IDS17-J. Prevent XML External Entity Attacks

Entity declarations define shortcuts to commonly used text or special characters and may.  An entity declaration may define either an internal or external entity. For internal entities, the content of the entity is given in the declaration. For external entities, the content is specified by a Uniform Resource Identifier (URI). 

Entities may be either parsed or unparsed. The contents of a parsed entity are referred to as its replacement text. An unparsed entity is a resource whose contents may or may not be text, and if text, may be other than XML. Parsed entities are invoked by name using an entity reference; unparsed entities by name.

According to XML W3C Recommendation [W3C 2008], section 4.4.3, "Included If Validating":

When an XML processor recognizes a reference to a parsed entity, to validate the document, the processor MUST include its replacement text. If the entity is external, and the processor is not attempting to validate the XML document, the processor MAY, but need not, include the entity's replacement text.

Because inclusion of replacement text from an external entity is optional, not all XML processors are vulnerable to external entity attacks during validation.

An XML external entity (XXE) attack occurs when XML input containing a reference to an external entity is processed by a improperly configured XML parser.  An attacker might use an XXE attack to gain access to sensitive information by manipulating the URI of the entity to refer to files on the local file system containing sensitive data such as passwords or private user data.  An attacker might launch a denial-of-service attack, for example, by specifying /dev/random or /dev/tty as input URIs can crash or indefinitely block a program.

Noncompliant Code Example

This noncompliant code example attempts to parse the file evil.xml, report any errors, and exit. However, a SAX (Simple API for XML) or a DOM (Document Object Model) parser will attempt to access the URI specified by the SYSTEM attribute, which means it will attempt to read the contents of the local /dev/tty file. On POSIX systems, reading this file causes the program to block until input data is supplied to the machine's console. Consequently, an attacker can use this malicious XML file to cause the program to hang.

class XXE {
  private static void receiveXMLStream(InputStream inStream,
                                       DefaultHandler defaultHandler)
      throws ParserConfigurationException, SAXException, IOException {
    SAXParserFactory factory = SAXParserFactory.newInstance();
    SAXParser saxParser = factory.newSAXParser();
    saxParser.parse(inStream, defaultHandler);
  }

  public static void main(String[] args)
      throws ParserConfigurationException, SAXException, IOException {
    receiveXMLStream(new FileInputStream("evil.xml"), 
                     new DefaultHandler());
  }
}

This program is subject to a remote XXE attack if the evil.xml file contains the following:

<?xml version="1.0"?>
<!DOCTYPE foo SYSTEM "file:/dev/tty">
<foo>bar</foo>

This noncompliant code example may also violate ERR01-J. Do not allow exceptions to expose sensitive information if the information contained in the exceptions is sensitive.

Compliant Solution (EntityResolver)

This compliant solution defines a CustomResolver class that implements the interface org.xml.sax.EntityResolver. This interface enables a SAX application to customize handling of external entities. The customized handler uses a simple whitelist for external entities. The resolveEntity() method returns an empty InputSource when an input fails to resolve to any of the specified, safe entity source paths.

import java.io.IOException;
import org.xml.sax.EntityResolver;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

class CustomResolver implements EntityResolver {
  public InputSource resolveEntity(String publicId, String systemId)
    throws SAXException, IOException {

    // Check for known good entities
    String entityPath = "/home/username/java/xxe/file";
    if (systemId.equals(entityPath)) {
      System.out.println("Resolving entity: " + publicId +
                         " " + systemId);
      return new InputSource(entityPath);
    } else {
      // Disallow unknown entities by returning a blank path 
      return new InputSource(); 
    }
  }
}

The setEntityResolver() method registers the instance with the corresponding SAX driver.  When parsing malicious input, the empty InputSource returned by the custom resolver causes a java.net.MalformedURLException to be thrown. Note that you must create an XMLReader object on which to set the custom entity resolver.

class XXE {
  private static void receiveXMLStream(InputStream inStream,
                                       DefaultHandler defaultHandler)
      throws ParserConfigurationException, SAXException, IOException {
    SAXParserFactory factory = SAXParserFactory.newInstance();
    SAXParser saxParser = factory.newSAXParser();

    // Create an XML reader to set the entity resolver.
    XMLReader reader = saxParser.getXMLReader();
    reader.setEntityResolver(new CustomResolver());
    reader.setContentHandler(defaultHandler);

    InputSource is = new InputSource(inStream);
    reader.parse(is);
  }

  public static void main(String[] args)
      throws ParserConfigurationException, SAXException, IOException {
    receiveXMLStream(new FileInputStream("evil.xml"), 
                     new DefaultHandler());
  }
}

Risk Assessment

Failure to sanitize user input before processing or storing it can result in injection attacks.

Automated Detection

Related Vulnerabilities

CVE-2008-2370 describes a vulnerability in Apache Tomcat 4.1.0 through 4.1.37, 5.5.0 through 5.5.26, and 6.0.0 through 6.0.16. When a RequestDispatcher is used, Tomcat performs path normalization before removing the query string from the URI, which allows remote attackers to conduct directory traversal attacks and read arbitrary files via a .. (dot dot) in a request parameter.

Related Guidelines

Android Implementation Details

This rule uses MS SQL Server as an example to show a database connection. However, on Android, DatabaseHelper from SQLite is used for a database connection. Because Android apps may receive untrusted data via network connections, the rule is applicable.

Bibliography