The review asked for a regression test pinning DTD-prohibited parsing. Writing
it showed the premise was wrong: XDocument.Parse permits an internal DTD subset
and expands its entities, so a DOCTYPE-bearing response body parsed fine and the
guard did not exist (entity-expansion DoS on external input).
Parse now goes through XmlReader with DtdProcessing.Prohibit and a null
XmlResolver. The regression test feeds a DOCTYPE + ENTITY payload shaped as a
well-formed EWS error response, so it fails if DtdProcessing is ever loosened
rather than passing for the unrelated-XML reason.
Joseph Doherty