This affects the package phpoffice/phpspreadsheet from 0.0.0. The library is vulnerable to XSS when creating an html output from an excel file by adding a comment on any cell. The root cause of this issue is within the HTML writer where user comments are concatenated as part of link and this is returned as HTML. A fix for this issue is available on commit 0ed5b800be2136bcb8fa9c1bdf59abc957a98845/master branch.

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.

securityScan() in PHPOffice PhpSpreadsheet through 1.5.0 allows a bypass of protection mechanisms for XXE via UTF-7 encoding in a .xlsx file