Security checklist for developers

This document is provided as a supplement to Security for developers . This is a list of common development tasks, and the security measures that need to be taken.

Security checklist

If you are working with ...	have you ...
Browser Cookies	reduced reviewer anxiety by using $wgRequest instead of $_COOKIE? fetched cookies using $wgRequest->getCookie(...)? set cookies using $wgRequest->response()->setCookie(...)? # Attempt to fetch the UserID cookie value. # Note: the value returned isn't trusted and is forced to be an int. $sId = intval( $wgRequest->getCookie( 'UserID' ) );
Dynamic code generation	Avoid using functions like eval() and create_function(), as well as the /e pattern modifier for preg_replace(). While powerful and convenient, these features are inherently insecure: it's easier to put arbitrary strings into text processed by regular expressions, which – when combined with the /e pattern modifier – can lead to code injection attacks. it is harder to read and maintain code that is part of a string. static analysis tools won't catch warnings and errors in the code. opcode caches (like APC) can't cache code mixed into strings. create_function() sometimes has garbage-collection issues. A loop which has a create_function() inside will create a new function on each iteration. Sometimes you really do need these features (obviously eval.php needs to run eval();) but in most cases, we'd rather see the function broken out and referred as a callback. Inline lambda functions will make it easier to make your callback inline while retaining the benefits of code that's written in native syntax instead of strings. Anything external that is used in part of regex should be escaped with preg_quote ($externalStr, $delimiter). It puts a backslash in front of every character that is part of the regular expression syntax, and escapes also the delimiter given as second parameter: $str = preg_replace( "!" . preg_quote( $externalStr, '!' ) . "!", $replacement, $str );
External programs	executed the program via Shell::command() from namespace MediaWiki\Shell? quoted all arguments to external programs using the above's secure parameter passing facilities (which is basically everything except for unsafeParams())? // Automatically escape any naughty characters $result = Shell::command( $cmd, '--version' ) ->params( 'some', 'extra', 'parameters' ) ->execute(); Note that old wfShellExec()/wfEscapeShellArg() are not recommended because they make it easier for developers to miss escaping a parameter.
Forms	used User ::getEditToken() to implement anti-CSRF measures? used User::matchEditToken() when checking the token to avoid timing attacks? reduced reviewer anxiety by using or extending MediaWiki's existing form functionality?
GET data	reduced reviewer anxiety by using $wgRequest instead of $_GET? # Check if the action parameter is set to 'purge' if ( $wgRequest->getVal( 'action' ) == 'purge' ) { ...
Output (API, CSS, JavaScript, HTML, XML, etc.) Any content that MediaWiki generates can be a vector for XSS attacks.	used the Html and Xml helper classes? # rawElement() escapes all attribute values # (which, in this case, is provided by $myClass) echo Html::rawElement( 'p', [ 'class' => $myClass ] ); reduced reviewer anxiety by using ResourceLoader to deliver CSS and JavaScript resources?
User provided CSS User provided CSS (Say for use in a style attribute) needs to be sanitized to prevent XSS, as well as to disallow insertion of tracking images (via background-image), etc	Use the Sanitizer::checkCss method for any css received from a user, possibly along with the Html class. # let $CSSFromUser be the user's CSS. echo Html::rawElement( 'p', [ 'style' => Sanitizer::checkCss( $CSSFromUser ) ] ); For CSS provided by the extension (and not the user), this is not needed (and will remove some valid things like background-image:). However, extension provided CSS should go in stylesheets loaded by ResourceLoader, and not in style attributes.
POST data	reduced reviewer anxiety by using $wgRequest instead of $_POST Always validate that any POST data received is what you expect it to be # Check if the action parameter is set to 'render' if ( $wgRequest->getVal( 'action' ) == 'render' ) { ...
Query strings	See #GET data above
Sessions
Reviewer anxiety	Clearly added comments to explain unexpected or odd parts of your code? # $wgRequest isn't yet available. Forced to use $_GET instead. if ( $_GET['setupTestSuite'] !== null ) { $setupTestSuiteName = $_GET['setupTestSuite']; ...
SQL queries	used MediaWiki's database wrappers?

Automated checking

Some of these issues can be checked with phan-taint-check-plugin, which is required for all MediaWiki code in Wikimedia production. This is of course just a tool, and it cannot detect all issue types, and may miss issues even in the issue types it can check for.

See also

• Security for developers