| Package | Affected versions | Patched versions |
|---|---|---|
| phpoffice/phpexcel | ≤ 1.8.2 | |
| phpoffice/phpspreadsheet | ≥ 2.0.0, < 2.1.1≥ 2.2.0, < 2.3.0< 1.29.1 | 2.1.12.3.01.29.1 |
| phpspreadsheet | ≥ 2.0.0, < 2.1.1≥ 2.2.0, < 2.3.0< 1.29.1 | 2.1.12.3.01.29.1 |
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.
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.
Needed:
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 [ ... ]>
<!ENTITY internal 'abc' >"
Resulting in:
PCFFTlRJVFkgaW50ZXJuYWwgJ2FiYycgID4K
<?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.
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.
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