{"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":0.26035,"ingestedAt":"2026-07-21T18:53:06.684Z","slug":"CVE-2026-59895","body":"## Overview\n\n### Summary\n\n`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).\n\n### Details\n\nBecause 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`.\n\n### Impact\n\nDuring 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.\n\nThis may lead to:\n\n- Execution of attacker-controlled script in the victim's browser session.\n- Session hijacking, credential theft, or actions performed on behalf of the victim.\n\nApplications are affected only if they render JSX server-side and pass untrusted input as a class name to `cx()`.\n\n## Affected packages\n\n- `hono >= 4.0.0, < 4.12.27`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `hono 4.12.27`","depth":"sunlit","depthScore":34,"depthScoreParts":{"impact":33.6,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}