CVE-2026-83618: xmldom RequireWellFormed DocType PublicId/systemId Validation Is Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vr34-hp96-76pp",
  "modified" : "2026-09-08T21:02:25Z",
  "published" : "2026-09-08T21:02:25Z",
  "aliases" : [ "CVE-2026-83618" ],
  "summary" : "xmldom: requireWellFormed DocType publicId/systemId validation is bypassable via an embedded line terminator",
  "details" : "## Summary\n\nAn embedded line terminator bypasses the `requireWellFormed` serializer check for a `DocumentType`'s\npublicId and systemId. The check was added to fix GHSA-f6ww-3ggp-fr8h; an id whose first line is a\nvalid literal slips past it and is emitted verbatim into the `<!DOCTYPE …>` declaration, so the markup\nafter the line terminator breaks out into the surrounding document. Callers who enabled\n`requireWellFormed` to neutralize DocumentType injection remain exposed.\n\n## Details\n\n`publicId` and `systemId` are stored as raw values **including their surrounding quotes**, and the\n`PubidLiteral`/`SystemLiteral` productions include those quotes. The serializer validates them with\n`g.PubidLiteral_match.test(publicId)` and `g.SystemLiteral_match.test(systemId)`, where both matchers\nare `reg('^', …, '$')` and inherit the `m` flag from xmldom's shared regexp builder. Under `m`, `$`\nmatches at an interior line terminator, so a value such as `\"valid pubid\"\\n\"><!ENTITY …>` satisfies\nthe matcher on its first line (`\"valid pubid\"` is a complete `PubidLiteral`) and the whole value —\nincluding the post-newline breakout — is emitted after `PUBLIC`/`SYSTEM`.\n\n### Root Cause\n\n1. A shared regexp builder compiles anchored productions with the `m` flag.\n2. `^…$` under `m` are line anchors, not string anchors.\n3. A full-string validator built on such a production (`.test()`) accepts any string with one\n   conforming line, so a complete, valid literal on the first line passes even though a line terminator\n   and breakout markup follow. `PubidChar` excluding `<`/`>` does not prevent it — the breakout is\n   appended *after* the literal, not embedded inside it.\n\nThe triggering line terminators are the ECMAScript `LineTerminator` set: U+000A, U+000D, U+2028, U+2029.\n\n## Affected Versions\n\nOnly `@xmldom/xmldom` 0.9.x is affected. The vulnerable matchers are built by `lib/grammar.js`'s\n`m`-flagged `reg()` builder, and the DocType `publicId`/`systemId` `requireWellFormed` check that\nconsumes them was introduced in 0.9.10 (the GHSA-f6ww-3ggp-fr8h fix); 0.9.10 and 0.9.11 carry it.\n`0.8.x` performs the same `requireWellFormed` check with inline, non-`m` regular expressions and is not\naffected. The unscoped `xmldom` package has no `grammar.js` and no `requireWellFormed` serializer, so\nthere is no check to bypass.\n\n## Proof of Concept\n\n```js\nconst { DOMImplementation, XMLSerializer } = require('@xmldom/xmldom');\nconst impl = new DOMImplementation();\n\n// publicId: complete literal on line 1, then newline + breakout\nconst dt = impl.createDocumentType('html', '\"valid pubid\"\\n\"><!ENTITY xxe SYSTEM \"file:///etc/passwd\">', '');\nconst doc = impl.createDocument(null, 'root', dt);\nconsole.log(new XMLSerializer().serializeToString(doc, { requireWellFormed: true }));\n// Observed (no throw):\n//   <!DOCTYPE html PUBLIC \"valid pubid\"\n//   \"><!ENTITY xxe SYSTEM \"file:///etc/passwd\">><root/>\n// Expected: InvalidStateError (publicId is not a valid PubidLiteral).\n// Control: a single-line invalid publicId (\"no-surrounding-quotes<>\") DOES throw InvalidStateError,\n// confirming the check is active and specifically bypassed by the line terminator.\n```\n\n## Impact\n\n- **Bypass of the GHSA-f6ww-3ggp-fr8h mitigation.** Applications that adopted `requireWellFormed:\n  true` to neutralize DocumentType injection remain exposed.\n- **XML structure injection into the DOCTYPE**, including injected markup / entity declarations after\n  the public or system identifier.\n\n## Fix Applied\n\nThe anchored `PubidLiteral`/`SystemLiteral` validators used by the `requireWellFormed`\nserializer no longer treat an interior line terminator as satisfying the `$` anchor, so a `publicId`\nor `systemId` containing any ECMAScript `LineTerminator` (U+000A, U+000D, U+2028, U+2029) is rejected\nwith `InvalidStateError`. Valid single-line identifiers serialize unchanged, and the default\nserialization path is unaffected.\n\n> **⚠ Opt-in required.** Protection is not automatic. Existing serialization calls remain vulnerable\n> unless `{ requireWellFormed: true }` is explicitly passed. Applications that serialize untrusted DOM\n> content should audit all `serializeToString()` call sites and add it.\n\n### Proof of Concept - fixed path\n\n```js\nconst { DOMImplementation, XMLSerializer } = require('@xmldom/xmldom');\nconst impl = new DOMImplementation();\nconst dt = impl.createDocumentType('html', '\"valid pubid\"\\n\"><!ENTITY xxe SYSTEM \"file:///etc/passwd\">', '');\nconst doc = impl.createDocument(null, 'root', dt);\n\n// Default path (requireWellFormed off) — unchanged, still emits verbatim:\nconsole.log(new XMLSerializer().serializeToString(doc));\n//   <!DOCTYPE html PUBLIC \"valid pubid\"\n//   \"><!ENTITY xxe SYSTEM \"file:///etc/passwd\">><root/>\n\n// Opt-in path — now throws instead of emitting the breakout:\nnew XMLSerializer().serializeToString(doc, { requireWellFormed: true });\n//   InvalidStateError: DocumentType publicId is not a valid PubidLiteral\n```\n\n### Why the default stays verbatim\n\nThe W3C DOM Parsing \"require well-formed\" flag defaults to false, and a browser `XMLSerializer` emits\nthe DOCTYPE verbatim. Unconditionally throwing on a malformed `publicId`/`systemId` would be an\nunjustified breaking change to the default path, so the fix tightens only the opt-in\n`requireWellFormed` validator, matching browser and spec defaults.\n\n### Residual limitation\n\nThe guarantee holds only for callers that pass `{ requireWellFormed: true }`; the default\nserialization path still emits `publicId`/`systemId` verbatim. `publicId` and `systemId` are not\nvalidated at creation (`createDocumentType`) or on direct property assignment\n(`documentType.publicId = …`) — the WHATWG DOM specification places no well-formedness constraint on\nthese fields at creation time, so the serializer is the spec-aligned enforcement point.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@xmldom/xmldom"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.9.10"
      }, {
        "fixed" : "0.9.12"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.9.11"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/xmldom/xmldom/security/advisories/GHSA-vr34-hp96-76pp"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-83618"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/xmldom/xmldom/pull/1071"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/xmldom/xmldom/commit/7b2ec67e1750daadd0bb06c92e875e726544a362"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/xmldom/xmldom"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/xmldom/xmldom/releases/tag/0.9.12"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-625", "CWE-91" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T21:02:25Z",
    "nvd_published_at" : "2026-09-01T15:17:40Z"
  }
}