This excerpt is from SQL Server Security, written by David Litchfield and published by McGraw-Hill/Osborne Media. Read the entire chapter here.
Most companies that have an online presence these days will have a firewall of some kind. The purpose of a firewall is to filter network traffic that passes into and out of an organization's network, limiting use of the network to permitted, "legitimate" users.
One of the conceptual problems with relying on a firewall for security is that the firewall operates at the level of IP addresses and network "ports"—in the OSI model of network protocols, the firewall is a service at layers 3, 4, and 5 (network, transport, and session layers, respectively). Consequently, a firewall doesn't understand the details of higher-level protocols such as HTTP (Hypertext Transfer Protocol, the protocol that runs the Web).
There is a whole class of attacks that operate at the application layer of the OSI model (OSI layer 7), and that by definition pass straight through firewalls. SQL injection is one of these attacks.
SQL Injection Basics
The basic idea behind SQL injection is that an attacker manipulates data passed into a web application to modify the query that is run in the back-end database. This might seem relatively innocuous at first sight, but it can be extremely damaging. The following sections describe SQL injection in depth and some of the steps that can be taken to defend against it.
One of the most worrying aspects of SQL injection is that the most straightforward way of querying a database from an application almost inevitably results in some form of SQL injection bug. The mistakes that result in SQL injection are very easy to make, even if you are aware of them (and most web developers aren't).
Another difficulty faced by organizations that want to rid their infrastructure of SQL injection bugs is that most of the publicly available advice on fixing the problem is flawed in some way, or omits crucial information. For example, the most common "fix" is to replace each occurrence of a single-quote character with two single-quote characters, effectively "escaping" the single quotes. This doesn't completely solve the problem, for reasons we'll go into later in this chapter. First, we'll look at how several SQL injection bugs were found in a real application.
This chapter will detail the basics of SQL injection, and best practices for defending SQL Server against such attacks. Read the entire chapter here.
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