Astro: XSS via unescaped spread attribute names in renderHTMLElement (incomplete fix for CVE-2026-54298)

Summary

The fix for CVE-2026-54298 (GHSA-jrpj-wcv7-9fh9) added an INVALID_ATTR_NAME_CHAR guard to addAttribute() so that spread-prop attribute names containing "' >/= or whitespace are dropped. A second attribute-rendering path, renderHTMLElement() in packages/astro/src/runtime/server/render/dom.ts, has its own inline attribute loop that does not go through addAttribute() and was not updated. It interpolates the attribute name unescaped and only escapes the value, so untrusted prop keys spread onto a native-HTMLElement-subclass component can still break out of the attribute context, resulting in XSS.

Details

renderHTMLElement builds attributes directly:

for (const attr in props) {
  attrHTML += ` ${attr}="${toAttributeString(await props[attr])}"`;
}

The attribute name (attr) is interpolated raw; only the value is escaped via toAttributeString. By contrast, the hardened addAttribute in util.ts rejects invalid names:

if (INVALID_ATTR_NAME_CHAR.test(key)) { return ''; } // /[\s"'>/=]/

renderHTMLElement is reached from component.ts when the component is a native HTMLElement subclass:

if (!renderer && typeof HTMLElement === 'function' && componentIsHTMLElement(Component)) {
  const output = await renderHTMLElement(result, Component, _props, slots);
}

where _props carries spread props verbatim.

Reachability

The branch only runs when typeof HTMLElement === 'function' at SSR time. In default Node SSR HTMLElement is undefined, so the branch is dead. It becomes reachable when the SSR runtime exposes a global HTMLElement (Deno, Bun with a DOM shim, or jsdom/happy-dom in Node) and a class extending HTMLElement is used directly as an Astro component that receives untrusted-keyed spread props.

Proof of Concept

Given malicious spread props:

const maliciousProps = {
  'onmouseover=alert(document.domain) x': 'y',
  'x><script>alert(1)</script>': 'z',
};

• addAttribute (post-fix) → <my-el></my-el> (key stripped — safe)
• renderHTMLElement → <my-el onmouseover=alert(document.domain) x="y" x><script>alert(1)</script>="z"></my-el> (handler + <script> injected — XSS)

Equivalent Astro template, served by an SSR runtime that defines a global HTMLElement:

---
import MyElement from '../MyElement.js'; // class MyElement extends HTMLElement {}
const userInput = Astro.url.searchParams;  // untrusted keys
---
<MyElement {...Object.fromEntries(userInput)} />

Impact

Cross-site scripting (CWE-79) via attribute-name breakout — the same vulnerability class as CVE-2026-54298, in a code path its fix did not cover. An attacker who controls the keys of an object spread onto a native-HTMLElement-subclass component can inject arbitrary event-handler attributes or sibling elements (including <script>) into the SSR output. Reachability is constrained by the runtime and component preconditions described above.