{"id":"CVE-2026-31873","aliases":["GHSA-5339-hvwr-7582"],"url":"https://o3.security/vulnerability/CVE-2026-31873","summary":"Unhead has a Bypass of URI Scheme Sanitization in makeTagSafe via Case-Sensitivity","details":"The `link.href` check in `makeTagSafe` (safe.ts, line 68-71) uses `String.includes()`, which is case-sensitive:\n\n```typescript\nif (key === 'href') {\n  if (val.includes('javascript:') || val.includes('data:')) {\n    return\n  }\n  next[key] = val\n}\n```\n\nBrowsers treat URI schemes case-insensitively. `DATA:text/css,...` is the same as `data:text/css,...` to the browser, but `'DATA:...'.includes('data:')` returns `false`.\n\n### PoC\n\n```javascript\nuseHeadSafe({\n  link: [{\n    rel: 'stylesheet',\n    href: 'DATA:text/css,body{display:none}'\n  }]\n})\n```\n\nSSR output:\n\n```html\n<link rel=\"stylesheet\" href=\"DATA:text/css,body{display:none}\">\n```\n\nThe 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.\n\nAny case variation works: `DATA:`, `Data:`, `dAtA:`, `JAVASCRIPT:`, etc.\n\n## Suggested fix\n\n```typescript\nif (key === 'href') {\n  const lower = val.toLowerCase()\n  if (lower.includes('javascript:') || lower.includes('data:')) {\n    return\n  }\n  next[key] = val\n}\n```","published":"2026-03-12T17:20:35.660Z","modified":"2026-08-12T03:51:36.263030969Z","cvss":{"score":0,"severity":"NONE","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"unhead","fixedVersion":"2.1.11"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31873.json"},{"type":"ADVISORY","url":"https://github.com/unjs/unhead/security/advisories/GHSA-5339-hvwr-7582"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-31873"},{"type":"PACKAGE","url":"https://github.com/unjs/unhead"},{"type":"WEB","url":"https://github.com/unjs/unhead/releases/tag/v2.1.11"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:36.263030969Z"}}