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. 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 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. 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. 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. 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 OpenSSL and Sodium, 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 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. The password functions provide a convenient way to hash sensitive data and work with these hashes. password_hash is used to hash a given string using the strongest algorithm currently available and password_verify checks whether the given password matches the hash stored in database. ]]> 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 an 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. ]]> 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. 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. ]]> 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. ]]> But if a malicious user submits the value ' or uid like'%admin% to $uid to change the admin's password, or simply sets $pwd to hehehe', trusted=100, admin='yes to gain more privileges, then, the query will be twisted: ]]> A frightening example of how operating system level commands can be accessed on some database hosts. ]]> 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. 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. A worked example of the issues regarding SQL Injection Image courtesy of xkcd 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'. 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. Use prepared statements with bound variables. They are provided by PDO, by MySQLi and by other libraries. 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 ctype_digit, or silently change its type using settype, or use its numeric representation by sprintf. ]]> 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. mysql_real_escape_string, sqlite_escape_string, etc.). Generic functions like addslashes are useful only in a very specific environment (e.g. MySQL in a single-byte character set with disabled NO_BACKSLASH_ESCAPES) so it is better to avoid them. 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.