If you are developing a WordPress theme or plugin then it is important to make it as secure and robust as possible. One way to do this is to properly escape values that you use in your project so that XSS (Cross-Site-Scripting) is prevented.
For instance, lets say that in your WordPress site you have created a custom piece of functionality that takes some user data, lets say something simple like a signup form:
And lets say that elsewhere in WordPress you output this into a table of form fields without any escaping (this is a super simplified example).
A malicious customer could enter there name as:
Malicious User" onclick="alert( 'Attack!' );"
When the data is loaded on your unescaped form, the first
" would end the
value="" attribute, and the
onclick="" would be inserted into the form, resulting in the following for the input is clicked:
True this would be a pretty mild piece of malicious code, but as you can imagine this could be exchanged for some very nasty code indeed.
Escape all the things
This horrible and malicious attack could have just been avoided if we had escaped the attribute using
esc_attr() like so:
<input type="text" value="<?php echo esc_attr( $my_value );?>"/>
There are a load of different escapes that you can use for a whole variety of scenarios (e.g. escaping URLs, escaping Email Addresses), and there is a fantastic section in the WordPress handbook all about Escaping.
Something that is equally important is sanitising your data, which you should do whenever you receive data from the user (remember to sanitise when you get it, escape when you output it). Information about sanitising is available in the WordPress handbook also.