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XPath injection occurs when an XML document is used for data storage in a manner similar to a relational database. This attack is similar to SQL injection or XML injection (see the appropriate parts of IDS00-J. Sanitize untrusted data passed across a trust boundary) wherein an attacker can enter valid SQL or XML constructs in the data fields of the query in use. Typically, the conditional field of the query resolves to a tautology or gives the attacker access to privileged information. This guideline is a specific example of the broadly scoped IDS52-J. Prevent code injection.

XML Path Injection Example

Consider the following XML schema.

<users>
  <user>
    <login>Utah</login>
    <password>C^f3</password>
  </user>
  <user>
    <login>Bohdi</login>
    <password>C@fe</password>
  </user>
  <user>
    <login>Busey</login>
    <password>cAf3</password>
  </user>
</users>

Untrusted code may attempt to retrieve user details from this file with an XPath statement constructed dynamically from user input.

//users/user[LoginID/text()='&LOGIN&' and password/text()='&PASSWORD&' ]

If an attacker knows that Utah is a valid login ID, he or she can specify an input login ID such as:

Utah' or '1'='1

This would yield the following query string:

//users/user[login/text()='Utah' or '1'='1' and password/text()='xxxx' ]

Because the '1'='1' is automatically true, the password is never validated. Consequently, the attacker is falsely logged in as user Utah without having to know the password.

To comply with MSC51-J. Store passwords using a hash function, the passwords would have to be encrypted. Unfortunately, on many small systems, they are not, so the password text added in the query string would match precisely what the user enters. An attacker could supply a password such as:

' or '1'='1

This would yield the following query string:

//users/user[password/text()='xxxx' and login/text()='' or '1'='1' ]

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

Noncompliant Code Example

In this noncompliant code example, a user name and password are read from the user and used to construct the query string. The password is passed as a char array, and then hashed, all to comply with MSC51-J. Store passwords using a hash function and MSC56-J. Limit the lifetime of sensitive data.

If passed the attack string for login described previously, the evaluate() method call returns the corresponding login node in the XML file, causing the login() method to return true and bypass any authorization.

private boolean doLogin(String loginID, char[] password)
  throws ParserConfigurationException, SAXException, IOException, XPathExpressionException {

  DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
  domFactory.setNamespaceAware(true);
  DocumentBuilder builder = domFactory.newDocumentBuilder();
  Document doc = builder.parse("users.xml");
  String pwd = hashPassword( password);

  XPathFactory factory = XPathFactory.newInstance();
  XPath xpath = factory.newXPath();
  XPathExpression expr = xpath.compile("//users/user[login/text()='" +
       loginID + "' and password/text()='" + pwd + "' ]");
  Object result = expr.evaluate(doc, XPathConstants.NODESET);
  NodeList nodes = (NodeList) result;

  // Print first names to the console
  for (int i = 0; i < nodes.getLength(); i++) {
    Node node = nodes.item(i).getChildNodes().item(1).getChildNodes().item(0);
    System.out.println( "Authenticated: " + node.getNodeValue());
  }

  return (nodes.getLength() >= 1);
}

Compliant Solution (XQuery)

XPath injection can be prevented by adopting defenses similar to those used to prevent SQL injection:

• Treat all user input as untrusted and perform appropriate sanitization.
• When sanitizing user input, verify the correctness of the data type, length, format, and content. For example, use a regular expression that checks for XML tags and special characters in user input. This corresponds to input sanitization. See IDS52-J. Prevent code injection for additional details.
• In a client-server application, perform validation at both the client and the server side.
• Extensively test applications that supply, propagate, or use user input.

An effective technique for preventing the related issue of SQL injection is parameterization, whereby user-specified data is passed to an API as a parameter, thus ensuring that user-specified data is never interpreted as executable logic. Unfortunately, Java SE currently lacks an analogous interface for XPath queries. SQL parameterization can be emulated by using an interface (such as XQuery) that supports specifying a query statement in a separate file that is supplied at runtime.

Input File: login.qry

declare variable $loginID as xs:string external;
declare variable $password as xs:string external;
//users/user[@loginID=$loginID and @password=$password]

This compliant solution uses a query specified in a text file by reading the file in the required format and then entering values for the user name and password in a Map. The XQuery library constructs the XML query from these elements.

private boolean doLogin(String loginID, String pwd)
  throws ParserConfigurationException, SAXException, IOException, XPathExpressionException {

  DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
  domFactory.setNamespaceAware(true);
  DocumentBuilder builder = domFactory.newDocumentBuilder();
  Document doc = builder.parse("users.xml");

  XQuery xquery = new XQueryFactory().createXQuery(new File("login.xry"));
  Map queryVars = new HashMap();
  queryVars.put("loginid", loginID);
  queryVars.put("password", pwd);
  NodeList nodes = xquery.execute(doc, null, queryVars).toNodes();

  // Print first names to the console
  for (int i = 0; i < nodes.getLength(); i++) {
    Node node = nodes.item(i).getChildNodes().item(1).getChildNodes().item(0);
    System.out.println( node.getNodeValue());
  }

  return (nodes.getLength() >= 1);
}

Using this method, the data specified in the loginID and password fields cannot be interpreted as executable content at runtime.

In addition, according to OWASP [OWASP 2005],

[Prevention of XPath injection] requires the following characters to be removed (ie, prohibited) or properly escaped:

• < > / ' = " to prevent straight parameter injection
• XPath queries should not contain any meta characters (such as ' = * ? // or similar)
• XSLT expansions should not contain any user input, or if they do, [you should] comprehensively test the existence of the file, and ensure that the files are within the bounds set by the Java 2 Security Policy.

Risk Assessment

Failure to validate user input may result in information disclosure and execution of unprivileged code.

Related Vulnerabilities

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

Bibliography

[Fortify 2008] "Input Validation and Representation: XML Injection"
[MITRE 2009] CWE ID 643 "Failure to Sanitize Data within XPath Expressions (aka 'XPath injection')"
[OWASP 2005] Testing for XPath Injection
[Sen 2007]
[Sun 2006] Ensure Data Security