hono/jsx renders plain strings unescaped in boundary components, leading to XSS

Summary

hono/jsx does not HTML-escape a plain string placed directly as a child or fallback of Suspense or ErrorBoundary, as the only child of a Context.Provider, or as the root value of renderToString() / renderToReadableStream() from hono/jsx/dom/server. Such a string is emitted as markup instead of text.

Details

These paths stringify their input and treat the result as already-escaped HTML, so a plain string passes through unchanged. Notable cases:

• Suspense: a string child, or a string fallback while a child suspends. With streaming, the fallback reaches the browser in the initial chunk.
• ErrorBoundary: a string child alongside an asynchronous sibling. The all-synchronous case was fixed in 4.11.7 (GHSA-9r54-q6cx-xmh5).
• Context.Provider: a single string child. Multiple children are escaped.
• hono/jsx/dom/server: a string, or an array containing strings, passed as the root.

A lone {children} forwarded by a wrapper component is enough to reach these paths. Strings wrapped in an element, values from raw() or the html helper, and client-side rendering with hono/jsx/dom are not affected.

Impact

An attacker who controls a string rendered in an affected position can inject arbitrary HTML into the server-rendered page, leading to cross-site scripting under the application's origin.

This issue affects applications that render untrusted strings directly in one of the positions above during server-side rendering.