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: ```js 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: ```js if (INVALID_ATTR_NAME_CHAR.test(key)) { return ''; } // /[\s"'>/=]/ ``` `renderHTMLElement` is reached from `component.ts` when the component is a native `HTMLElement` subclass: ```js 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: ```js const maliciousProps = { 'onmouseover=alert(document.domain) x': 'y', 'x>': 'z', }; ``` - `addAttribute` (post-fix) → `` (key stripped — safe) - `renderHTMLElement` → `="z">` (handler + `

CVE-2026-59729 Blast Radius

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