CVE-2026-31873: Unhead Vulnerable to Bypass of URI Scheme Sanitization in makeTagSafe via Case-Sensitivity

March 12, 2026

The link.href check in makeTagSafe (safe.ts, line 68-71) uses String.includes(), which is case-sensitive:

if (key === 'href') {
if (val.includes('javascript:') || val.includes('data:')) {
return
}
next[key] = val
}

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.

References

github.com/advisories/GHSA-5339-hvwr-7582
github.com/unjs/unhead
github.com/unjs/unhead/releases/tag/v2.1.11
github.com/unjs/unhead/security/advisories/GHSA-5339-hvwr-7582
nvd.nist.gov/vuln/detail/CVE-2026-31873

Code Behaviors & Features

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