# 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 the LIBXML_DTDLOAD and LIBXML_DTDATTR settings for the XML Readers by default.
Should you ever need to change these settings, the following method is available through the PHPExcel_Settings:
Allowing you to specify the XML loader settings that those that you want to use instead.
> While PHPExcel protects you with its default settings, if you do change these settings yourself, then you're responsible for ensuring that your XML-based formats aren't open to XXE injection.
