SQL injection vulnerabilities arise in applications where elements of a SQL query originate from an untrusted source. Without precautions, the untrusted data may maliciously alter the query, resulting in information leaks or data modification. The primary means of preventing SQL injection are sanitization and validation, which are typically implemented as parameterized queries and stored procedures.

Suppose a system authenticates users by issuing the following query to a SQL database. If the query returns any results, authentication succeeds; otherwise, authentication fails.

SELECT * FROM db_user WHERE username='<USERNAME>' AND 
                            password='<PASSWORD>'

Suppose an attacker can substitute arbitrary strings for <USERNAME> and <PASSWORD>. In that case, the authentication mechanism can be bypassed by supplying the following <USERNAME> with an arbitrary password:

validuser' OR '1'='1

The authentication routine dynamically constructs the following query:

SELECT * FROM db_user WHERE username='validuser' OR '1'='1' AND password='<PASSWORD>'

If validuser is a valid user name, this SELECT statement yields the validuser record in the table. The password is never checked because username='validuser' is true; consequently, the items after the OR are not tested. As long as the components after the OR generate a syntactically correct SQL expression, the attacker is granted the access of validuser.

Similarly, an attacker could supply the following string for <PASSWORD> with an arbitrary username:

' OR '1'='1

producing the following query:

SELECT * FROM db_user WHERE username='<USERNAME>' AND password='' OR '1'='1'

'1'='1' always evaluates to true, causing the query to yield every row in the database. In this scenario, the attacker would be authenticated without needing a valid username 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.

Unfortunately, this code example permits a SQL injection attack by incorporating the unsanitized input argument username into the SQL command, allowing an attacker to inject validuser' OR '1'='1. The password argument cannot be used to attack this program because it is passed to the hashPassword() function, which also sanitizes the input.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

class Login {
  public Connection getConnection() throws SQLException {
    DriverManager.registerDriver(new
            com.microsoft.sqlserver.jdbc.SQLServerDriver());
    String dbConnection = 
      PropertyManager.getProperty("db.connection");
    // Can hold some value like
    // "jdbc:microsoft:sqlserver://<HOST>:1433,<UID>,<PWD>"
    return DriverManager.getConnection(dbConnection);
  }

  String hashPassword(char[] password) {
    // Create hash of password
  }

  public void doPrivilegedAction(String username, char[] password)
                                 throws SQLException {
    Connection connection = getConnection();
    if (connection == null) {
      // Handle error
    }
    try {
      String pwd = hashPassword(password);

      String sqlString = "SELECT * FROM db_user WHERE username = '" 
                         + username +
                         "' AND password = '" + pwd + "'";
      Statement stmt = connection.createStatement();
      ResultSet rs = stmt.executeQuery(sqlString);

      if (!rs.next()) {
        throw new SecurityException(
          "User name or password incorrect"
        );
      }

      // Authenticated; proceed
    } finally {
      try {
        connection.close();
      } catch (SQLException x) {
        // Forward to handler
      }
    }
  }
}

Noncompliant Code Example (PreparedStatement)

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 correctly. This code example modifies the doPrivilegedAction() method to use a PreparedStatement instead of java.sql.Statement. However, the prepared statement still permits a SQL injection attack by incorporating the unsanitized input argument username into the prepared statement.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

class Login {
  public Connection getConnection() throws SQLException {
    DriverManager.registerDriver(new
            com.microsoft.sqlserver.jdbc.SQLServerDriver());
    String dbConnection = 
      PropertyManager.getProperty("db.connection");
    // Can hold some value like
    // "jdbc:microsoft:sqlserver://<HOST>:1433,<UID>,<PWD>"
    return DriverManager.getConnection(dbConnection);
  }

  String hashPassword(char[] password) {
    // Create hash of password
  }

  public void doPrivilegedAction(
    

Many programs accept data from untrusted sources, and then pass the (modified or unmodified) data to some subsystem. Often the data is in the form of a string with some internal syntactic structure, and the subsystem must parse the string.

Such data must be sanitized, both because the subsystem may be unprepared to handle the malformed input, and also because unsanitized input may include an injection attack.

As a result, it is necessary to sanitize all string data passed to parsers or command interpreters so that the resulting string is innocuous in the context in which it will be parsed or interpreted.

