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.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-12331
• https://github.com/PHPOffice/PhpSpreadsheet/blob/master/CHANGELOG.md#180---2019-07-01
• https://github.com/PHPOffice/PhpSpreadsheet/releases/tag/1.8.0
• https://herolab.usd.de/security-advisories/usd-2019-0046/
• PHPOffice/PhpSpreadsheet#1041
• PHPOffice/PhpSpreadsheet@0e6238c
• https://github.com/FriendsOfPHP/security-advisories/blob/master/phpoffice/phpspreadsheet/CVE-2019-12331.yaml