Security for developers



As a MediaWiki developer, you have a responsibility to 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, and periodically re-familiarize themselves with this material. Additionally, every developer should carefully read the articles on Cross-site scripting (XSS), Cross-site request forgery (CSRF), and SQL injection, which each provide more detailed explanations of each of these common vulnerability types. The security checklist for developers provides a useful reference for common development tasks.

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.

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.

Note that output encoding (escaping) is context sensitive. So be aware of the intended output context and encode appropriately (e.g. HTML entity, URL, JavaScript, etc.)

The OWASP Abridged XSS Prevention Cheat Sheet is a useful and up to date quick reference guide for mitigating XSS issues.

If you are writing JavaScript, please ensure that you understand DOM-based XSS, and how to prevent it in your code.

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.

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.

serialize/unserialize
unserialize can lead to arbitrary code execution. To prevent this MediaWiki developers should always use json instead of php serialization in new code.

Primary defenses: Use FormatJson::encode and FormatJson::decode instead of PHP's native serialization.

Historical

 * register_globals, has not been an issue since MediaWiki 1.24+.

Books

 * Tobias Wassermann: "Sichere Webanwendungen mit PHP". ISBN 9783826617546