Injection Theory

Author: Jeff Williams
Contributor(s): Jmanico, kingthorin

Injection is an attacker’s attempt to send data to an application in a way that will change the meaning of commands being sent to an interpreter. For example, the most common example is SQL injection, where an attacker sends “101 OR 1=1” instead of just “101”. When included in a SQL query, this data changes the meaning to return ALL records instead of just one. There are lots of interpreters in the typical web environment, such as SQL, LDAP, Operating System, XPath, XQuery, Expression Language, and many more. Anything with a “command interface” that combines data into a command is susceptible. Even XSS is really just a form of HTML injection.

Frequently these interpreters run with a lot of access, so a successful attack can easily result in significant data breaches, or even loss of control of a browser, application, or server. Taken together, injection attacks are a huge percentage of the serious application security risk. Many organizations have poorly thought through security controls in place to prevent injection attacks. Vague recommendations for input validation and output encoding are not going to prevent these flaws. Instead, we recommend a strong set of controls integrated into your application frameworks. The goal is to make injections impossible for developers.

Why Injection Happens to Good Developers

Injection can be complex. The subtleties of data flow, parsers, contexts, capabilities, and escaping are overwhelming even for security specialists. In the following sections we will outline these topics to make it clear how injection can happen in a variety of different technologies.

Untrusted Data

First we need to consider the vehicle for injection attacks – untrusted data.

Untrusted data is most often data that comes from the HTTP request, in the form of URL parameters, form fields, headers, or cookies. But data that comes from databases, web services, and other sources is frequently untrusted from a security perspective. That is, untrusted data is input that can be manipulated to contain a web attack payload. The OWASP Code Review Guide has a decent list of methods that return untrusted data in various languages, but you should be careful about your own methods as well.

Untrusted data should always be treated as though it contains an attack. That means you should not send it anywhere without taking steps to make sure that any attacks are detected and neutralized. As applications get more and more interconnected, the likelihood of a buried attack being decoded or executed by a downstream interpreter increases rapidly.

As untrusted data flows through an application, it is frequently split into parts, combined with safe data, transformed, validated, and encoded in a variety of ways. A single piece of data could go through dozens of these steps before it gets to an interpreter. This makes identifying injection problems very difficult. Tools have a difficult time tracing the entire data flow and understanding exactly what data can run the gauntlet and what cannot.

Injection Context

When untrusted data is used by an application, it is often inserted into a command, document, or other structure. We will call this the injection context. For example, consider a SQL statement constructed with SELECT * FROM users WHERE name='" + request.getParameter( "name" ) + "'"; In this example, the name is data from a potentially hostile user, and so could contain an attack. But the attack is constrained by the injection context. In this case, inside single quotes '. That’s why single quotes are so important for SQL injection.

Consider a few of the types of commands and documents that might allow for injection…

  • SQL queries
  • LDAP queries
  • Operating system command interpreters
  • Any program invocation
  • XML documents
  • HTML documents
  • JSON structures
  • HTTP headers
  • File paths
  • URLs
  • A variety of expression languages
  • etc…

In all of these cases, if the attacker can “break out” of the intended injection context and modify the meaning of the command or document, they might be able to cause significant harm.


Every interpreter has a parser. Injection attacks target those parsers – attempting to trick them into interpreting data as commands. Understanding how a particular interpreter’s parser works is the key to successful injection attacks – and, ultimately, the path to creating defenses against injection.

If you are a student of application security, you should learn as much as you can about how real parsers work. Learn about grammars, and how to read BNF. Beware, though, that the grammar may not match the implementation. Real world parsers have many corner cases and flaws that may not match the spec. A scientific approach to testing the real behavior of a parser is the best course forward.

TBD. Describe different types of parsers, tokens (particularly control characters), BNF.

Injection into References

A “reference” could be a database key, a URL, a filename, or some other kind of lookup index. While injecting into these references doesn’t typically allow for command execution, it’s interesting because the parsers for these references aren’t typically too complicated. However, URLs and filenames can become quite complex. See the “jar:” scheme for examples of non-intuitive syntax begging for injection.

