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A 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 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 sanitizing and validating untrusted input and parameterizing queries sanitization and validation, which are typically implemented as parameterized queries and stored procedures.

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

A SQL command to authenticate a user might take the following form:

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>'

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

Suppose However, if an attacker can substitute arbitrary strings for <USERNAME> and <PASSWORD>, they can perform a SQL injection by using the following string for <USERNAME>. In that case, the authentication mechanism can be bypassed by supplying the following <USERNAME> with an arbitrary password:

validuser' OR '1'='1

When injected into the command, the command becomes

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 selects 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.

LikewiseSimilarly, an attacker could supply a the following string for <PASSWORD> such as with an arbitrary username:

' OR '1'='1

This would yield producing the following commandquery:

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

This time, the '1'='1' tautology disables both user name and password validation, and the attacker is falsely logged in without a correct login ID  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 because the SQL statement sqlString accepts unsanitized input arguments. The attack scenario outlined previously would work as describedby 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)

Fortunately, the 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 is an example of component-based sanitization.This compliant solution 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(
    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;      
      PreparedStatement stmt = connection.prepareStatement(sqlString);

      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
      }
    }
  }
}

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);

      // EnsureValidate that theusername length of user name is legitimate
      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 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 MS 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 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"

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