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GHSA-8266-84wp-wv5c: Svelte has a potential mXSS vulnerability due to improper HTML escaping

Summary

A potential XSS vulnerability exists in Svelte for versions prior to 4.2.19.

Details

Svelte improperly escapes HTML on server-side rendering. It converts strings according to the following rules:

  • If the string is an attribute value:
    • " -> "
    • & -> &
    • Other characters -> No conversion
  • Otherwise:
    • < -> &lt;
    • & -> &amp;
    • Other characters -> No conversion

The assumption is that attributes will always stay as such, but in some situation the final DOM tree rendered on browsers is different from what Svelte expects on server-side rendering. This may be leveraged to perform XSS attacks. More specifically, this can occur when injecting malicious content into an attribute within a <noscript> tag.

PoC

A vulnerable page (+page.svelte):

<script>
import { page } from "$app/stores"

// user input
let href = $page.url.searchParams.get("href") ?? "https://example.com";
</script>

<noscript>
  <a href={href}>test</a>
</noscript>

If a user accesses the following URL,

http://localhost:4173/?href=</noscript><script>alert(123)</script>

then, alert(123) will be executed.

Impact

XSS, when using an attribute within a noscript tag

ghsa
Open in Source
#xss#vulnerability#js#perl

Summary

A potential XSS vulnerability exists in Svelte for versions prior to 4.2.19.

Details

Svelte improperly escapes HTML on server-side rendering. It converts strings according to the following rules:

  • If the string is an attribute value:
    • " -> "
    • & -> &
    • Other characters -> No conversion
  • Otherwise:
    • < -> <
    • & -> &
    • Other characters -> No conversion

The assumption is that attributes will always stay as such, but in some situation the final DOM tree rendered on browsers is different from what Svelte expects on server-side rendering. This may be leveraged to perform XSS attacks. More specifically, this can occur when injecting malicious content into an attribute within a <noscript> tag.

PoC

A vulnerable page (+page.svelte):

<script> import { page } from “$app/stores”

// user input let href = $page.url.searchParams.get(“href”) ?? "https://example.com"; </script>

<noscript> <a href={href}>test</a> </noscript>

If a user accesses the following URL,

http://localhost:4173/?href=</noscript><script>alert(123)</script>

then, alert(123) will be executed.

Impact

XSS, when using an attribute within a noscript tag

References

  • GHSA-8266-84wp-wv5c
  • sveltejs/svelte@83e96e0
  • https://nvd.nist.gov/vuln/detail/CVE-2024-45047

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