GHSA-vr34-hp96-76pp: Npm/@xmldom/xmldom vulnerability
Summary
An embedded line terminator bypasses the requireWellFormed serializer check for a DocumentType's publicId and systemId. The check was added to fix GHSA-f6ww-3ggp-fr8h; an id whose first line is a valid literal slips past it and is emitted verbatim into the <!DOCTYPE …> declaration, so the markup after the line terminator breaks out into the surrounding document. Callers who enabled requireWellFormed to neutralize DocumentType injection remain exposed.
Details
publicId and systemId are stored as raw values including their surrounding quotes, and the PubidLiteral/SystemLiteral productions include those quotes. The serializer validates them with g.PubidLiteralmatch.test(publicId) and g.SystemLiteralmatch.test(systemId), where both matchers are reg('^', …, '$') and inherit the m flag from xmldom's shared regexp builder. Under m, $ matches at an interior line terminator, so a value such as "valid pubid"\n"><!ENTITY …> satisfies the matcher on its first line ("valid pubid" is a complete PubidLiteral) and the whole value — including the post-newline breakout — is emitted after PUBLIC/SYSTEM.
Root Cause
1. A shared regexp builder compiles anchored productions with the m flag. 2. ^…$ under m are line anchors, not string anchors. 3. A full-string validator built on such a production (.test()) accepts any string with one conforming line, so a complete, valid literal on the first line passes even though a line terminator and breakout markup follow. PubidChar excluding </> does not prevent it — the breakout is appended after the literal, not embedded inside it.
The triggering line terminators are the ECMAScript LineTerminator set: U+000A, U+000D, U+2028, U+2029.
Affected Versions
Only @xmldom/xmldom 0.9.x is affected. The vulnerable matchers are built by lib/grammar.js's m-flagged reg() builder, and the DocType publicId/systemId requireWellFormed check that consumes them was introduced in 0.9.10 (the GHSA-f6ww-3ggp-fr8h fix); 0.9.10 and 0.9.11 carry it. 0.8.x performs the same requireWellFormed check with inline, non-m regular expressions and is not affected. The unscoped xmldom package has no grammar.js and no requireWellFormed serializer, so there is no check to bypass.
Proof of Concept
js const { DOMImplementation, XMLSerializer } = require('@xmldom/xmldom'); const impl = new DOMImplementation();
// publicId: complete literal on line 1, then newline + breakout const dt = impl.createDocumentType('html', '"valid pubid"\n"><!ENTITY xxe SYSTEM "file:///etc/passwd">', ''); const doc = impl.createDocument(null, 'root', dt); console.log(new XMLSerializer().serializeToString(doc, { requireWellFormed: true })); // Observed (no throw): // <!DOCTYPE html PUBLIC "valid pubid" // "><!ENTITY xxe SYSTEM "file:///etc/passwd">><root/> // Expected: InvalidStateError (publicId is not a valid PubidLiteral). // Control: a single-line invalid publicId ("no-surrounding-quotes<>") DOES throw InvalidStateError, // confirming the check is active and specifically bypassed by the line terminator.
Impact
- Bypass of the GHSA-f6ww-3ggp-fr8h mitigation. Applications that adopted requireWellFormed: true to neutralize DocumentType injection remain exposed. - XML structure injection into the DOCTYPE, including injected markup / entity declarations after the public or system identifier.
Fix Applied
The anchored PubidLiteral/SystemLiteral validators used by the requireWellFormed serializer no longer treat an interior line terminator as satisfying the $ anchor, so a publicId or systemId containing any ECMAScript LineTerminator (U+000A, U+000D, U+2028, U+2029) is rejected with InvalidStateError. Valid single-line identifiers serialize unchanged, and the default serialization path is unaffected.
⚠ Opt-in required. Protection is not automatic. Existing serialization calls remain vulnerable unless { requireWellFormed: true } is explicitly passed. Applications that serialize untrusted DOM content should audit all serializeToString() call sites and add it.
Proof of Concept - fixed path
js const { DOMImplementation, XMLSerializer } = require('@xmldom/xmldom'); const impl = new DOMImplementation(); const dt = impl.createDocumentType('html', '"valid pubid"\n"><!ENTITY xxe SYSTEM "file:///etc/passwd">', ''); const doc = impl.createDocument(null, 'root', dt);
// Default path (requireWellFormed off) — unchanged, still emits verbatim: console.log(new XMLSerializer().serializeToString(doc)); // <!DOCTYPE html PUBLIC "valid pubid" // "><!ENTITY xxe SYSTEM "file:///etc/passwd">><root/>
// Opt-in path — now throws instead of emitting the breakout: new XMLSerializer().serializeToString(doc, { requireWellFormed: true }); // InvalidStateError: DocumentType publicId is not a valid PubidLiteral
Why the default stays verbatim
The W3C DOM Parsing "require well-formed" flag defaults to false, and a browser XMLSerializer emits the DOCTYPE verbatim. Unconditionally throwing on a malformed publicId/systemId would be an unjustified breaking change to the default path, so the fix tightens only the opt-in requireWellFormed validator, matching browser and spec defaults.
Residual limitation
The guarantee holds only for callers that pass { requireWellFormed: true }; the default serialization path still emits publicId/systemId verbatim. publicId and systemId are not validated at creation (createDocumentType) or on direct property assignment (documentType.publicId = …) — the WHATWG DOM specification places no well-formedness constraint on these fields at creation time, so the serializer is the spec-aligned enforcement point.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/@xmldom/xmldomto a version that resolves this vulnerability.Fixed in 0.9.12 - Upgrade
Upgrade
@xmldom/xmldomto a version that resolves this vulnerability.Fixed in 0.9.10 - Upgrade
Upgrade
@xmldom/xmldomto a version that resolves this vulnerability.Fixed in 0.9.11 - Configuration
Pass `{ requireWellFormed: true }` to `new XMLSerializer().serializeToString(doc, { requireWellFormed: true })` (opt-in) to enforce the serializer’s `DocumentType` publicId/systemId validation and prevent DOCTYPE injection bypass that occurs when `requireWellFormed` is not enabled.
@xmldom/xmldom XMLSerializer requireWellFormed = true
Event History
Frequently Asked Questions
Which applications remain exposed despite enabling the serializer’s well-formedness check?
Applications that enable requireWellFormed and serialize DocumentType nodes whose publicId or systemId can contain attacker-controlled values remain exposed. The bypass specifically affects values that include their surrounding quotes and contain an embedded line terminator.
What does an attacker need to supply to bypass the validation?
The attacker needs control of a DocumentType publicId or systemId value such that its first line is a valid quoted PubidLiteral or SystemLiteral. Content after a line terminator can then be emitted verbatim into the DOCTYPE declaration and break out into the surrounding markup.
How can I determine whether my application is at risk?
Review code paths that serialize DocumentType nodes with requireWellFormed enabled. They are at risk if publicId or systemId values may originate from untrusted input and can include newline or other line-terminator characters.