CVE-2026-83618

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

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