---
id: CVE-2026-59895
aliases:
  - GHSA-w62v-xxxg-mg59
title: 'Hono: Server-Side XSS via JSX Escaping Bypass in cx() Utility'
summary: 'Hono: Server-Side XSS via JSX Escaping Bypass in cx() Utility'
severity: medium
cvss: 6.1
cwe:
  - CWE-79
  - CWE-116
vendor: hono
product: hono
ecosystem: npm
affected:
  - 'hono >= 4.0.0, < 4.12.27'
patched:
  - hono 4.12.27
published: '2026-07-21'
updated: '2026-07-21'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-w62v-xxxg-mg59'
references:
  - url: 'https://github.com/honojs/hono/security/advisories/GHSA-w62v-xxxg-mg59'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-59895'
  - url: >-
      https://github.com/honojs/hono/commit/cd3f6f7194f0e5c9d4b26ae0cf232018d0f388fc
  - url: 'https://github.com/honojs/hono/releases/tag/v4.12.27'
  - url: 'https://github.com/advisories/GHSA-w62v-xxxg-mg59'
tags:
  - ghsa
  - npm
epss: 0.00327
epssPercentile: 0.23037
ingestedAt: '2026-07-21T18:53:06.684Z'
---

## Overview

### 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()`.

## Affected packages

- `hono >= 4.0.0, < 4.12.27`

## Remediation

Upgrade to a patched release:

- `hono 4.12.27`