How to Fix Xss Vulnerability in Astro

Fixing XSS Vulnerabilities in Astro

Astro auto-escapes expressions in templates by default, but XSS vulnerabilities can still appear when using set:html, handling user input in API endpoints, or embedding content from external sources.

How XSS Sneaks In

• Using set:html with unsanitized user content
• API routes returning HTML without encoding
• Client-side scripts injecting unescaped data

The Fix

Sanitize when you must use raw HTML:

---
import DOMPurify from 'isomorphic-dompurify';

const userComment = Astro.props.comment;
const safeHtml = DOMPurify.sanitize(userComment, {
  ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'a', 'p'],
  ALLOWED_ATTR: ['href'],
});
---

<!-- Safe: auto-escaped -->
<p>{userComment}</p>

<!-- When you need HTML rendering, sanitize first -->
<div set:html={safeHtml} />

<!-- API endpoint: always encode -->

Never use set:html with raw user input. Astro's default expression escaping handles most cases — only reach for set:html when rendering trusted or sanitized HTML.

Avoiding Recurrence

Once you fix this error, add a regression test that reproduces the exact scenario. Document the root cause in your team's knowledge base so others can recognize the pattern. Configure monitoring alerts for early detection if the issue appears again in a different part of the codebase.

Bugsly for Astro

Bugsly flags potential XSS vectors in error reports when unsanitized HTML is detected in responses, helping you catch vulnerabilities before they're exploited.