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The extensible markup language (XML) is designed to help store, structure, and transfer data. Because of its platform independence, flexibility, and relative simplicity, the extensible markup language ( XML ) has found use in applications ranging from remote procedure calls to systematic storage, exchange, and retrieval of dataa wide range of applications. However, because of its versatility, XML is vulnerable to a wide spectrum of attacks. One such attack is called , including XML injection.

A user who has the ability to provide structured XML as input can override the contents of input string data that is incorporated into an XML document by injecting can inject XML tags in data fields. These tags are interpreted and classified by an the XML parser as executable content and , as a result, may cause certain data members to be overridden.

Consider the following XML code snippet from an An online store application , designed primarily to query a back-end database. The user has the ability that allows the user to specify the quantity of an item available for purchase .might generate the following XML document:

<item>
  <description>Widget</description>
  <price>500.0</price>
  <quantity>1</quantity>
</item>

A malicious user An attacker might input the following string instead of a simple number in count for the quantity field.:

1</quantity><price>1.0</price><quantity>1

ConsequentlyIn this case, the XML resolves to the following block:

<item>
  <description>Widget</description>
  <price>500.0</price>
  <quantity>1</quantity><price>1.0</price><quantity>1</quantity>
</item>

A Simple API for XML (SAX) parser (org.xml.sax and javax.xml.parsers.SAXParser) interprets the XML An XML parser may interpret the XML in this example such that the second price field overrides the first, leaving changing the price of the item as to $1. Even when it is not possible to perform such an attackAlternatively, the attacker may be able to inject special characters, such as comment blocks and CDATA delimiters, which corrupt the meaning of the XML.

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Depending on the specific data and command interpreter or parser to which data is being sent, appropriate methods must be used to sanitize untrusted user input. This compliant solution validates that quantity is an unsigned integer.:

import java.io.BufferedOutputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class OnlineStore {
  private static void createXMLStream(final BufferedOutputStream outStream,
      final String quantity) throws IOException, NumberFormatException {
    // Write XML string only if quantity is an unsigned integer (count).
    int count = Integer.parseUnsignedInt(quantity);
    String xmlString = "<item>\n<description>Widget</description>\n"
        + "<price>500</price>\n" + "<quantity>" + count + "</quantity></item>";
    outStream.write(xmlString.getBytes());
    outStream.flush();
  }
} 

...

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="item">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="description" type="xs:string"/>
      <xs:element name="price" type="xs:decimal"/>
      <xs:element name="quantity" type="xs:integernonNegativeInteger"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>
</xs:schema>

The schema is available as the file schema.xsd. This compliant solution employs this schema to prevent XML injection from succeeding. It also relies on the CustomResolver class defined in IDS17-J. Prevent XML External Entity Attacks to prevent XML external entity (XXE) attacks. This class, as well as XXE attacks, are described in the subsequent code examples.

private void createXMLStream(BufferedOutputStream outStream,
                      import java.io.BufferedOutputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.io.StringReader;

import javax.xml.XMLConstants;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
import javax.xml.transform.stream.StreamSource;
import javax.xml.validation.Schema;
import javax.xml.validation.SchemaFactory;

import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.DefaultHandler;

public class OnlineStore {

  private static void createXMLStream(final BufferedOutputStream outStream,
      final String quantity) throws IOException {
    String xmlString;
    xmlString = "<item>\n<description>Widget</description>\n" +
             + "<price>500.0</price>\n" +
 "<quantity>"      + quantity
       "<quantity>" + quantity + "</quantity></item>";
    InputSource xmlStream = new InputSource(
    new StringReader(xmlString));
  );

  // Build a validating SAX parser using our schema
    SchemaFactory sf = SchemaFactory
     =   SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
    DefaultHandler defHandler = new DefaultHandler() {
      public void warning(SAXParseException s)
 throws SAXParseException {
     throws  SAXParseException {throw s;
      }
      public void error(SAXParseException s)
 throws SAXParseException {
      throws SAXParseException {throw s;
      }
      public void fatalError(SAXParseException s)
 throws SAXParseException {
      throws SAXParseException {throw s;
      }
    };
    StreamSource ss = new StreamSource(new File("schema.xsd"));
    try {
      Schema schema = sf.newSchema(ss);
      SAXParserFactory spf = SAXParserFactory.newInstance();
      spf.setSchema(schema);
      SAXParser saxParser = spf.newSAXParser();
      // To set the custom entity resolver,
      // an XML reader needs to be created
      XMLReader reader = saxParser.getXMLReader();
 
     reader.setEntityResolver(new CustomResolver());
      saxParser.parse(xmlStream, defHandler);
    } catch (ParserConfigurationException x) {
      throw new IOException("Unable to validate XML", x);
    } catch (SAXException x) {
      throw new IOException("Invalid quantity", x);
    }

    // Our XML is valid, proceed
    outStream.write(xmlString.getBytes());
    outStream.flush();
  }
}

Using a schema or DTD to validate XML is convenient when receiving XML that may have been loaded with unsanitized input. If such an XML string has not yet been built, sanitizing input before constructing XML yields better performance.

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

Bibliography

[OWASP 2005]	A Guide to Building Secure Web Applications and Web Services
[OWASP 2007]	OWASP Top 10 for Java EE
[OWASP 2008]	Testing for XML Injection (OWASP-DV-008)
[Seacord 2015]	Image Added IDS00-J. Prevent SQL Injection LiveLesson
[W3C 2008]	Section 4.4.3, "Included If Validating"

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