GHSA-f6ww-3ggp-fr8h

Описание

Summary

The package serializes DocumentType node fields (internalSubset, publicId, systemId) verbatim without any escaping or validation. When these fields are set programmatically to attacker-controlled strings, XMLSerializer.serializeToString can produce output where the DOCTYPE declaration is terminated early and arbitrary markup appears outside it.

Details

DOMImplementation.createDocumentType(qualifiedName, publicId, systemId, internalSubset) validates only qualifiedName against the XML QName production. The remaining three arguments are stored as-is with no validation.

The XMLSerializer emits DocumentType nodes as:

All fields are pushed into the output buffer verbatim — no escaping, no quoting added.

internalSubset injection: The serializer wraps internalSubset with [ and ]. A value containing ]> closes the internal subset and the DOCTYPE declaration at the injection point. Any content after ]> in internalSubset appears outside the DOCTYPE in the serialized output as raw XML markup. Reported by @TharVid (GHSA-f6ww-3ggp-fr8h). Affected: @xmldom/xmldom ≥ 0.9.0 via createDocumentType API; 0.8.x only via direct property write.

publicId injection: The serializer emits publicId verbatim after PUBLIC with no quoting added. A value containing an injected system identifier (e.g., "pubid" SYSTEM "evil") breaks the intended quoting context, injecting a fake SYSTEM entry into the serialized DOCTYPE declaration. Identified during internal security research. Affected: both branches, all versions back to 0.1.0.

systemId injection: The serializer emits systemId verbatim. A value containing > terminates the DOCTYPE declaration early; content after > appears as raw XML markup outside the DOCTYPE context. Identified during internal security research. Affected: both branches, all versions back to 0.1.0.

The parse path is safe: the SAX parser enforces the PubidLiteral and SystemLiteral grammar productions, which exclude the relevant characters, and the internal subset parser only accepts a subset it can structurally validate. The vulnerability is reachable only through programmatic createDocumentType calls with attacker-controlled arguments.

Affected code

lib/dom.js — createDocumentType (lines 898–910):

lib/dom.js — serializer DOCTYPE case (lines 2948–2964):

PoC

internalSubset injection

publicId quoting context break

systemId injection

Impact

An application that programmatically constructs DocumentType nodes from user-controlled data and then serializes the document can emit a DOCTYPE declaration where the internal subset is closed early or where injected SYSTEM entities or other declarations appear in the serialized output.

Downstream XML parsers that re-parse the serialized output and expand entities from the injected DOCTYPE declarations may be susceptible to XXE-class attacks if they enable entity expansion.

Fix Applied

⚠ Opt-in required. Protection is not automatic. Existing serialization calls remain vulnerable unless { requireWellFormed: true } is explicitly passed. Applications that pass untrusted data to createDocumentType() or write untrusted values directly to a DocumentType node's publicId, systemId, or internalSubset properties should audit all serializeToString() call sites and add the option.

XMLSerializer.serializeToString() now accepts an options object as a second argument. When { requireWellFormed: true } is passed, the serializer validates the DocumentType node's publicId, systemId, and internalSubset fields before emitting the DOCTYPE declaration and throws InvalidStateError if any field contains an injection sequence:

• publicId: throws if non-empty and does not match the XML PubidLiteral production (XML 1.0 [12])
• systemId: throws if non-empty and does not match the XML SystemLiteral production (XML 1.0 [11])
• internalSubset: throws if it contains ]> (which closes the internal subset and DOCTYPE declaration early)

All three checks apply regardless of how the invalid value entered the node — whether via createDocumentType arguments or a subsequent direct property write.

PoC — fixed path

The guard also covers post-creation property writes:

Why the default stays verbatim

The W3C DOM Parsing and Serialization spec §3.2.1.3 defines a require well-formed flag whose default value is false. With the flag unset, the spec permits verbatim serialization of DOCTYPE fields. Unconditionally throwing would be a behavioral breaking change with no spec justification. The opt-in requireWellFormed: true flag allows applications that require injection safety to enable strict mode without breaking existing deployments.

Residual limitation

createDocumentType(qualifiedName, publicId, systemId[, internalSubset]) does not validate publicId, systemId, or internalSubset at creation time. This creation-time validation is a breaking change and is deferred to a future breaking release.

When the default serialization path is used (without requireWellFormed: true), all three fields are still emitted verbatim. Applications that do not pass requireWellFormed: true remain exposed.