Unhead Vulnerable to Bypass of URI Scheme Sanitization in makeTagSafe via Case-Sensitivity

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

0.0 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:N

EPSS

• CVE-2026-31873: 0.25% estimated exploitation activity in the next 30 days; percentile 16.01 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

The link.href check in makeTagSafe (safe.ts, line 68-71) 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. PoC SSR output: 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. Suggested fix

Published by the source

12 Mar 2026, 14:19 UTC

Last modified by the source

14 Mar 2026, 06:26 UTC

Source last checked

26 Oct 2026, 12:46 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-5339-hvwr-7582

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-5339-hvwr-7582

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# Patch Next advisory brief: CVE-2026-31873

Advisory: Unhead Vulnerable to Bypass of URI Scheme Sanitization in makeTagSafe via Case-Sensitivity
IDs: CVE-2026-31873, GHSA-5339-hvwr-7582
Record: https://patchnext.com/advisories/cve-2026-31873 (revision 2, source checked 26 Oct 2026, 12:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Unrated (CVSS 0.0)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: unhead 2.1.11. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm unhead: affected <= 2.1.10; fixed in 2.1.11

## Suggested inspection (read-only)
- `npm ls unhead --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on unhead.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-5339-hvwr-7582 (OSV record GHSA-5339-hvwr-7582 lists affected ranges: npm unhead >=0 <2.1.11 (fixed))
- Verified: https://github.com/advisories/GHSA-5339-hvwr-7582 (GitHub-reviewed advisory GHSA-5339-hvwr-7582 lists affected ranges: npm unhead >=0 <=2.1.10)

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