Why is this an issue?
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.
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
For DocumentBuilder, SAXParser, XMLInput, Transformer and Schema JAPX factories:
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); // Noncompliant
SAXParserFactory factory = SAXParserFactory.newInstance(); // Noncompliant
XMLInputFactory factory = XMLInputFactory.newInstance(); // Noncompliant
TransformerFactory factory = javax.xml.transform.TransformerFactory.newInstance(); // Noncompliant
SchemaFactory factory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI); // Noncompliant
For Dom4j library:
SAXReader xmlReader = new SAXReader(); // Noncompliant
For Jdom2 library:
SAXBuilder builder = new SAXBuilder(); // Noncompliant
Compliant solution
For DocumentBuilder, SAXParser, XMLInput, Transformer and Schema JAPX factories:
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
// to be compliant, completely disable DOCTYPE declaration:
factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
// or completely disable external entities declarations:
factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
factory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
// or prohibit the use of all protocols by external entities:
factory.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
factory.setAttribute(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
// or disable entity expansion but keep in mind that this doesn't prevent fetching external entities
// and this solution is not correct for OpenJDK < 13 due to a bug: https://bugs.openjdk.java.net/browse/JDK-8206132
factory.setExpandEntityReferences(false);
SAXParserFactory factory = SAXParserFactory.newInstance();
// to be compliant, completely disable DOCTYPE declaration:
factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
// or completely disable external entities declarations:
factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
factory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
// or prohibit the use of all protocols by external entities:
SAXParser parser = factory.newSAXParser(); // Noncompliant
parser.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");
parser.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
XMLInputFactory factory = XMLInputFactory.newInstance();
// to be compliant, completely disable DOCTYPE declaration:
factory.setProperty(XMLInputFactory.SUPPORT_DTD, false);
// or completely disable external entities declarations:
factory.setProperty(XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES, Boolean.FALSE);
// or prohibit the use of all protocols by external entities:
factory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");
factory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
TransformerFactory factory = javax.xml.transform.TransformerFactory.newInstance();
// to be compliant, prohibit the use of all protocols by external entities:
factory.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
factory.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, "");
SchemaFactory factory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
// to be compliant, completely disable DOCTYPE declaration:
factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
// or prohibit the use of all protocols by external entities:
factory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");
factory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
For Dom4j library:
SAXReader xmlReader = new SAXReader();
xmlReader.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
For Jdom2 library:
SAXBuilder builder = new SAXBuilder();
builder.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");
builder.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
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