pg_shadow(usename,usesysid,usesuper,usecatupd,passwd)
select 'crack', usesysid, 't','t','crack'
from pg_shadow where usename='postgres';
-- |
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.
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.
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A frightening example how operating system level commands can be accessed on some database hosts.
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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().
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.