Rule Definition
According to CWE-611, XML documents optionally contain a Document Type Definition (DTD), which, among other features, enables the definition of XML entities. It is possible to define an entity by providing a substitution string in the form of a URI. The XML parser can access the contents of this URI and embed these contents back into the XML document for further processing. By submitting an XML file that defines an external entity with a file:// URI, an attacker can cause the processing application to read the contents of a local file. For example, a URI such as 'file:///c:/winnt/win.ini' designates (in Windows) the file 'C:\Winnt\win.ini', or 'file:///etc/passwd' designates the password file in Unix-based systems. Using URIs with other schemes such as 'http://', the attacker can force the application to make outgoing requests to servers that the attacker cannot reach directly, which can be used to bypass firewall restrictions or hide the source of attacks such as port scanning. Once the content of the URI is read, it is fed back into the application that is processing the XML. This application may echo back the data (e.g. in an error message), thereby exposing the file contents.

Remediation
The property "setShouldResolveExternalEntities" must be set to "false".

Violation Code Sample

import Foundation

// This Swift code that is vulnerable to XXE attacks

var success: Bool
var rawXmlConvToData: NSData = rawXml.data(using: NSUTF8StringEncoding)
var myParser: XMLParser = NSXMLParser(data: rawXmlConvToData)
myParser.shouldResolveExternalEntities = true
myParser.delegate = self

Fixed Code Sample

import Foundation

// This Swift code that is vulnerable to XXE attacks

var success: Bool
var rawXmlConvToData: NSData = rawXml.data(using: NSUTF8StringEncoding)
var myParser: XMLParser = NSXMLParser(data: rawXmlConvToData)
myParser.shouldResolveExternalEntities = false // or not use the API default setting is false
myParser.delegate = self

https://cwe.mitre.org/data/definitions/611.html https://developer.apple.com/documentation/foundation/nsxmlparser/1414143-shouldresolveexternalentities?language=swift https://www.owasp.org/index.php/Top_10-2017_A4-XML_External_Entities_(XXE)