TBD. This is for URLs, paths, and other simple forms. Focus on the parser. Could be as simple as Double.parseDouble (mark of the beast)

Injection into Commands

TBD. Recursive descent or LALR parsers.

Injecting in Hierarchical Documents

To really understand what’s going on with XSS, you have to consider injection into the hierarchical structure of the HTML DOM. Given a place to insert data into an HTML document (that is, a place where a developer has allowed untrusted data to be included in the DOM), there are two ways to inject code:

  • Injecting UP:The most common way is to close the current context and start a new code context. For example, this is what you do when you close an HTML attribute with a "\> and start a new <script> tag. This attack closes the original context (going up in the hierarchy) and then starts a new tag that will allow script code to execute. Remember that you may be able to skip many layers up in the hierarchy when trying to break out of your current context. For example, a </script> tag may be able to terminate a script block even if it is injected inside a quoted string inside a method call inside the script. This happens because the HTML parser runs before the JavaScript parser.

  • Injecting DOWN:The less common way to perform XSS injection is to introduce a code subcontext without closing the current context. For example, if the attacker is able to change <img src="...UNTRUSTED DATA HERE..." /> into <img src="<javascript:alert(document.cookie>)" /> they do not have to break out of the HTML attribute context. Instead, they introduce a subcontext that allows scripting within the src attribute (in this case a javascript url). Another example is the expression() functionality in CSS properties. Even though you may not be able to escape a quoted CSS property to inject up, you may be able to introduce something like xss:expression(document.write(document.cookie)) without ever leaving the current context.

There’s also the possibility of injecting directly in the current context. For example, if you take untrusted input and put it directly into a JavaScript context. While insane, accepting code from an attacker is more common than you might think in modern applications. Generally it is impossible to secure untrusted code with escaping (or anything else). If you do this, your application is just a conduit for attacker code to get running in your users’ browsers.

The rules in this document have been designed to prevent both UP and DOWN varieties of XSS injection. To prevent injecting up, you must escape the characters that would allow you to close the current context and start a new one. To prevent attacks that jump up several levels in the DOM hierarchy, you must also escape all the characters that are significant in all enclosing contexts. To prevent injecting down, you must escape any characters that can be used to introduce a new sub-context within the current context.

Injection with Multiple Nested Parsers

XSS is a form of injection where the interpreter is the browser and attacks are buried in an HTML document. HTML is easily the worst mashup of code and data of all time, as there are so many possible places to put code and so many different valid encodings. HTML is particularly difficult because it is not only hierarchical, but also contains many different parsers (XML, HTML, JavaScript, VBScript, CSS, URL, etc…).




Traditionally, input validation has been the preferred approach for handling untrusted data. However, input validation is not a great solution for injection attacks. First, input validation is typically done when the data is received, before the destination is known. That means that we don’t know which characters might be significant in the target interpreter. Second, and possibly even more importantly, applications must allow potentially harmful characters in. For example, should poor Mr. O’Malley be prevented from registering in the database simply because SQL considers ‘ a special character?

While input validation is important and should always be performed, it is not a complete solution for injection attacks. It’s better to think of input validation as defense in depth and use of escaping as described below as the primary defense.

Using Safe Interfaces

TBD - parameterized interfaces with strong typing

Escaping (aka Output Encoding)

Escaping” is a technique used to ensure that characters are treated as data, not as characters that are relevant to the interpreter’s parser. There are lots of different types of escaping, sometimes confusingly called output “encoding.” Some of these techniques define a special “escape” character, and other techniques have a more sophisticated syntax that involves several characters.

Do not confuse output escaping with the notion of Unicode character encoding, which involves mapping a Unicode character to a sequence of bits. This level of encoding is automatically decoded, and does not defuse attacks. However, if there are misunderstandings about the intended charset between the server and browser, it may cause unintended characters to be communicated, possibly enabling XSS attacks. This is why it is still important to specify the Unicode character encoding (charset), such as UTF-8, for all communications.

Escaping is the primary means to make sure that untrusted data can’t be used to convey an injection attack. There is no harm in escaping data properly - it will still render in the browser properly. Escaping simply lets the interpreter know that the data is not intended to be executed, and therefore prevents attacks from working.