CVE-2026-59895Medium· 6.1▾ SunlitHono: Server-Side XSS via JSX Escaping Bypass in cx() Utility
▾ Sunlit zone — Low / medium · no exploitation signal
impact 33.6 · likelihood 0.1 · exploitation 0
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Exploit-prediction probability, daily snapshots since Jul 21.
Disclosure to exploitation, from the record and what we observed since indexing it.
Disclosed via GHSA
0.2%
0.2% → 0.3%
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).
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.
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:
Applications are affected only if they render JSX server-side and pass untrusted input as a class name to cx().
hono >= 4.0.0, < 4.12.27Upgrade to a patched release:
hono 4.12.27Connected by shared product, vendor, weakness, or advisory.
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