XSS is an abbreviation for cross-site scripting. It is a web security flaw that enables an attacker to control how users interact with a vulnerable application. It enables an attacker to circumvent the same-origin policy, which is intended to keep websites separate. Cross-site scripting flaws allow an attacker to impersonate a victim user and perform any actions or access any of the user's data. If the victim user has privileged access to the application, the attacker may be able to fully control its functionality and data.
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Depending on the sophistication of the application and how it handles user-controllable data, preventing cross-site scripting can be simple in some cases and much more difficult in others. In general, preventing XSS vulnerabilities will almost certainly necessitate a combination of the following steps:
- Filter the input when it arrives. Filter user input as precisely as possible at the point of reception, based on what is expected or valid input.
- Encode the data on the output. To avoid being perceived as active content, encode user-controllable data in HTTP responses at the point of output. The output context may necessitate a combination of HTML, URL, JavaScript, and CSS encoding.
- Use relevant headers for the response. You can use the Content-Type and X-Content-Type-Options headers to ensure that browsers read HTTP responses correctly, preventing XSS in HTTP responses that aren't supposed to contain any HTML or JavaScript.
- Content Security Policy To mitigate the severity of any remaining XSS issues, you can use Content Security Policy (CSP) as a last line of defence.