GHSA-w62v-xxxg-mg59 Hono: Server-Side XSS via JSX Escaping Bypass in cx() Utility

• npm hono: >= 4.0.0, < 4.12.27 → fixed in 4.12.27

Summary `cx()` in `hono/css` composes class names from plain strings but marks the result as already-escaped without HTML-escaping the input. When the result is used as a JSX `class` attribute during server-side rendering, the value is written into the attribute unescaped, so untrusted input can break out of the `class` attribute and inject arbitrary markup, leading to Cross-Site Scripting (XSS). Details Because the composed value is treated as pre-escaped, the HTML attribute escaping normally applied to interpolated values is skipped, and characters such as `"` pass through unescaped — allowing a value to terminate the attribute and add further attributes or elements. This arises when an application passes untrusted, user-controlled input as a class name to `cx()`, for example when merging a base class with an externally provided `className`. Impact During server-side rendering, an attacker who controls a value passed to `cx()` can inject arbitrary HTML into the page, resulting in stored or reflected XSS in the victim's browser. This may lead to: • Execution of attacker-controlled script in the victim's browser session. • Session hijacking, credential theft, or actions performed on behalf of the victim. Applications are affected only if they render JSX server-side and pass untrusted input as a class name to `cx()`.