GHSA-6H8R-XR42-GP59
Vulnerability from github – Published: 2026-09-08 21:02 – Updated: 2026-09-08 21:02Summary
xmldom's parser silently accepts a not-well-formed end tag whose valid name is followed by
trailing content — e.g. </a⏎junk>. The element is closed, the trailing content is discarded, and no
error is reported, even though the XML end-tag production allows only optional whitespace after the
name and both Chromium and Firefox reject such input as application/xml. An application that relies
on xmldom to reject not-well-formed input therefore receives a false "valid" result for a document the
specification and browsers consider malformed.
Details
Across every affected version, an end tag whose valid Name is followed by trailing content before
> is silently accepted: the element is closed, the residue is dropped, and no error is reported. How
much leaks differs by line (see Affected Versions), but the observable weakness is the same.
On the current (0.9.x) line, the parser validates the end-tag name against the XML ETag production
with an anchored regular expression (^ QName S? $). That expression is compiled with the m
(multiline) flag by a shared builder, so $ matches at an interior line terminator: a valid name on
the first line satisfies the anchored production and any content after the line break escapes the
check. On 0.9.x the whitespace-separated variant (</a junk>) is already rejected; only the
line-terminator variant leaks. Older lines have no anchored end-tag validator at all, so they accept
both the line-terminator and the whitespace variant.
This is not content injection — the trailing content is dropped, and the resulting DOM is a normal
single-root document (<a/>). The security-relevant property is the silent acceptance of
not-well-formed input: xmldom's parse result disagrees with the specification and with browser XML
parsers, so any control that treats "xmldom parsed it without error" as "well-formed" is bypassed.
Root Cause
On the 0.9.x line, where the line-terminator variant specifically leaks:
- A shared regexp builder compiles anchored productions with the
mflag. ^…$undermare line anchors, not string anchors.- The anchored end-tag production
^ QName S? $is therefore satisfied by the first line alone, so trailing content after a line terminator is neither matched nor rejected — the malformed end tag is accepted and the residue silently discarded.
The triggering line terminators are the ECMAScript LineTerminator set: U+000A, U+000D, U+2028, U+2029.
U+2028 and U+2029 are not XML whitespace, so they are non-conforming trailing content that nonetheless
leaks because the JavaScript $ anchor treats them as line boundaries under m.
Affected Versions
Both maintained versions are affected and fixed:
0.9.x(>= 0.9.0, <= 0.9.11) →0.9.12: the anchored end-tag validator'smflag leaks the line-terminator variant. The whitespace variant (</a junk>) is already rejected on this line.0.8.x(>= 0.8.0, <= 0.8.14) →0.8.15: no anchored end-tag validator at all — both the line-terminator and the whitespace variant are silently accepted; the fix adds a residue check.
release-0.7.x (<= 0.7.13) and the unscoped xmldom package (range *, last release 0.6.0) are
affected but will not be patched — they are end-of-life / unmaintained. They share the older-line
behavior (both variants silently accepted, residue dropped).
Proof of Concept
const { DOMParser, XMLSerializer } = require('@xmldom/xmldom');
const doc = new DOMParser().parseFromString('<a></a\njunk>', 'text/xml'); // no error
console.log(new XMLSerializer().serializeToString(doc));
// Observed: <a/> — the malformed end tag is accepted, "junk" silently discarded, no error reported.
// Expected (per XML spec / Chromium / Firefox): a parse error — the input is not well-formed.
Impact
- Silent acceptance of not-well-formed XML: a document the specification and browser XML parsers reject is parsed without error.
- Bypass of a well-formedness / parse-before-trust gate: an application relying on xmldom to reject malformed input treats such a document as valid. Note this is an input-validation / parser-differential issue, not content injection — the trailing content is discarded.
Fix Applied
The parser now reports a not-well-formed end tag whose valid name is followed by trailing content, on both 0.9.12 and 0.8.15 — previously it was accepted silently — and parsing recovers to the byte-identical DOM as before.\
On 0.9.12 the anchored end-tag validator is corrected so a line break followed by trailing content no longer satisfies it, reported as a recoverable error when parsing as XML and a warning when parsing as HTML.\
On 0.8.15, which previously performed no end-tag residue validation, an equivalent residue check is added, reported as a recoverable error in both XML and HTML.\
Well-formed documents are unaffected.
