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XML EXTERNAL ENTITIES

<?xml version="1.0"encoding="UTF-8"?>
<!DOCTYPE contato ..
	<!ENTITY meunome TEXT "Guilherme"> # External entity
<contato>
	<nome>&meunome;</nome>
	<telefone tipo="residencial">3199554565</telefone>
	<telefone tipo="comercial">8156754565</telefone>
</contato>

External entity exploring

<!ENTITY meunome SYSTEM "file://etc/passwd">
<!ENTITY meunome SYSTEM "https://192.168.1.1/private">
<!ENTITY meunome SYSTEM "file:///dev/random">
  • The name entity will be the file with the system passwords
  • Access server local files LFI
  • Get network informations
  • Denial of Service (DOS) with number generator

How Prevent

  • Use less complex data formats such as JSON
  • Update all XML processors and libraries
  • Not serialize sensitive data
  • Disable XML external entity / DTD processing
  • Use XSD to validate XML / XSL files