{"id":"CVE-2026-59729","aliases":["GHSA-f48w-9m4c-m7f5"],"title":"Astro: XSS via unescaped spread attribute names in renderHTMLElement (incomplete fix for CVE-2026-54298)","summary":"Astro: XSS via unescaped spread attribute names in renderHTMLElement (incomplete fix for CVE-2026-54298)","severity":"medium","cwe":["CWE-79"],"vendor":"astro","product":"astro","ecosystem":"npm","affected":["astro < 7.0.6"],"patched":["astro 7.0.6"],"published":"2026-07-20","updated":"2026-07-20","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-f48w-9m4c-m7f5","references":[{"url":"https://github.com/withastro/astro/security/advisories/GHSA-f48w-9m4c-m7f5"},{"url":"https://github.com/withastro/astro/pull/17251"},{"url":"https://github.com/withastro/astro/commit/5240e26c9dd91f9bc7140dcfacdb48d5a132830d"},{"url":"https://github.com/withastro/astro/releases/tag/astro@7.0.6"},{"url":"https://github.com/advisories/GHSA-f48w-9m4c-m7f5"}],"tags":["ghsa","npm"],"ingestedAt":"2026-07-20T23:44:02.336Z","epss":0.00538,"epssPercentile":0.42846,"slug":"CVE-2026-59729","body":"## Overview\n\n## Summary\n\nThe 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.\n\n## Details\n\n`renderHTMLElement` builds attributes directly:\n\n```js\nfor (const attr in props) {\n  attrHTML += ` ${attr}=\"${toAttributeString(await props[attr])}\"`;\n}\n```\n\nThe attribute name (`attr`) is interpolated raw; only the value is escaped via `toAttributeString`. By contrast, the hardened `addAttribute` in `util.ts` rejects invalid names:\n\n```js\nif (INVALID_ATTR_NAME_CHAR.test(key)) { return ''; } // /[\\s\"'>/=]/\n```\n\n`renderHTMLElement` is reached from `component.ts` when the component is a native `HTMLElement` subclass:\n\n```js\nif (!renderer && typeof HTMLElement === 'function' && componentIsHTMLElement(Component)) {\n  const output = await renderHTMLElement(result, Component, _props, slots);\n}\n```\n\nwhere `_props` carries spread props verbatim.\n\n### Reachability\n\nThe 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.\n\n## Proof of Concept\n\nGiven malicious spread props:\n\n```js\nconst maliciousProps = {\n  'onmouseover=alert(document.domain) x': 'y',\n  'x><script>alert(1)</script>': 'z',\n};\n```\n\n- `addAttribute` (post-fix) → `<my-el></my-el>` (key stripped — safe)\n- `renderHTMLElement` → `<my-el onmouseover=alert(document.domain) x=\"y\" x><script>alert(1)</script>=\"z\"></my-el>` (handler + `<script>` injected — XSS)\n\nEquivalent Astro template, served by an SSR runtime that defines a global `HTMLElement`:\n\n```astro\n---\nimport MyElement from '../MyElement.js'; // class MyElement extends HTMLElement {}\nconst userInput = Astro.url.searchParams;  // untrusted keys\n---\n<MyElement {...Object.fromEntries(userInput)} />\n```\n\n## Impact\n\nCross-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.\n\n## Affected packages\n\n- `astro < 7.0.6`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `astro 7.0.6`","depth":"sunlit","depthScore":28,"depthScoreParts":{"impact":27.5,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}