Medium severity · published Mar 12, 2026
CVE-2026-31873
Unhead has a Bypass of URI Scheme Sanitization in makeTagSafe via Case-Sensitivity
6.1
CVSS
0.2%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Confidentiality
Low
Integrity
Low
Availability
None
Scope
Changed
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 15% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
UN
unhead · unjs
< 2.1.11 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-31873
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-31873
Exploit
https://github.com/unjs/unhead/security/advisories/GHSA-5339-hvwr-7582
More from unhead
6.1
CVE-2026-39315
Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe()
6.1
CVE-2026-31860
Unhead has a XSS bypass in `useHeadSafe` via attribute name injection and case-sensitive protocol check