Cross-Site Scripting, commonly known as XSS, is a prevalent vulnerability type that is widely spread and easily detectable. It is one of the significant vulnerabilities listed in the OWASP TOP 10. As a professional penetration tester (pentester), understanding and teaching about XSS is an essential part of my role.
What is XSS?
XSS is a vulnerability where an attacker can inject untrusted snippets of JavaScript into your application without validation. This JavaScript is then executed by the victim who is visiting the target site. XSS is classified into three types: Reflected XSS, Stored XSS, and DOM-Based XSS.
Reflected XSS
In Reflected XSS, an attacker sends the victim a link to the target application through email, social media, etc. This link has a script that executes when visiting the target site.
Stored XSS
In Stored XSS, the attacker can plant a persistent script in the target website which will execute when anyone visits it.
DOM-Based XSS
With DOM-Based XSS, no HTTP request is required; the script is injected as a result of modifying the DOM of the target site in the client-side code in the victim’s browser and is then executed.
Potential Risks of Cross-Site Scripting
The risks associated with XSS are significant. The attacker can compromise or take over the victim’s user account in the application. They could retrieve data from the target web application, modify content on the target page, redirect the victim to another malicious or spoof site, or use it to install other malware on the victim’s system. The consequences of any of the above can seriously impact your ability to conduct business, your customers, and your organization’s reputation.
Defenses against Cross-Site Scripting
As a pentester, it’s crucial to teach about the defenses against XSS. Here are some key points to consider:
- What input do we trust? Does it adhere to expected patterns?
- Never reflect untrusted data. This applies to data within our database too.
- Encoding of context (Java/attribute/HTML/CSS) is essential.
Understanding and mitigating XSS vulnerabilities is a critical aspect of maintaining secure web applications. As a pro pentester, it’s my job not only to identify these vulnerabilities but also to educate others about them. By spreading knowledge about these threats, we can collectively work towards a safer digital world.
