Hono: Server-Side XSS via JSX Escaping Bypass in cx() Utility

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-59895: 0.33% estimated exploitation activity in the next 30 days; percentile 25.84 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

21 Jul 2026, 18:33 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 12:57 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-w62v-xxxg-mg59

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-w62v-xxxg-mg59

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-59895

Advisory: Hono: Server-Side XSS via JSX Escaping Bypass in cx() Utility
IDs: CVE-2026-59895, GHSA-w62v-xxxg-mg59
Record: https://patchnext.com/advisories/cve-2026-59895 (revision 2, source checked 26 Oct 2026, 12:57 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: hono 4.12.27. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm hono: affected >= 4.0.0, < 4.12.27; fixed in 4.12.27

## Suggested inspection (read-only)
- `npm ls hono --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on hono.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-w62v-xxxg-mg59 (OSV record GHSA-w62v-xxxg-mg59 lists affected ranges: npm hono >=4.0.0 <4.12.27 (fixed))
- Verified: https://github.com/advisories/GHSA-w62v-xxxg-mg59 (GitHub-reviewed advisory GHSA-w62v-xxxg-mg59 lists affected ranges: npm hono >=4.0.0 <4.12.27)

Commands are suggestions: review each one before running it.