CVE-2026-31873

The link.href check in makeTagSafe (safe.ts, line 68-71) uses String.includes(), which is case-sensitive:

if (key === 'href') {
  if (val.includes('javascript:') || val.includes('data:')) {
    return
  }
  next[key] = val
}

Browsers treat URI schemes case-insensitively. DATA:text/css,... is the same as data:text/css,... to the browser, but 'DATA:...'.includes('data:') returns false.

PoC

useHeadSafe({
  link: [{
    rel: 'stylesheet',
    href: 'DATA:text/css,body{display:none}'
  }]
})

SSR output:

<link rel="stylesheet" href="DATA:text/css,body{display:none}">

The browser loads this as a CSS stylesheet. An attacker can inject arbitrary CSS for UI redressing or data exfiltration via CSS attribute selectors with background-image callbacks.

Any case variation works: DATA:, Data:, dAtA:, JAVASCRIPT:, etc.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

Kodem intelligence

if (key === 'href') {
  const lower = val.toLowerCase()
  if (lower.includes('javascript:') || lower.includes('data:')) {
    return
  }
  next[key] = val
}

1. What is CVE-2026-31873? CVE-2026-31873 is a low-severity cross-site scripting (XSS) vulnerability in unhead (npm), affecting versions <= 2.1.10. It is fixed in 2.1.11. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. Which versions of unhead are affected by CVE-2026-31873? unhead (npm) versions <= 2.1.10 is affected.
3. Is there a fix for CVE-2026-31873? Yes. CVE-2026-31873 is fixed in 2.1.11. Upgrade to this version or later.
4. Is CVE-2026-31873 exploitable, and should I be worried? Whether CVE-2026-31873 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-31873 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-31873? Upgrade unhead to 2.1.11 or later.