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vendor/phpoffice/phpexcel/Documentation/markdown/ReadingSpreadsheetFiles/02-Security.md

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+# PHPExcel User Documentation – Reading Spreadsheet Files
+
+
+## Security
+
+XML-based formats such as OfficeOpen XML, Excel2003 XML, OASIS and Gnumeric are susceptible to XML External Entity Processing (XXE) injection attacks (for an explanation of XXE injection see http://websec.io/2012/08/27/Preventing-XEE-in-PHP.html) when reading spreadsheet files. This can lead to:
+
+ - Disclosure whether a file is existent
+ - Server Side Request Forgery
+ - Command Execution (depending on the installed PHP wrappers)
+ 
+
+To prevent this, PHPExcel sets `libxml_disable_entity_loader` to `true` for the XML-based Readers by default.