XML External Entity (XXE) vulnerability in the XML data handler

TL;DR

This vulnerability only affects Kirby sites that use the Xml data handler (e.g. Data::decode($string, 'xml')) or the Xml::parse() method in site or plugin code. The Kirby core does not use any of the affected methods.

If you use an affected method and cannot rule out XML input controlled by an attacker, we strongly recommend to update to a patch release.

Introduction

XML External Entities (XXE) is a little used feature in the XML markup language that allows to include data from external files in an XML structure. If the name of the external file can be controlled by an attacker, this becomes a vulnerability that can be abused for various system impacts like the disclosure of internal or confidential data that is stored on the server (arbitrary file disclosure) or to perform network requests on behalf of the server (server-side request forgery, SSRF).

Impact

Kirby’s Xml::parse() method used PHP’s LIBXML_NOENT constant, which enabled the processing of XML external entities during the parsing operation.

The Xml::parse() method is used in the Xml data handler (e.g. Data::decode($string, 'xml')).

Both the vulnerable method and the data handler are not used in the Kirby core. However they may be used in site or plugin code, e.g. to parse RSS feeds or other XML files. If those files are of an external origin (e.g. uploaded by a user or retrieved from an external URL), attackers may be able to include an external entity in the XML file that will then be processed in the parsing process.

Kirby sites that don’t use XML parsing in site or plugin code are not affected.

Patches

The problem has been patched in Kirby 3.5.8.3, Kirby 3.6.6.3, Kirby 3.7.5.2, Kirby 3.8.4.1 and Kirby 3.9.6. Please update to one of these or a later version to fix the vulnerability.

In all of the mentioned releases, we have removed the LIBXML_NOENT constant as processing of external entities is out of scope of the parsing logic. This protects all uses of the method against the described vulnerability.

Credits

Thanks to Alexandre Zanni (@noraj) at ACCEIS and Patrick Falb (@dapatrese) at FORMER 03 for responsibly reporting the identified issue.