GHSA-6h8r-xr42-gp59

## Summary

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:

1. A shared regexp builder compiles anchored productions with the `m` flag. 2. `^…$` under `m` are line anchors, not string anchors. 3. 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's `m` flag 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

```js 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](https://github.com/xmldom/xmldom/issues/1074).