Vulnerability-Lookup

CVE-2026-59729 (GCVE-0-2026-59729)

Vulnerability from cvelistv5 – Published: 2026-07-27 19:38 – Updated: 2026-07-28 13:48

Title

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

Summary

Astro is a web framework for content-driven websites. Versions prior to 7.0.6 are vulnerable to XSS through unescaped spread attribute names in renderHTMLElement. 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. This issue has been fixed in version 7.0.6.

Severity

5.1 (Medium)


                        
                          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

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Assigner

GitHub_M

References

4 references

URL	Tags
https://github.com/withastro/astro/security/advis…	x_refsource_CONFIRM
https://github.com/withastro/astro/pull/17251	x_refsource_MISC
https://github.com/withastro/astro/commit/5240e26…	x_refsource_MISC
https://github.com/withastro/astro/releases/tag/a…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
withastro	astro	Affected: < 7.0.6

Show details on NVD website

JSON

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GHSA-F48W-9M4C-M7F5

Vulnerability from github – Published: 2026-07-20 23:21 – Updated: 2026-07-20 23:21

Summary

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

Details

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.

Severity

5.1 (Medium)


                  
                    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

Show details on source website

JSON

To clipboard

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  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "astro"
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      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
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          "type": "ECOSYSTEM"
        }
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  ],
  "aliases": [
    "CVE-2026-59729"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-20T23:21:58Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "## 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 `\"\u0027 \u003e/=` 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 \u0027\u0027; } // /[\\s\"\u0027\u003e/=]/\n```\n\n`renderHTMLElement` is reached from `component.ts` when the component is a native `HTMLElement` subclass:\n\n```js\nif (!renderer \u0026\u0026 typeof HTMLElement === \u0027function\u0027 \u0026\u0026 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 === \u0027function\u0027` 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  \u0027onmouseover=alert(document.domain) x\u0027: \u0027y\u0027,\n  \u0027x\u003e\u003cscript\u003ealert(1)\u003c/script\u003e\u0027: \u0027z\u0027,\n};\n```\n\n- `addAttribute` (post-fix) \u2192 `\u003cmy-el\u003e\u003c/my-el\u003e` (key stripped \u2014 safe)\n- `renderHTMLElement` \u2192 `\u003cmy-el onmouseover=alert(document.domain) x=\"y\" x\u003e\u003cscript\u003ealert(1)\u003c/script\u003e=\"z\"\u003e\u003c/my-el\u003e` (handler + `\u003cscript\u003e` injected \u2014 XSS)\n\nEquivalent Astro template, served by an SSR runtime that defines a global `HTMLElement`:\n\n```astro\n---\nimport MyElement from \u0027../MyElement.js\u0027; // class MyElement extends HTMLElement {}\nconst userInput = Astro.url.searchParams;  // untrusted keys\n---\n\u003cMyElement {...Object.fromEntries(userInput)} /\u003e\n```\n\n## Impact\n\nCross-site scripting (CWE-79) via attribute-name breakout \u2014 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 `\u003cscript\u003e`) into the SSR output. Reachability is constrained by the runtime and component preconditions described above.",
  "id": "GHSA-f48w-9m4c-m7f5",
  "modified": "2026-07-20T23:21:59Z",
  "published": "2026-07-20T23:21:58Z",
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      "url": "https://github.com/withastro/astro/pull/17251"
    },
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      "url": "https://github.com/withastro/astro/commit/5240e26c9dd91f9bc7140dcfacdb48d5a132830d"
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  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Astro: XSS via unescaped spread attribute names in renderHTMLElement (incomplete fix for CVE-2026-54298)"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.

Action not permitted

CVE-2026-59729 (GCVE-0-2026-59729)

GHSA-F48W-9M4C-M7F5

Summary

Details

Reachability

Proof of Concept

Impact

Tags

Sightings

Nomenclature