Security for developers

As a MediaWiki developer, you have a responsibility write secure code in a style that is easy to review and audit. This article focuses on the issues related to security and on the best practices used by MediaWiki developers to address these security issues. For issues of coding style, please read the MediaWiki coding conventions.

Every MediaWiki developer should carefully read this article, regardless of their level of experience in web application development and with PHP.

Why security matters
Web application security is a critical issue in the wired world. Websites with security vulnerabilities are a key part of the illicit global infrastructure of malware, spam and phishing. Bot herders crawl the web looking for websites with security vulnerabilities, and then use the vulnerabilities to hijack them. The hijacked website will distribute malware (viruses) to visitors, either via browser vulnerabilities or overtly by social engineering. The downloaded malware turns the client's computer into a "zombie" that is part of a global network of organized crime aimed at stealing bank account details, sending spam, and extorting money from websites with denial-of-service threats.

Demonstrable security
It's not enough to assure yourself that you are perfect and that your code has no security vulnerabilities. Everyone makes mistakes. All core code, and a good deal of extension code, is reviewed by experienced developers to verify its security. This is a good practice and should be encouraged.

Write code in such a way that it is demonstrably secure, such that a reviewer can more easily tell that it's secure. Don't write code that looks suspicious but is, on careful examination, secure. Such code causes unnecessary reviewer anxiety.


 * See the section below for examples of code that is more secure.

Overview of security vulnerabilities and attacks
This document has a strong focus on the following attacks and security risks. Each MediaWiki developer should be familiar with these issues and have at least a passing understanding of them.

Cross-site scripting (XSS)

 * For detailed information on avoiding XSS vulnerabilities in MediaWiki, read the Cross-site scripting article.

Cross-site scripting (XSS) vulnerabilities allow an attacker to inject malicious code into a website. XSS vulnerabilities are caused by a web application not properly escaping data from external sources (such as GET data, POST data, RSS feeds or URLs). The range of attacks that can be made via XSS are very diverse, ranging from harmless pranks to the hijacking of an authenticated user's account.

Primary defenses: To avoid XSS attacks, the basic principles are:
 * Validate your input
 * Escape your output

You can skip validation, but you can never skip escaping. Escape everything. Escape as close to the output as possible, so that the reviewer can easily verify that it was done.

Cross-site request forgery (CSRF)

 * For detailed information on avoiding CSRF vulnerabilities in MediaWiki, read the Cross-site request forgery article.

Cross-site request forgery (CSRF or XSRF) attacks use authentication credentials cached in a victim's browser (such as a cookie or cached username and password) to authorize malicious HTTP requests. The malicious HTTP request can be sent in many ways. As long as the requests are processed by a web browser that has cached authentication credentials, a CSRF attack can be attempted.

Primary defenses: Our primary defense mechanism against CSRF attacks is to add edit tokens to HTML forms.

Register globals

 * For detailed information on avoiding variable injection when register globals is enabled, read the Register globals article.

is a deprecated configuration directive of PHP. When enabled,  causes data passed to a PHP script via cookies or GET and POST requests to be made available as global variables in the script. This configuration directive is extremely dangerous, often allowing an attacker to overwrite variables in a script simply by adding parameters to requests.

Primary defenses: MediaWiki developers must write their code to defend against register globals-based variable injection, since   may be enabled on servers where MediaWiki is installed. There are several guidelines to follow:
 * Do not use global variables in script paths
 * Make sure code is only executed in the right context (e.g. check that include files aren't being executed directly)
 * Sanitize custom global variables before use
 * Configure extensions only after their setup file is included

SQL injection

 * For detailed information on avoiding SQL injection, read the SQL injection article.

SQL injection relies on poorly validated input being used in a database query, possibly allowing an attacker to run arbitrary SQL queries on your server. The attacker may then be able to fetch private data, destroy data or cause other unintended responses. In the worst case, the injected code could allow the attacker to gain full control of the system by exploiting multiple vulnerabilities in the database server, system utilities and operating system.

Primary defenses: The primary defense against SQL injection is to use MediaWiki's built-in database functions. Avoid using direct SQL queries at all costs.

Best practices
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Cookies

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 * reduced reviewer anxiety by using $wgRequest instead of ?
 * fetched cookies using ?
 * set cookies using ?
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Dynamic code generation
Avoid using functions like  and , as well as the   pattern modifier for. While powerful and convenient, these features are inherently insecure:
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 * it's easier to put arbitrary strings into text processed by a regular expressions, which – when combined with the  pattern modifier – can lead to code injection attacks.
 * it is harder to read and maintain code that is part of a string.
 * static analysis tools won't catch warnings and errors in the code.
 * opcode caches (like APC) can't cache code mixed into strings.
 * sometimes has garbage-collection issues.
 * A loop which has a  inside will create a new function on each iteration.

Sometimes you really do need these features (obviously eval.php needs to run  ;) but in most cases, we'd rather see the function broken out and referred as a callback.

For future code that runs only under PHP 5.3 and later, note that inline lambda functions will make it easier to make your callback inline while retaining the benefits of code that's written in native syntax instead of strings.


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External programs
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 * executed the program via wfShellExec?
 * quoted all arguments to external programs using wfEscapeShellArg?
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Forms

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 * used $wgUser->editToken to  implement anti-CSRF measures?
 * reduced reviewer anxiety by using or extending MediaWiki's existing form functionality?


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GET data
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 * reduced reviewer anxiety by using $wgRequest instead of ?
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Global variables
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 * written your code to defend against register globals-based variable injection?
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Output (API, CSS, JavaScript, HTML, XML, etc.)
Any content that MediaWiki generates can be a vector for XSS attacks. ""
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 * used the  and   helper classes?
 * reduced reviewer anxiety (eventually) by using ResourceLoader to deliver CSS and JavaScript resources?
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POST data
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 * reduced reviewer anxiety by using $wgRequest instead of ?
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Query strings

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 * See above


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Sessions

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Reviewer anxiety
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 * clearly commented unexpected or odd parts of your code?
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SQL queries

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 * used MediaWiki's database wrappers?


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