Sanitization Techniques

Blacklisting

Blacklisting is the process of examining input data, looking for components that are known to be invalid. One advantage of this approach is that detection of known invalid input is often straightforward. A disadvantage is that the set of all possible invalid inputs may be unknown, or too large to enumerate fully.

Depending on the language and subsystem in question, certain characters and character sequences are frequently considered to be invalid input when encountered in strings. A common set of such characters includes:

A blacklist of invalid inputs would forbid the appearance of any of these characters in their raw form. Note that determination of what constitutes invalid input can be difficult. For example, input validation of textual data using a black-listing approach requires enumerating not only the invalid characters shown above, but also the alternate Unicode representations of these characters in differing locales.

Whitelisting

The whitelisting approach to input validation consists of building a list of valid input elements (such as characters) and ensuring that all untrusted input elements appear on that list. Whitelisting is easier than blacklisting when it is easier to enumerate valid input elements than to detect and reject all instances of invalid input elements. But this advantage over blacklisting fails to apply when the set of valid input elements is difficult or impossible to enumerate and creating a subset of valid input elements is not a viable solution.

Component-based Sanitization

Many parsers and command interpreters provide their own sanitization and validation APIs. When available, their use is preferred over homegrown sanitization techniques, as homegrown sanitization can often neglect special cases or hidden complexities in the parser.

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 validating and sanitizing user input, and parameterizing the query.

Suppose a database contains usernames and passwords to allow users to authenticate to the system. The usernames have a string size limit of 8. The passwords have a size limit of 20.

An SQL command to authenticate a user would take the form:

SELECT * FROM db_user WHERE username=<USERNAME> AND password=<PASSWORD>

If it returns any records, the username and password are valid.

However, if an attacker can substitute any strings for <USERNAME> and <PASSWORD>, they can perform an SQL injection by using the following string for <USERNAME>:

validuser' OR '1'='1

When injected into the command, the command becomes:

SELECT * FROM db_user WHERE username='validuser' OR '1'='1' AND password=<PASSWORD>

If validuser is actually a valid user name, this will select the validuser record in the table. The hashed password will never be checked because the expression '1'='1' is always true. Consequently the attacker is granted the access of validuser.

In order to comply with guideline MSC05-J. Store passwords using a hash function, the passwords would have to be encrypted. Unfortunately, on many small systems, they are not, and so the password text added in the query string would match precisely what the user enters. An attacker could supply a string for <PASSWORD> such as:

' or '1'='1

This would yield the following command:

SELECT * FROM db_user WHERE username='anything' AND password='' OR '1'='1'

This time, the '1'='1' tautology disables both username 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, all to comply with MSC05-J. Store passwords using a hash function and MSC10-J. Limit the lifetime of sensitive data.

Unfortunately, this code example permits an SQL injection attack, because the SQL statement sqlString accepts unsanitized input arguments. The attack scenario outlined above would work as described.

class Login {
  public Connection getConnection() throws SQLException {
    DriverManager.registerDriver(new com.microsoft.jdbc.sqlserver.SQLServerDriver());
    String dbConnection = PropertyManager.getProperrty("db.connection");
    // can hold some value like "jdbc:microsoft:sqlserver://<HOST>:1433,<UID>,<PWD>"
    return DriverManager.getConnection( dbConnection);
  }

  String hashPassword(char[] password) {
    // create hash of password
  }

  public void doPrivilegedAction(String username, char[] password) throws SQLException {
    Connection connection = getConnection(); 
    if (connection == null) {
      // handle error
    }
    String pwd = hashPassword( password);

    String sqlString = "SELECT * FROM db_user WHERE username = '" + username + 
                       "' AND password = '" + pwd + "'";
    Statement stmt = connection.createStatement();
    ResultSet rs = stmt.executeQuery( sqlString);
     
    if (!rs.next()) {
    	 throw new SecurityException("User name or Password incorrect");
    } 

    // Authenticated; proceed
  } 
}

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.