Because the report is recoverable, the fix is non-breaking: no previously-parsed document begins to
throw and no serialized output changes. Consumers that want strict rejection can escalate the reported
error to a fatal one via the parser's error handler (onError in 0.9.12, errorHandler in
0.8.15). The two versions also differ in the whitespace-separated variant (</a junk>): 0.9.12
already rejected it with a fatal error in XML and continues to, while 0.8.15 — which validated
neither variant — now emits the same recoverable report for both the whitespace and line-break
variants.
Residual limitation
By default the parser still recovers (it does not reject the document); the reported condition is a
recoverable error/warning, not a fatal error, to avoid changing the parsed output in a patch
release. Converting this and the other not-well-formed-acceptance cases to a consistent fatalError —
including the broader end-tag leniency on the older versions — is deferred to the next breaking release,
tracked at xmldom/xmldom#1074.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 0.8.14"
},
"package": {
"ecosystem": "npm",
"name": "@xmldom/xmldom"
},
"ranges": [
{
"events": [
{
"introduced": "0.7.0"
},
{
"fixed": "0.8.15"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 0.9.11"
},
"package": {
"ecosystem": "npm",
"name": "@xmldom/xmldom"
},
"ranges": [
{
"events": [
{
"introduced": "0.9.0"
},
{
"fixed": "0.9.12"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "npm",
"name": "xmldom"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"last_affected": "0.6.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-83611"
],
"database_specific": {
"cwe_ids": [
"CWE-1286"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-08T21:02:13Z",
"nvd_published_at": "2026-09-01T15:17:39Z",
"severity": "MODERATE"
},
"details": "## Summary\n\nxmldom\u0027s parser silently accepts a **not-well-formed end tag** whose valid name is followed by\ntrailing content \u2014 e.g. `\u003c/a\u23cejunk\u003e`. The element is closed, the trailing content is discarded, and no\nerror is reported, even though the XML end-tag production allows only optional whitespace after the\nname and both Chromium and Firefox reject such input as `application/xml`. An application that relies\non xmldom to reject not-well-formed input therefore receives a false \"valid\" result for a document the\nspecification and browsers consider malformed.\n\n## Details\n\nAcross every affected version, an end tag whose valid `Name` is followed by trailing content before\n`\u003e` is silently accepted: the element is closed, the residue is dropped, and no error is reported. How\nmuch leaks differs by line (see Affected Versions), but the observable weakness is the same.\n\nOn the current (`0.9.x`) line, the parser validates the end-tag name against the XML `ETag` production\nwith an anchored regular expression (`^ QName S? $`). That expression is compiled with the `m`\n(multiline) flag by a shared builder, so `$` matches at an interior line terminator: a valid name on\nthe first line satisfies the anchored production and any content after the line break escapes the\ncheck. On 0.9.x the whitespace-separated variant (`\u003c/a junk\u003e`) is already rejected; only the\nline-terminator variant leaks. Older lines have no anchored end-tag validator at all, so they accept\nboth the line-terminator and the whitespace variant.\n\nThis is **not** content injection \u2014 the trailing content is dropped, and the resulting DOM is a normal\nsingle-root document (`\u003ca/\u003e`). The security-relevant property is the silent acceptance of\nnot-well-formed input: xmldom\u0027s parse result disagrees with the specification and with browser XML\nparsers, so any control that treats \"xmldom parsed it without error\" as \"well-formed\" is bypassed.\n\n### Root Cause\n\nOn the `0.9.x` line, where the line-terminator variant specifically leaks:\n\n1. A shared regexp builder compiles anchored productions with the `m` flag.\n2. `^\u2026$` under `m` are line anchors, not string anchors.\n3. The anchored end-tag production `^ QName S? $` is therefore satisfied by the first line alone, so\n trailing content after a line terminator is neither matched nor rejected \u2014 the malformed end tag is\n accepted and the residue silently discarded.\n\nThe triggering line terminators are the ECMAScript `LineTerminator` set: U+000A, U+000D, U+2028, U+2029.\nU+2028 and U+2029 are not XML whitespace, so they are non-conforming trailing content that nonetheless\nleaks because the JavaScript `$` anchor treats them as line boundaries under `m`.