# 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.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **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:** Yes
- **Published:** 9 Apr 2026
- **Updated:** 13 Apr 2026
- **Also known as:** CLSA-2026-1789031967, CLSA-2026-1792160937, CLSA-2026-1792161538, CLSA-2026-1792161580, CLSA-2026-1792161645, CLSA-2026-1792162015, GHSA-95h2-gj7x-gx9w

## 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 (Incomplete List of Disallowed Inputs)

## Which versions are affected by CVE-2026-39315?

2 open source packages ship an affected version, including unhead. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `unhead` | npm | All versions | 2.1.13 |
| `unhead` | npm | All versions | 2.1.13 |

Products: Unjs Unhead.

## How do I fix CVE-2026-39315?

Upgrade unhead to 2.1.13 or later.

## 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

- [CVE-2026-31860](https://insights.sbomify.com/CVE-2026-31860/): Unhead has a XSS bypass in `useHeadSafe` via attribute name injection and case-sensitive protocol check (medium 5.3)
- [CVE-2026-31873](https://insights.sbomify.com/CVE-2026-31873/): Unhead has a Bypass of URI Scheme Sanitization in makeTagSafe via Case-Sensitivity (not scored)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39315 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39315 exists.

Affected packages and fixed versions:
- unhead (npm): affected all versions; fixed in 2.1.13
- unhead (npm): affected all versions; fixed in 2.1.13

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-39315/ (Markdown: https://insights.sbomify.com/CVE-2026-39315/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39315
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39315
- OSV: https://osv.dev/vulnerability/CVE-2026-39315

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39315/
Summaries are generated and may contain mistakes; the official description above is the record.