CVE-2019-12331: Improper Restriction of XML External Entity Reference

November 7, 2019 (updated November 13, 2019)

PHPOffice PhpSpreadsheet 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

nvd.nist.gov/vuln/detail/CVE-2019-12331

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