CVSS Vector: AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Cross-site scripting is the injection of client-side scripts into web applications, which is enabled by a lack of validating and correctly encoding user input. The malicious scripts are executed within the browser of the end users and enable a variety of attacks from stealing the end users session to monitoring and altering all actions performed by the end users on the affected website. There are different types of cross-site scripting attacks, which distinguish if the malicious scripts could be injected in a non-persistent or persistent fashion. Furthermore there is differentiation made between the vulnerability being caused by a flawed input validation on the client- or server-side.
There are different types of cross-site scripting attacks, which are introduced in the following sections.
A stored cross-site scripting vulnerability allows an attacker to inject a malicious script persistently in a web application. For example the script might have been submitted via an input field to the web server, which did not perform a sufficient validation and stores the script persistently in the database. The consequence of this might be, that this script is now being delivered to all users visiting the web application and e.g. able to gain access to the session cookies of the user.
A reflected cross-site scripting vulnerability appears if unvalidated input is directly displayed to the user. For example the input of a search form is reflected on the page to show what the search key was. An attacker may craft an URL that contains malicious code and spread the URL via e-mail or social media. A user who clicks on this link opens the (valid) web application, which then runs the malicious code in the user's browser.
To prevent XSS attacks treat all user input as potentially malicious and follow some programming guidelines:
In cases where any untrusted input is shown as normal text inside a HTML tag, filter out the characters which allow an attacker to insert a <script> tag in the page. Use the following functions for that:
In cases where user input needs to be inserted into tag attributes or inside a script, you will need to use stronger escape mechanisms. Refer to the XSS Prevention Cheat Sheet for more information. This is the case if you plan to allow user input in cases such as: