# CVE-2026-31873: Unhead has a Bypass of URI Scheme Sanitization in makeTagSafe via Case-Sensitivity

> Attackers can bypass URI sanitization to inject malicious CSS, enabling UI redressing or data exfiltration in applications using Unhead.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 12 Mar 2026
- **Updated:** 12 Mar 2026
- **Also known as:** GHSA-5339-hvwr-7582

## What is CVE-2026-31873?

CVE-2026-31873 is a cross-site scripting vulnerability in unjs unhead, published on 12 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Unhead is a document head and template manager. Prior to 2.1.11, The link.href check in makeTagSafe (safe.ts) uses String.includes(), which is case-sensitive. 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. An attacker can inject arbitrary CSS for UI redressing or data exfiltration via CSS attribute selectors with background-image callbacks. This vulnerability is fixed in 2.1.11.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-31873?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `unhead` | npm | All versions | 2.1.11 |
| `unhead` | npm | All versions | 2.1.11 |

Products: Unjs Unhead.

## How do I fix CVE-2026-31873?

Upgrade unhead to 2.1.11 or later.

## Is CVE-2026-31873 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-31873 exists.

## Other vulnerabilities in unjs unhead

- [CVE-2026-39315](https://insights.sbomify.com/CVE-2026-39315/): Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe() (medium 6.1)
- [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)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31873 and fix it if it is affected.

What it is: CVE-2026-31873 is a cross-site scripting vulnerability in unjs unhead, published on 12 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-31873 exists.

Affected packages and fixed versions:
- unhead (npm): affected all versions; fixed in 2.1.11
- unhead (npm): affected all versions; fixed in 2.1.11

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-2026-31873/ (Markdown: https://insights.sbomify.com/CVE-2026-31873/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31873
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31873
- OSV: https://osv.dev/vulnerability/CVE-2026-31873

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31873/
Summaries are generated and may contain mistakes; the official description above is the record.