Skip to content

xmldom: requireWellFormed DocType publicId/systemId validation is bypassable via an embedded line terminator

High severity GitHub Reviewed Published Aug 21, 2026 in xmldom/xmldom • Updated Sep 8, 2026

Package

npm @xmldom/xmldom (npm)

Affected versions

>= 0.9.10, <= 0.9.11

Patched versions

0.9.12

Description

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.PubidLiteral_match.test(publicId) and g.SystemLiteral_match.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

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

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.

References

@karfau karfau published to xmldom/xmldom Aug 21, 2026
Published by the National Vulnerability Database Sep 1, 2026
Published to the GitHub Advisory Database Sep 8, 2026
Reviewed Sep 8, 2026
Last updated Sep 8, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(25th percentile)

Weaknesses

XML Injection (aka Blind XPath Injection)

The product does not properly neutralize special elements that are used in XML, allowing attackers to modify the syntax, content, or commands of the XML before it is processed by an end system. Learn more on MITRE.

Permissive Regular Expression

The product uses a regular expression that does not sufficiently restrict the set of allowed values. Learn more on MITRE.

CVE ID

CVE-2026-83618

GHSA ID

GHSA-vr34-hp96-76pp

Source code

Credits

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.