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Many command interpreters and parsers provide their own sanitization and validation methods. When available, their use is preferred over custom sanitization techniques because homegrown sanitization can often neglect special cases or hidden complexities in the parser. Another problem with custom sanitization code is that it may not be adequately maintained when new capabilities are added to the command interpreter or parser software.

SQL Injection

An SQL injection vulnerability arises when the original SQL query can be altered to form an altogether different query. Execution of this altered query may result in information leaks or data modification. The primary means of preventing SQL injection are sanitizing and validating untrusted input and parameterizing the query.

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This time, the '1'='1' tautology disables both user name and password validation, and the attacker is falsely logged in without knowing a login ID or password.

Noncompliant Code Example

This noncompliant code example shows JDBC code to authenticate a user to a system. The password is passed as a char array, the database connection is created, and then the passwords are hashed to comply with MSC05-J. Store passwords using a hash function and MSC10-J. Limit the lifetime of sensitive data.

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This code example also does not address how to safely shut down the connection. For information on how to do this, see rule FIO06-J. Close resources when they are no longer needed.

Compliant Solution (PreparedStatement)

Fortunately, the JDBC library provides an API for building SQL commands that sanitize untrusted data. The java.sql.PreparedStatement class properly escapes input strings, preventing SQL injection when used properly. This is an example of component-based sanitization.

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This code example also does not address how to safely shut down the connection. For information on how to do this, see rule FIO06-J. Close resources when they are no longer needed.

XML Injection

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 data. However, because of its versatility, XML is vulnerable to a wide spectrum of attacks. One such attack is called XML Injection.

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A SAX parser (org.xml.sax and javax.xml.parsers.SAXParser) interprets the above XML such that the second price field overrides the first, leaving the price of the item as $1. Even when it is not possible to perform such an attack, the attacker may be able to inject special characters, such as comment blocks and CDATA delimiters, which distort 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.

private void createXMLStream(BufferedOutputStream outStream, String quantity)
             throws IOException {
  String xmlString;
  xmlString = "<item>\n<description>Widget</description>\n<price>500.0</price>\n" +
              "<quantity>" + quantity + "</quantity></item>";
  outStream.write(xmlString.getBytes());
  outStream.flush();
}

Compliant Solution (Whitelisting)

Depending on the specific data and command interpreter or parser to which data is being sent, different methods must be used to sanitize untrusted user input. This compliant solution uses whitelisting to sanitize the input. In this compliant solution, the method requires that the quantity field must be a number between 0 and 9.

private void createXMLStream(BufferedOutputStream outStream, String quantity)
             throws IOException {
  // Write XML string if quantity contains numbers only (whitelisting)
  // Blacklisting of invalid characters can also be done in conjunction

  if (!Pattern.matches("[0-9]+", quantity)) {
    // Format violation
  }

  String xmlString = "<item>\n<description>Widget</description>\n<price>500</price>\n" +
                     "<quantity>" + quantity + "</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 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:

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

XML External Entity Attacks (XXE)

An XML document can be dynamically constructed from smaller logical blocks called entities. Entities can be either internal, external, or parameter-based. External entities allow the inclusion of XML data from external files.

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An attacker may attempt to cause denial of service or program crashes by manipulating the URI of the entity to refer to special files existing on the local file system, for example, by specifying /dev/random or /dev/tty as input URIs. This may crash or block the program indefinitely. This is called an XML External Entity (XXE) attack. Because inclusion of replacement text from an external entity is optional, not all XML processors are vulnerable to external entity attacks.

Noncompliant Code Example

This noncompliant code example attempts to parse the file evil.xml, reports any errors, and exits. However, a SAX or a DOM parser will attempt to access the URL specified by the SYSTEM attribute, which means it will attempt to read the contents of the local /dev/tty file. On POSIX system, 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.

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This noncompliant code example may also violate rule ERR06-J. Do not allow exceptions to expose sensitive information if the information contained in the exceptions is considered sensitive.

Compliant Solution (EntityResolver)

This compliant solution defines a CustomResolver class that implements the interface org.xml.sax.EntityResolver. This enables a SAX application to implement customized handling of external entities. The setEntityResolver() method registers the implementation with the corresponding SAX driver. 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. Consequently, 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.

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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 {
      return new InputSource(); // Disallow unknown entities, by returning blank path
    }
  }
}

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

    // To set the Entity Resolver, an XML reader needs to be created
    XMLReader reader = saxParser.getXMLReader();
    reader.setEntityResolver(new CustomResolver());
    reader.setErrorHandler(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 lead to injection attacks.

Related Guidelines

Search for other vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

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