XML standard allows the use of entities, declared in the DOCTYPE of the document, which can be internal or external.

When parsing the XML file, the content of the external entities is retrieved from an external storage such as the file system or network, which may lead, if no restrictions are put in place, to arbitrary file disclosures or server-side request forgery (SSRF) vulnerabilities.

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE person [
  <!ENTITY file SYSTEM "file:///etc/passwd">
  <!ENTITY ssrf SYSTEM "https://internal.network/sensitive_information">
]>

<person>
  <name>&file;</name>
  <city>&ssrf;</city>
  <age>18</age>
</person>

It’s recommended to limit resolution of external entities by using one of these solutions:

• If DOCTYPE is not necessary, completely disable all DOCTYPE declarations.
• If external entities are not necessary, completely disable their declarations.
• If external entities are necessary then:
  • Use XML processor features, if available, to authorize only required protocols (eg: https).
  • And use an entity resolver (and optionally an XML Catalog) to resolve only trusted entities.

Noncompliant Code Example

SimpleXML object:

$xml = file_get_contents("xxe.xml");
$doc = simplexml_load_string($xml, "SimpleXMLElement", LIBXML_NOENT); // Noncompliant (LIBXML_NOENT enable external entities substitution)

DOMDocument object:

$doc = new DOMDocument();
$doc->load("xxe.xml", LIBXML_NOENT); // Noncompliant (LIBXML_NOENT enable external entities substitution)

XMLReader object:

$reader = new XMLReader();
$reader->open("xxe.xml");
$reader->setParserProperty(XMLReader::SUBST_ENTITIES, true); // Noncompliant (SUBST_ENTITIES enable external entities substitution)

Compliant Solution

SimpleXML object:

$xml = file_get_contents("xxe.xml");
$doc = simplexml_load_string($xml, "SimpleXMLElement"); // Compliant (external entities substitution are disabled by default)

DOMDocument object:

$doc = new DOMDocument();
$doc->load("xxe.xml"); // Compliant (external entities substitution are disabled by default)

XMLReader object:

$reader = new XMLReader();
$reader->open("xxe.xml");
$reader->setParserProperty(XMLReader::SUBST_ENTITIES, false); // Compliant (SUBST_ENTITIES set to false)

See

• OWASP Top 10 2021 Category A5 - Security Misconfiguration
• OWASP Top 10 2017 Category A4 - XML External Entities (XXE)
• OWASP XXE Prevention Cheat Sheet
• MITRE, CWE-611 - Information Exposure Through XML External Entity Reference
• MITRE, CWE-827 - Improper Control of Document Type Definition