{
"document": {
"category": "csaf_base",
"csaf_version": "2.0",
"distribution": {
"tlp": {
"label": "WHITE"
}
},
"lang": "en",
"notes": [
{
"category": "legal_disclaimer",
"text": "The Netherlands Cyber Security Center (henceforth: NCSC-NL) maintains this portal to enhance access to its information and vulnerabilities. The use of this information is subject to the following terms and conditions:\n\nThe vulnerabilities disclosed in this portal are gathered by NCSC-NL from a variety of open sources, which the user can retrieve from other platforms. NCSC-NL makes every reasonable effort to ensure that the content of this portal is kept up to date, and that it is accurate and complete. Nevertheless, NCSC-NL cannot entirely rule out the possibility of errors, and therefore cannot give any warranty in respect of its completeness, accuracy or real-time keeping up-to-date. NCSC-NL does not control nor guarantee the accuracy, relevance, timeliness or completeness of information obtained from these external sources. The vulnerabilities disclosed in this portal are intended solely for the convenience of professional parties to take appropriate measures to manage the risks posed to the cybersecurity. No rights can be derived from the information provided therein.\n\nNCSC-NL and the Kingdom of the Netherlands assume no legal liability or responsibility for any damage resulting from either the use or inability of use of the vulnerabilities disclosed in this portal. This includes damage resulting from the inaccuracy of incompleteness of the information contained in it.\nThe information on this page is subject to Dutch law. All disputes related to or arising from the use of this portal regarding the disclosure of vulnerabilities will be submitted to the competent court in The Hague. This choice of means also applies to the court in summary proceedings."
}
],
"publisher": {
"category": "coordinator",
"contact_details": "cert@ncsc.nl",
"name": "National Cyber Security Centre",
"namespace": "https://www.ncsc.nl/"
},
"title": "CVE-2026-31873",
"tracking": {
"current_release_date": "2026-03-23T11:03:07.886135Z",
"generator": {
"date": "2026-02-17T15:00:00Z",
"engine": {
"name": "V.E.L.M.A",
"version": "1.7"
}
},
"id": "CVE-2026-31873",
"initial_release_date": "2026-03-12T14:50:35.481964Z",
"revision_history": [
{
"date": "2026-03-12T14:50:35.481964Z",
"number": "1",
"summary": "CVE created.| Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| References created (3).| CWES updated (1)."
},
{
"date": "2026-03-12T14:50:44.825049Z",
"number": "2",
"summary": "NCSC Score created."
},
{
"date": "2026-03-12T17:38:39.211807Z",
"number": "3",
"summary": "Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| Products created (1).| References created (1).| CWES updated (1)."
},
{
"date": "2026-03-12T17:38:45.040218Z",
"number": "4",
"summary": "NCSC Score updated."
},
{
"date": "2026-03-12T18:25:57.363199Z",
"number": "5",
"summary": "Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| References created (1).| CWES updated (1)."
},
{
"date": "2026-03-12T18:26:01.629862Z",
"number": "6",
"summary": "NCSC Score updated."
},
{
"date": "2026-03-12T18:38:41.136555Z",
"number": "7",
"summary": "Unknown change."
},
{
"date": "2026-03-12T20:12:12.716393Z",
"number": "8",
"summary": "References created (1)."
},
{
"date": "2026-03-13T14:13:57.122390Z",
"number": "9",
"summary": "Source created.| CVE status created. (valid)| EPSS created."
},
{
"date": "2026-03-13T14:14:03.693714Z",
"number": "10",
"summary": "NCSC Score updated."
},
{
"date": "2026-03-16T18:27:24.885310Z",
"number": "11",
"summary": "Products connected (1).| Product Identifiers created (1).| Exploits created (1)."
},
{
"date": "2026-03-16T18:27:28.883104Z",
"number": "12",
"summary": "NCSC Score updated."
},
{
"date": "2026-03-19T15:30:22.826558Z",
"number": "13",
"summary": "Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| References created (4).| CWES updated (1)."
