Cross-Site Scripting (XSS) is an attack vector where hackers inject malicious code into a vulnerable web application. XSS differs from other web attack vectors in that it does not directly target the application itself; instead, users of the web application are the ones at risk. Cross-site scripting vulnerabilities normally allow attackers to masquerade as a victim user, carrying out any actions that the user is able to perform and accessing any of the user’s data. If the user has privileged (administrator) access within the application, then the attacker might be able to obtain complete control over all of the system’s functionality and data. This would cause most, if not all, user accounts to be compromised. Trojan horse malware can also be activated and page content modified, tricking users into voluntarily giving out their private data. Furthermore, session cookies could be revealed, allowing the hacker to imitate authentic users and exploit their private accounts.
Source: Imperva, PortSwigger
Additional Reading: What is an XSS Attack? – FedTech Magazine
Related Terms: Cross-Site Request Forgery (CSRF)
What does this mean for an SMB?
If your SMB doesn’t develop your own software or do coding, you don’t need to concern yourself with XSS other than knowing it exists. However, if you do development, read on.
Preventing cross-site scripting is difficult in some cases and can be more complex depending on the design of the application and how it handles user-controllable data. In general, effectively preventing XSS vulnerabilities is likely to involve a combination of the following measures, listed below. If your organization has IT staff or hires a third party to support your systems, work with them to improve your security by implementing these measures to defend against XSS attacks:
- Filter input on arrival. At the point where user input is received, filter as strictly as possible based on what is expected or valid input.
- Encode data on output. At the point where user-controllable data is output in HTTP responses, encode the output to prevent it from being interpreted as active content. Depending on the output context, this might require applying combinations of HTML, URL, JavaScript, and CSS encoding.
- Use appropriate response headers. To prevent XSS in HTTP responses that aren’t intended to contain any HTML or JavaScript, you can use the
Content-TypeandX-Content-Type-Optionsheaders to ensure that browsers interpret the responses in the way you intend. - Content Security Policy. As a last line of defense, you can use Content Security Policy (CSP) to reduce the severity of any XSS vulnerabilities that still occur.