This compliant solution uses a PreparedStatement instead of java.sql.Statement. This code also validates the length of the username argument, preventing an attacker from submitting an arbitrarily long username.

class Login {
  public void doPrivilegedAction(String username, char[] password
  ) throws SQLException {
    Connection connection = getConnection(); 
    if (connection == null) {
      // handleHandle error
    }
    try {
      String pwd = hashPassword( password);
   
   String // Ensure that the length of username and password is legitimate  
sqlString = "select * from db_user where username=" + 
        if ((username.length() >= 8) {
      // Handle error
 + " and password =" + pwd;      }
	    
  PreparedStatement stmt String= connection.prepareStatement(sqlString);

 = "select * from db_user where username=? and password=?";
    PreparedStatement stmt = connection.prepareStatement(sqlStringResultSet rs = stmt.executeQuery();
    stmt.setString(1, username);
    stmt.setString(2, pwd);
    ResultSet rs = stmt.executeQuery();
      if (!rs.next()) {
        throw new SecurityException("User name or Passwordpassword incorrect");
      } 

      // Authenticated; proceed
    } finally {
}

Use PreparedStatement's set*() methods to enforce strong type checking. This mitigates the SQL injection vulnerability because the input is properly escaped by automatic entrapment within double quotes. Note that Prepared Statements must be used even with queries that insert data into the database.

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.

When a user has the ability to add structured XML as input, he can override the contents of an XML document by injecting XML tags in data fields. These tags are interpreted and classified by an XML parser as executable content and as a result, may cause certain data members to be unintentionally overridden.

Consider the following XML code snippet from an online store application, designed primarily to query a backend database. The user has the ability to specify the quantity of an item, available for purchase.

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

A malicious user inputs the following string instead of a simple number in the quantity field.

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

Consequently, 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 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. Since the method performs no validation of the input, XML injection becomes possible.

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 application, different methods may be used to sanitize the user-specified data. 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) {
  // Write XML string if quantity contains numbers only (white-listing)
  // Black-listing 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();
}

XML External Entity Attacks

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.

According to XML W3C Recommendation \[[W3C 2008|AA. Bibliography#W3C 08]\], 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.

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 severely affects the behavior of the program by crashing or blocking it 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 try 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.

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());
  }
}

Unfortunately, this program is subject to a remote XXE attack. Suppose that 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 guideline EXC06-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.

This is an example of component-based sanitization.

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.

      try {
        connection.close();
      } catch (SQLException x) {
        // Forward to handler
      }
    }
  }
}

Compliant Solution (PreparedStatement)

This compliant solution uses a parametric query with a ? character as a placeholder for the argument. This code also validates the length of the username argument, preventing an attacker from submitting an arbitrarily long user name.

  public void doPrivilegedAction(
    String username, char[] password
  ) throws SQLException {
    Connection connection = getConnection();
    if (connection == null) {
      // Handle error
    }
    try {
      String pwd = hashPassword(password);

      // Validate username length
      if (username.length() > 8) {
        // Handle error
      }

      String sqlString = 
        "select * from db_user where username=? and password=?";
      PreparedStatement stmt = connection.prepareStatement(sqlString);
      stmt.setString(1, username);
      stmt.setString(2, pwd);
      ResultSet rs = stmt.executeQuery();
      if (!rs.next()) {
        throw new SecurityException("User name or password incorrect");
      }

      // Authenticated; proceed
    } finally {
      try {
        connection.close();
      } catch (SQLException x) {
        // Forward to handler
      }
    }
  }

Use the set*() methods of the PreparedStatement class to enforce strong type checking. This technique mitigates the SQL injection vulnerability because the input is properly escaped by automatic entrapment within double quotes. Note that prepared statements must be used even with queries that insert data into the database.

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 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 Microsoft 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

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

...

Image Added Image Added Image Added

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

Bibliography

\[[OWASP 2008|AA. Bibliography#OWASP 08]\] [Testing for XML Injection (OWASP-DV-008)|http://www.owasp.org/index.php/Testing_for_XML_Injection_%28OWASP-DV-008%29]

\[[OWASP 2005|AA. Bibliography#OWASP 05]\] 
\[[OWASP 2007|AA. Bibliography#OWASP 07]\]
\[[W3C 2008|AA. Bibliography#W3C 08]\] 4.4.3 Included If Validating

13. Input Validation and Data Sanitization (IDS)      13. Input Validation and Data Sanitization (IDS)      IDS02-J. Normalize strings before validating them