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.