\n\n## Affected Versions\n\nBoth maintained versions are affected and fixed:\n\n- **`0.9.x`** (`\u003e= 0.9.0, \u003c= 0.9.11`) \u2192 **`0.9.12`**: the anchored end-tag validator\u0027s `m` flag leaks\n the line-terminator variant. The whitespace variant (`\u003c/a junk\u003e`) is already rejected on this line.\n- **`0.8.x`** (`\u003e= 0.8.0, \u003c= 0.8.14`) \u2192 **`0.8.15`**: no anchored end-tag validator at all \u2014 both the\n line-terminator and the whitespace variant are silently accepted; the fix adds a residue check.\n\n`release-0.7.x` (`\u003c= 0.7.13`) and the unscoped `xmldom` package (range `*`, last release 0.6.0) are\n**affected but will not be patched** \u2014 they are end-of-life / unmaintained. They share the older-line\nbehavior (both variants silently accepted, residue dropped).\n\n## Proof of Concept\n\n```js\nconst { DOMParser, XMLSerializer } = require(\u0027@xmldom/xmldom\u0027);\nconst doc = new DOMParser().parseFromString(\u0027\u003ca\u003e\u003c/a\\njunk\u003e\u0027, \u0027text/xml\u0027); // no error\nconsole.log(new XMLSerializer().serializeToString(doc));\n// Observed: \u003ca/\u003e \u2014 the malformed end tag is accepted, \"junk\" silently discarded, no error reported.\n// Expected (per XML spec / Chromium / Firefox): a parse error \u2014 the input is not well-formed.\n```\n\n## Impact\n\n- **Silent acceptance of not-well-formed XML**: a document the specification and browser XML parsers\n reject is parsed without error.\n- **Bypass of a well-formedness / parse-before-trust gate**: an application relying on xmldom to\n reject malformed input treats such a document as valid. Note this is an input-validation /\n parser-differential issue, not content injection \u2014 the trailing content is discarded.\n\n## Fix Applied\n\nThe parser now reports a not-well-formed end tag whose valid name is followed by trailing content, on both `0.9.12` and `0.8.15` \u2014 previously it was accepted silently \u2014 and parsing recovers to the byte-identical DOM as before.\\\nOn `0.9.12` the anchored end-tag validator is corrected so a line break followed by trailing content no longer satisfies it, reported as a recoverable `error` when parsing as XML and a `warning` when parsing as HTML.\\\nOn `0.8.15`, which previously performed no end-tag residue validation, an equivalent residue check is added, reported as a recoverable `error` in both XML and HTML.\\\nWell-formed documents are unaffected.\n\nBecause the report is recoverable, the fix is non-breaking: no previously-parsed document begins to\nthrow and no serialized output changes. Consumers that want strict rejection can escalate the reported\n`error` to a fatal one via the parser\u0027s error handler (`onError` in `0.9.12`, `errorHandler` in\n`0.8.15`). The two versions also differ in the whitespace-separated variant (`\u003c/a junk\u003e`): `0.9.12`\nalready rejected it with a fatal error in XML and continues to, while `0.8.15` \u2014 which validated\nneither variant \u2014 now emits the same recoverable report for both the whitespace and line-break\nvariants.\n\n### Residual limitation\n\nBy default the parser still **recovers** (it does not reject the document); the reported condition is a\nrecoverable `error`/`warning`, not a fatal error, to avoid changing the parsed output in a patch\nrelease. Converting this and the other not-well-formed-acceptance cases to a consistent `fatalError` \u2014\nincluding the broader end-tag leniency on the older versions \u2014 is deferred to the next breaking release,\ntracked at [xmldom/xmldom#1074](https://github.com/xmldom/xmldom/issues/1074).",
"id": "GHSA-6h8r-xr42-gp59",
"modified": "2026-09-08T21:02:13Z",
"published": "2026-09-08T21:02:13Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/xmldom/xmldom/security/advisories/GHSA-6h8r-xr42-gp59"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-83611"
},
{
"type": "WEB",
"url": "https://github.com/xmldom/xmldom/pull/1071"
},
{
"type": "WEB",
"url": "https://github.com/xmldom/xmldom/pull/1072"
},
{
"type": "WEB",
"url": "https://github.com/xmldom/xmldom/commit/4430189660b0d380ee9c9ee7550a1358688e8828"
},
{
"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/0.8.15"
},
{
"type": "WEB",
"url": "https://github.com/xmldom/xmldom/releases/tag/0.9.12"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "xmldom: Parser silently accepts a not-well-formed end tag whose name is followed by a line break and trailing content"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.