xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. Prior to @xmldom/xmldom versions 0.8.15 and 0.9.12, and in xmldom version 0.6.0 and earlier, DOMParser.parseFromString() can silently accept an end tag such as </a\njunk>, close the element, and discard the trailing content. On 0.9.x, the lib/sax.js end-tag validator inherits the multiline flag from reg(), allowing the first line to satisfy the anchored XML ETag production; older lines have no equivalent residue validation. This parser differential can bypass a parse-before-trust well-formedness gate, although it does not inject the discarded content; onError on 0.9.x and errorHandler on 0.8.x are the relevant reporting interfaces. This issue is fixed in @xmldom/xmldom versions 0.8.15 and 0.9.12; no fixed version is available for xmldom.

Affected: npm: @xmldom/xmldom, npm: xmldom

## 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 whit…

Affected: Ubuntu:20.04:LTS: node-xmldom, Ubuntu:22.04:LTS: node-xmldom, Ubuntu:24.04:LTS: node-xmldom, Ubuntu:26.04:LTS: node-xmldom

xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. Prior to @xmldom/xmldom versions 0.8.15 and 0.9.12, and in xmldom version 0.6.0 and earlier, DOMParser.parseFromString() can silently accept an end tag such as </a\njunk>, close the element, and discard the trailing content. On 0.9.x, the lib/sax.js end-tag validator inherits the multiline flag from reg(), allowing the first line to satisfy the anchored XML ETag production; older lines have no equivalent residue validation. This parser differential can bypass a parse-before-trust well-formedness gate, although it does not inject the discarded content; onError on 0.9.x and errorHandler on 0.8.x are the relevant reporting interfaces. This issue is fixed in @xmldom/xmldom versions 0.8.15 and 0.9.12; no fixed version is available for xmldom.