PhpSpreadsheet: XmlScanner Bypass via Double-Encoded unentity() TOCTOU Allows XXE and Denial of Service

PhpSpreadsheet is a pure PHP library for reading and writing spreadsheet files. From 3.3.0 until 3.10.8 and 5.10.0, Reader\Xml passes stored SpreadsheetML XML through XmlScanner::scan in Xml::canRead and Xml::trySimpleXMLLoadStringPrivate, while Xml::unentity decodes numeric entities only after each DOCTYPE pattern check. A double-encoded less-than character is reduced by the first callback to another encoded character and by the second callback to a literal less-than sign, so SimpleXML receives an unchecked DOCTYPE and entity declaration. Processing attacker-controlled SpreadsheetML XML can cause entity-expansion denial of service and can permit local file access or SSRF on PHP before 8.0 or when LIBXML_NOENT is enabled. This issue is fixed in versions 3.10.8 and 5.10.0.