Vulnerability · Published Apr 9, 2026 · Assigned by GitHub_M
CVE-2026-39315Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe()
Medium 6.1
In plain English
A medium-severity security flaw in unhead from unjs. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 22% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
Unhead is a document head and template manager. Prior to 2.1.13, useHeadSafe() is the composable that Nuxt's own documentation explicitly recommends for rendering user-supplied content in <head> safely. Internally, the hasDangerousProtocol() function in packages/unhead/src/plugins/safe.ts decodes HTML entities before checking for blocked URI schemes (javascript:, data:, vbscript:). The decoder uses two regular expressions with fixed-width digit caps. The HTML5 specification imposes no limit on leading zeros in numeric character references. When a padded entity exceeds the regex digit cap, the decoder silently skips it. The undecoded string is then passed to startsWith('javascript:'), which does not match. makeTagSafe() writes the raw value directly into SSR HTML output. The browser's HTML parser decodes the padded entity natively and constructs the blocked URI. This vulnerability is fixed in 2.1.13.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| npm | unhead | 2.1.13 | GHSA-95h2-gj7x-gx9w |
Affected products (as reported)
Weakness: CWE-184 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
References
- https://github.com/unjs/unhead/security/advisories/GHSA-95h2-gj7x-gx9w x_refsource_CONFIRM
- https://github.com/unjs/unhead/commit/961ea781e091853812ffe17f8cda17105d2d2299 x_refsource_MISC
- https://github.com/unjs/unhead/releases/tag/v2.1.13 x_refsource_MISC