{"id":"GHSA-f48w-9m4c-m7f5","revision":"9d74e32e6fc9431fa3b41d7e1a9bef9d","records":[{"revision":"9d74e32e6fc9431fa3b41d7e1a9bef9d","observedAt":"2026-10-24T13:17:17.793+00:00","statement":{"source":"osv","id":"GHSA-f48w-9m4c-m7f5","aliases":["CVE-2026-59729"],"title":"Astro: XSS via unescaped spread attribute names in renderHTMLElement (incomplete fix for CVE-2026-54298)","description":"## 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.","state":0,"publishedAt":"2026-07-20T23:21:58+00:00","modifiedAt":"2026-08-12T20:45:07.6868475+00:00","affected":[{"ecosystem":"npm","name":"astro","ranges":[{"type":"SEMVER","events":[{"kind":"introduced","version":"0"},{"kind":"fixed","version":"7.0.6"}]}],"versions":[],"rawVersionEvidence":null}],"metrics":[{"system":"CVSS_V4","vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N","score":null,"severity":null},{"system":"Publisher rating","vector":null,"score":null,"severity":"MODERATE"}],"references":["https://github.com/withastro/astro/security/advisories/GHSA-f48w-9m4c-m7f5","https://nvd.nist.gov/vuln/detail/CVE-2026-59729","https://github.com/withastro/astro/pull/17251","https://github.com/withastro/astro/commit/5240e26c9dd91f9bc7140dcfacdb48d5a132830d","https://github.com/withastro/astro","https://github.com/withastro/astro/releases/tag/astro@7.0.6"],"sourceUrl":"https://github.com/advisories/GHSA-f48w-9m4c-m7f5","license":"GitHub Advisory Database · CC-BY-4.0","key":"osv:GHSA-f48w-9m4c-m7f5","identifiers":["GHSA-f48w-9m4c-m7f5","CVE-2026-59729"]},"retained":false,"historicalAffected":[]}],"coverage":[]}