Cross-site scripting (XSS) is a client-side code injection attack. The web security vulnerability allows an attacker to compromise the interactions that users have with a vulnerable application.
In XSS attacks, a hacker executes malicious scripts in a victim’s web browser by including malicious code in a legitimate web page or web application. In this case, the cybercriminal circumvents the same-origin policy designed to segregate different websites from each other.
The actual attack occurs when a victim visits the web page or web application that executes the malicious code. The web page or web app becomes a vehicle to deliver the malicious script to the user’s browser. Other methods used to launch XSS attacks include forums, message boards, and web pages that allow comments.
How Cross-Site Scripting Works
A web page or web app is vulnerable to XSS if it uses unsanitized user input in the output it generates. The victim’s browser then parses the user input.
Types Cross-Site Scripting Attacks
There are three main types of XSS attacks.
- Reflected XSS – a malicious script comes from the current HTTP request
- Stored XSS – a malicious script comes from the website’s database
- DOM-based XSS – the vulnerability exists in the client-side code rather than server-side code
Impacts of Cross-Site Scripting
In some cases, attackers use XSS to deface a website. They use injected scripts to change the site’s content or redirect a user browser to another web page containing malicious code.
XSS vulnerabilities allow an attacker to masquerade as a victim user to carry out any actions that a user can perform and access any user’s data. In case the victim has privileged access within the application, then the attacker might gain full control over all the application’s functionalities and data.
Overall, cybercriminals can combine cross-site scripting with other attacks such as social engineering to pull off advanced attacks, including cookie theft, planting trojans, phishing, identity theft, and keylogging.
Are You Vulnerable to Cross-Site Scripting?
The Open Web Application Security Project (OWASP) organization lists XSS vulnerability as the second most prevalent issue in their OWASP Top 10 2017.
You can test if your web application or website is vulnerable to XSS by running automated web scans using web vulnerability scanners. You can also use a systematic manual set of tests to detect XSS vulnerability.
Preventing Cross-Site Scripting
Follow these steps to prevent XSS vulnerability:
- Sanitize your input to stay safe from XSS. Your web application code should never output data received as input directly to the browser without checking it for malicious code
- Train everyone involved in building the web application to create awareness of XSS risks
- Deploy a Content Security Policy (CSP), which is an HTTP response header that allows you to declare the dynamic resources that are permitted to load depending on the request source
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