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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, XML has found use in a wide range of applications. However, because of its versatility, XML is vulnerable to a wide spectrum of attacks, including XML injection.

A user who has the ability to provide input string data that is incorporated into an XML document can inject XML tags. These tags are interpreted by the XML parser and may cause data to be overridden.

An online store application 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>

An attacker might input the following string instead of a count for the quantity:

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

In this case, the XML resolves to the following:

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

An XML parser may interpret the XML in this example such that the second price field overrides the first, changing the price of the item to $1. Alternatively, the attacker may be able to inject special characters, such as comment blocks and CDATA delimiters, which corrupt the meaning of the XML.

Noncompliant Code Example

In this noncompliant code example, a client method uses simple string concatenation to build an XML query to send to a server. XML injection is possible because the method performs no input validation.

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

public class OnlineStore {
  private static void createXMLStreamBad(final BufferedOutputStream outStream,
      final String quantity) throws IOException {
    String xmlString = "<item>\n<description>Widget</description>\n"
        + "<price>500</price>\n" + "<quantity>" + quantity
        + "</quantity></item>";
    outStream.write(xmlString.getBytes());
    outStream.flush();
  }
} 

Compliant Solution (Input Validation)

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();
  }
} 

Compliant Solution (XML Schema)

A more general mechanism for checking XML for attempted injection is to validate it using a Document Type Definition (DTD) or schema. The schema must be rigidly defined to prevent injections from being mistaken for valid XML. Here is a suitable schema for validating our XML snippet:

<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:nonNegativeInteger"/>
    </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.

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></item>";
    InputSource xmlStream = new InputSource(new StringReader(xmlString));
    // Build a validating SAX parser using our schema
    SchemaFactory sf = SchemaFactory
        .newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
    DefaultHandler defHandler = new DefaultHandler() {
      public void warning(SAXParseException s) throws SAXParseException {
        throw s;
      }
      public void error(SAXParseException s) throws SAXParseException {
        throw s;
      }
      public void fatalError(SAXParseException s) 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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

IDS16-J

High

Probable

Medium

P12

L1

Automated Detection

ToolVersionCheckerDescription
The Checker Framework

2.1.3

Tainting CheckerTrust and security errors (see Chapter 8)
Fortify1.0

Missing_XML_Validation

Implemented
Parasoft Jtest 10.3 BD.SECURITY.TDXML 

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

 


 

 

7 Comments

  1. It is unclear what the format violation is in this CS:

     

      if (!Pattern.matches("[0-9]+", quantity)) {
        // Format violation
      }
    but if it is going to throw an exception anyway why not just do this:
          int quant = Integer.parseUnsignedInt(quantity);

    Parses the string argument as an unsigned decimal integer. The characters in the string must all be decimal digits, except that the first character may be an an ASCII plus sign '+' ('\u002B'). The resulting integer value is returned, exactly as if the argument and the radix 10 were given as arguments to the parseUnsignedInt(java.lang.String, int) method.

    Parameters:s - a String containing the unsigned int representation to be parsed

    Returns:the unsigned integer value represented by the argument in decimal.

    Throws:NumberFormatException - if the string does not contain a parsable unsigned integer.

    I'm sort of assuming this method will fail if the integer is out of range, which the current solution does not.
    It also generates an integer so further validation can be performed, like is the quantity greater than zero, less than some max, or less than the number of items in stock.
    1. I made this change; please review.

  2. It seems odd to me that we comment on SAX but not on DOM which I assume behaves the same?

    1. Maybe, but not necessarily. DOM builds a tree of the parsed XML, and it is up to the program how to interpret the tree. Neither DOM nor SAX specify how to handle multiple fields like {{<price>}} when only one is expected. So it is possible for an XML injection to fail because the injected price gets ignored.

      Clearly to illustrate DOM we would need separate code examples.

      1. It might be worthwhile to also have a DOM example.  We could also just test it out and then note if it behaves the same or not without including the example.

        1. Sorry, let me be more precise:

          The NCCE is noncompliant because it enables the resulting XML string to contain 2 prices. Both CS's prevent this, hence they are good.

          None of the code examples show how a program uses SAX or DOM to parse the XML. Well, the last CS does use a SAX parser, but only to confirm that the XML conforms to the schema. We could add SAX or DOM parsing to the other code samples, but its not necessary for the rule. Once your XML has two prices, you may be pwned; that's what the rule is trying to prevent. The rule doesn't address how to 'correctly' parse XML, using SAX or DOM...once the injection occurs, there isn't a safe way to parse the XML.

          So I took out the paragraph describing SAX, as it doesn't really contribute to the rule.

          1. That SAX paragraph was part of the book; I just cleaned it up a little.  Anyway, I think you went a little too far in removing the description of the vulnerability so I added it back but just referenced a generic XML parser.