PHPOffice PhpSpreadsheet before 1.8.0 has an XXE issue. The XmlScanner decodes the sheet1.xml from an .xlsx to utf-8 if something else than UTF-8 is declared in the header. This was a security measurement to prevent CVE-2018-19277 but the fix is not sufficient. By double-encoding the the xml payload to utf-7 it is possible to bypass the check for the string ‚<!ENTITY‘ and thus allowing for an xml external entity processing (XXE) attack.
PHPExcel before 1.8.0, as used in ownCloud Server before 5.0.15 and 6.0.x before 6.0.2, does not disable external entity loading in libxml, which allows remote attackers to read arbitrary files, cause a denial of service, or possibly have other impact via an XML External Entity (XXE) attack.