CVE-2026-39315 Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe()
Attackers can bypass protocol filters in Unhead's useHeadSafe() using padded HTML entities to inject malicious scripts into SSR output.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-39315?
CVE-2026-39315 is a medium-severity cross-site scripting vulnerability in unjs unhead, published on 9 Apr 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
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.
Weakness: CWE-184
Which versions are affected by CVE-2026-39315?
2 open source packages ship an affected version, including unhead. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
unhead npm | All versions | 2.1.13 |
unhead npm | All versions | 2.1.13 |
Products include Unjs Unhead.
How do I fix CVE-2026-39315?
Upgrade unhead to 2.1.13 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-39315 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39315 exists.
Other vulnerabilities in unjs unhead
Sources
Also known as CLSA-2026-1789031967, CLSA-2026-1792160937, CLSA-2026-1792161538, CLSA-2026-1792161580, CLSA-2026-1792161645, CLSA-2026-1792162015 and 1 more.