CVE-2026-108357: PhpSpreadsheet: XmlScanner Bypass via Double-Encoded unentity() TOCTOU Allows XXE and Denial of Service

The SpreadsheetML XML reader (Reader\Xml) scans file content twice through XmlScanner::scan(), and each scan runs an unentity() callback that calls html_entity_decode() after the pattern check. By double-encoding the < character as <, the first scan decodes & to & (forming <), and the second scan decodes < to < (forming <!DOCTYPE). Neither scan’s pattern check detects the <!DOCTYPE string because the callback runs after each check. This enables Denial of Service via entity expansion on all PHP versions, and XXE/SSRF on PHP < 8.0 or when LIBXML_NOENT is used.