programming secure extensions

While mostly the functionality of an extension is set of great value, the safety aspect of the programmed code is clearly less respected. In this section we will make you sensible for safety relevant aspects you should take care of during extension development. In addition we will show you some concepts implemented by extbase that increase the safety of an extension.

A basic principle that you don't have to disregard when programming extensions is, that you should never trust the user input. All input data your extension gets from the user can be potentially malicious. That applies for all data that are transferred via GET and POST over from a form. But also cookies should be classified as malicious, because they can be manipulated by the user.

In the daily programming, all the data that comes from the user should be treated with carefulness - check always if the format of the data corresponds with the format you expected. For example you should check for a field that contains an email address, that a valid email address was entered and not any other text. Here is the validating framework of extbase, you have learned about in the past section, much helpful.

Especially critical are the positions where directly communicated with the database, e.g. with the SQL query language. In the next section we will show what is to care of with it. After this we present some concepts that extbase and fluid uses internally to increase the security of an extension. We will show you how queries that changes data are to be secured by extbase. Next we addict to the Cross Site Scripting and illustrate how to secure your own extensions.

create own database queries

Even though you will mostly use the query language of extbase (see section "implementing custom queries" in chapter 6) to formulate database queries, there is an option to directly formulate SQL queries. That is very helpful for example when you need performance optimization. Always create your own SQL queries in repository classes, to have the potential unsafe code at a defined place.

If you create own SQL queries you always have to convert the input data to the desired format, for example to a number with the use of intval().

Tip

More hints for safety programming with PHP you find also in the PHP handbook at http://php.net/security .

Now we want to present some concepts that are used by extbase and fluid to increase the security of an extension. First we explain how requests that changes data are verified by extbase. After that we explain Cross Site Scripting in order that you can secure your extension for that effect.

Trusted Properties

In the section "mapping arguments" above in this chapter we have explained the transparent argument mapping. For this all properties that are to be send, were changed transparent on the object. Certainly this implies a safety risk, that we will explain with an example: Assume we have a form to edit a user object. This object has the properties username, email, password and description. We want to provide the user a form to change all properties, except the username (because the username should not be changed in our system).

The form looks (shortened) like this:

<f:form name="user" object="{user}" action="update">
   <f:form.textbox property="email" />
   <f:form.textbox property="password" />
   <f:form.textbox property="description" />
</f:form>

If the form is sent, the argument mapping for the user object gets this array:

[
   __identity => ...
   email =>  ...
   password => ...
   description => ...
],

Because the __identity property and further properties are set, the argument mapper gets the object from the persistence layer, makes a copy and then applies the changed properties to the object. After this normally we call the method update($user) for the corresponding repository to make the changes persistent.

What happened if an attacker manipulates the form data and transfers an additional field username to the server? In this case the argument mapping would also change the $username property of the cloned object - although we actual said that this property should not be changed by the user itself.

To avoid this problem fluid creates a hidden form field __trustedProperties which contains information about what properties are to be trusted. Once a request reaches the server, the property mapper of Extbase compares the incoming fields with the property names, defined by the __trustedProperties argument.

As the content of said field could also be manipulated by the client, the field does not only contain a serialized array of trusted properties but also a hash of that array. On the server side, the hash is also compared to ensure the data has not been tampered with on the client side.

So only the form fields that are generated by Fluid with the appropriate ViewHelpers are transferred to the server. If an attacker tries, like described above, to add a field on the client side, this is detected by the property mapper and an exception will be thrown.

In general __trustedProperties should work completely transparent for you, you don't have to know how it works in detail. You have to know this background knowledge only if you want to change data via JavaScript or webservices.

Prevent Cross Site Scripting

Fluid contains some integrated techniques to secure web applications per default. One of the important parts for this is the automatic prevention against cross site scripting, that counts to the most used attack against web applications. In this section we give you a problem description and show how you can avoid cross site scripting (XSS).

Assume you have programmed a forum. An "evil" user will get access to the admin account. For this he posted following harmful looking message in the forum to try to embed JavaScript code:

<script type="text/javascript">alert("XSS");</script>

When he let display the forum post he gets, if the programmer of the forum has made no additional preventions, a JavaScript popup "XSS". The attacker now knows that every JavaScript he write in a post, is executed when displaying the post - the forum is vulnerable for cross site scripting. Now the attacker can replace the code with a more complex JavaScript program, that for example can read the cookies of the visitors of the forum and send them to a certain URL.

If an administrator retrieve this prepared forum post, his session ID (that is stored in a cookie) is transferred to the attacker. By setting the cookie at the attacker himself, in the worst case he can get administrator privileges.

How can we prevent this now? The forum post don't have to put out unchanged - before we have to mask out all special characters with a call of htmlspecialchars(). With this instead of <script>..</script> the safe result is delivered to the browser: &amp;lt;script&amp;gt;...&amp;lt;/script&amp;gt;. So the content of the script tag is no longer executed as JavaScript, but only displayed.

But there is a problem with this: If you miss only at one place the clean masking of the data, a XSS hole exists in the system.

In Fluid the output of every object accessor that occurs in a template is automatically processed by htmlspecialchars(). But Fluid uses htmlspecialchars() only for templates with the extension .html, e.g. if the output format is set to HTML. If you use other output formats it is disabled and you have to make sure to mask the special characters correct. Also deactivated is is it for object accessors that are used in arguments of a ViewHelper. A short example for this:

{variable1}
<f:format.crop append="{variable2}">a very long text</f:format.crop>

The content of {variable1} is send thru htmlspecialchars(), instead the content of {variable2} is not changed. The ViewHelper must get the unchanged data because ewe can not foresee what he will be done with the data. For this reason ViewHelper that output parameter directly have to mask them correct.