CVE-2026-126361

When a dynamic tag (const Tag = ...; ) resolves to a string, Astro renders it as a raw HTML element name. The name was filtered to remove characters that could inject attributes, but the filter did not handle /. Because HTML parsers treat / inside a tag as an attribute separator, a tag name such as img/src/onerror=alert(1) was rendered as an element with an onerror` attribute. The JSX renderer used for MDX did not filter string tag names at all.

For example:

rendered ?as=img/src/onerror=alert(1) as live HTML.

Reflected or stored cross-site scripting, but only in applications that use untrusted input (such as a request parameter or an uncontrolled CMS field) as all or part of an element name. Astro itself never derives element names from request data.

Applications that let content choose an element should map that input to a fixed set of known tags. Letting untrusted input choose an element name is unsafe even with a correct filter, because it can still select elements such as…