},
{
"date": "2026-03-20T09:32:31.273874Z",
"number": "14",
"summary": "Source connected.| CVE status created. (valid)| EPSS created."
}
],
"status": "interim",
"version": "14"
}
},
"product_tree": {
"branches": [
{
"branches": [
{
"branches": [
{
"category": "product_version_range",
"name": "vers:unknown/<2.1.11",
"product": {
"name": "vers:unknown/<2.1.11",
"product_id": "CSAFPID-5810534",
"product_identification_helper": {
"cpe": "cpe:2.3:a:unjs:unhead:*:*:*:*:*:*:*:*"
}
}
}
],
"category": "product_name",
"name": "unhead"
}
],
"category": "vendor",
"name": "unjs"
}
]
},
"vulnerabilities": [
{
"cve": "CVE-2026-31873",
"cwe": {
"id": "CWE-79",
"name": "Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')"
},
"notes": [
{
"category": "description",
"text": "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\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```",
"title": "github - https://github.com/advisories/GHSA-5339-hvwr-7582"
},
{
"category": "description",
"text": "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.",
"title": "cveprojectv5 - https://www.cve.org/CVERecord?id=CVE-2026-31873"
},
{
"category": "description",
"text": "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.",
"title": "nvd - https://nvd.nist.gov/vuln/detail/CVE-2026-31873"
},
{
"category": "description",
"text": "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\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```",
"title": "github - https://api.github.com/advisories/GHSA-5339-hvwr-7582"
},
{
"category": "other",
"text": "0.00036",
"title": "EPSS"
},
{
"category": "other",
"text": "4.0",
"title": "NCSC Score"
},
{
"category": "other",
"text": "The value of the most recent CVSS (V3) score, There is exploit data available from source Nvd, Exploit code publicly available",
"title": "NCSC Score top decreasing factors"
}
],
"product_status": {
"known_affected": [
"CSAFPID-5810534"
]
},
"references": [
{
"category": "external",
"summary": "Source - github",
"url": "https://github.com/advisories/GHSA-5339-hvwr-7582"
},
{
"category": "external",
"summary": "Source raw - github",
"url": "https://api.github.com/advisories/GHSA-5339-hvwr-7582"
},
{
"category": "external",
"summary": "Source - cveprojectv5",
"url": "https://www.cve.org/CVERecord?id=CVE-2026-31873"
},
{
"category": "external",
"summary": "Source raw - cveprojectv5",
"url": "https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2026/31xxx/CVE-2026-31873.json"
},
{
"category": "external",
"summary": "Source - nvd",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-31873"
},
{
"category": "external",
"summary": "Source raw - nvd",
"url": "https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2026-31873"
},
{
"category": "external",
"summary": "Source - first",
"url": "https://api.first.org/data/v1/epss?cve=CVE-2026-31873"
},
{
"category": "external",
"summary": "Source raw - first",
"url": "https://api.first.org/data/v1/epss?limit=10000&offset=0"
},
{
"category": "external",
"summary": "Source - github",
"url": "https://api.github.com/advisories/GHSA-5339-hvwr-7582"
},
{
"category": "external",
"summary": "Source - first",
"url": "https://api.first.org/data/v1/epss?limit=10000&offset=0"
},
{
"category": "external",
"summary": "Reference - cveprojectv5; github; nvd",
"url": "https://github.com/unjs/unhead/security/advisories/GHSA-5339-hvwr-7582"
},
{
"category": "external",
"summary": "Reference - github",
"url": "https://github.com/unjs/unhead/releases/tag/v2.1.11"
},
{
"category": "external",
"summary": "Reference - github",
"url": "https://github.com/advisories/GHSA-5339-hvwr-7582"
},
{
"category": "external",
"summary": "Reference - github",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-31873"
}
],
"scores": [
{
"cvss_v3": {
"version": "3.1",
"vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:N",
"baseScore": 0.0,
"baseSeverity": "NONE"
},
"products": [
"CSAFPID-5810534"
]
}
],
"title": "CVE-2026-31873"
}
]
}