CVE-2026-59727

When a transition:persist, transition:scope, or transition:persist-props directive is applied to a client-hydrated (client:*) component, Astro copied the directive value onto the rendered <astro-island> element without HTML-escaping it. If a developer reflects attacker-controlled input into one of these directives, an attacker can break out of the attribute and inject arbitrary HTML/JavaScript into the server-rendered output, resulting in reflected cross-site scripting (XSS).

Severity

Although a generic reflected XSS scores in the Medium range, exploitation here requires the application developer to have written a non-idiomatic pattern, passing untrusted, request-derived input directly into a transition directive. Astro applications that do not route untrusted input into these directives are unaffected. This mitigating precondition places the real-world severity at Low.

Details

In generateHydrateScript() (packages/astro/src/runtime/server/hydration.ts), every island property is HTML-escaped before serialization, the attrs, props, and opts assignments all pass through escapeHTML(). The transition directives, however, were copied verbatim:

transitionDirectivesToCopyOnIsland.forEach((name) => {
  if (typeof props[name] !== 'undefined') {
    island.props[name] = props[name]; // not escaped
  }
});

The <astro-island> element is serialized via renderElement('astro-island', island, false) with shouldEscape=false, and toAttributeString() returns the value unchanged in that mode. As a result there is no downstream re-escaping, and the raw directive value reaches the HTML response. This is the same output sink previously addressed for slot names in GHSA-8hv8-536x-4wqp.

The affected directives are:

• data-astro-transition-scope (transition:scope)
• data-astro-transition-persist (transition:persist)
• data-astro-transition-persist-props (transition:persist-props)

Note that transition:persist is typed boolean | string, so passing a string value is a supported use of the API.

Proof of Concept

A component that reflects a query parameter into a transition directive:

---
const persist = Astro.url.searchParams.get('persist') ?? 'default';
---
<Island client:load transition:persist={persist} />

Request:

https://example.com/?persist="><img src=x onerror=alert(document.domain)>

Rendered output (before the fix):

<astro-island … data-astro-transition-persist=""><img src=x onerror=alert(document.domain)>></astro-island>

The " closes the attribute and the injected <img onerror=…> executes in the victim's browser.

Affected Versions

astro >= 3.10.0, < 7.0.4 (introduced in 3.10.0, PR #7861).

Patched Versions

astro >= 7.0.4. Fixed in PR #17212 by HTML-escaping transition directive values before they are rendered onto the island element.

Workarounds

Do not pass untrusted or request-derived input into transition:persist, transition:scope, or transition:persist-props. If such input is required, HTML-escape or strictly validate it before passing it to the directive. Upgrading to [email protected] or later removes the need for manual mitigation.

Credits

Reported by @jlgore.