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CHAPTER 5 STATEMENT AND PREPAREDSTATEMENT
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String verifyStmtString = "select count(*) from user_info " + "where username = "+ " and password = "; System.out.println("verify statement: " + verifyStmtString ); // prepare the statement pstmt = conn.prepareStatement( verifyStmtString ); // bind the values pstmt.setString(1, username ); pstmt.setString(2, password ); // execute the statement rset = pstmt.executeQuery(); while( rset.next() ) { int count = rset.getInt(1); if( count == 0 ) System.out.println("Invalid username and password - access denied!"); else System.out.println("Congratulations! You have been " + "authenticated successfully!"); } } finally { // release JDBC related resources in the finally clause. JDBCUtil.close( rset ); JDBCUtil.close( pstmt ); } } The program ends after defining the _validateProgramInputs() method: // check command-line parameters. private static void _validateProgramInputs( String[] args ) { if( args.length != 3 ) { System.out.println(" Usage: java <program_name> " + "<bind|nobind> <username> <password>"); System.exit(1); } if( !( NO_BIND.equals( args[0] ) || BIND.equals( args[0] ) ) ) { System.out.println(" Usage: java <program_name> " + "<bind|nobind> <username> <password>"); System.exit(1); } }
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private static final String NO_BIND= "nobind"; private static final String BIND= "bind"; } // end of program When we execute the preceding program with the nobind option while giving a valid username and password, it works fine: B:\code\book\ch05>java DemoSQLInjection nobind user1 password1 URL:jdbc:oracle:thin:@(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(PORT=1521)(HOST=rmeno n-lap))(CONNECT_DATA=(SID=ora10g))) verify statement: select count(*) from user_info where username = 'user1' and pa ssword = 'password1' Congratulations! You have been authenticated successfully! If we use the same option, but give a wrong username password combination, we are denied access, as expected: B:\>java DemoSQLInjection nobind user1 password2 URL:jdbc:oracle:thin:@(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp) (PORT=1521)(HOST=rmenon-lap))(CONNECT_DATA=(SID=ora10g))) verify statement: select count(*) from user_info where username = 'user1' and pa ssword = 'password2' Invalid username and password - access denied!! So far, the program looks rock-solid even if we don t use bind variables. Unfortunately, that is not really the case. Consider the following invocation with the option of nobind: B:\> java DemoSQLInjection nobind invalid_user "junk_password' or 'x'='x" URL:jdbc:oracle:thin:@(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp) (PORT=1521)(HOST=rmenon-lap))(CONNECT_DATA=(SID=ora10g))) verify statement: select count(*) from user_info where username = 'invalid_user' and password = 'junk_password' or 'x'='x' Congratulations! You have been authenticated successfully! Even though an invalid username and password was given, the authentication was successful. What happened A careful examination reveals that the input was engineered in such a way that the where clause of the query had the criterion " or 'x' = 'x'" appended to the end. And since this last criterion is always true, the executing select statement will always return a nonzero count, resulting in a successful authentication. Let s see what happens if we use the same input parameters, but choose the bind option this time: B:\code\book\ch05>java DemoSQLInjection bind invalid_user "junk_password' or 'x' ='x" URL:jdbc:oracle:thin:@(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp) (PORT=1521)(HOST=localhost))(CONNECT_DATA=(SID=ora10g))) verify statement: select count(*) from user_info where username = and password = Invalid username and password - access denied!
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The hacker would be disappointed in this case. When we use bind variables, the query itself remains the same, since we use in place of actual parameter values. Hence, it does not matter what the input parameter values are the program will work as expected. The SQL injection attack has caused a lot of grief at many websites that use relational databases. Note that the SQL injection attack is not specific to the Oracle database. Much has been written on this topic, as a simple search on Google will reveal. In Oracle, using bind variables can protect applications from this dangerous attack most of the time. Now that you have seen how to use bind variables in the PreparedStatement and all the benefits of using bind variables, let s move on to look at some nuances related to bind variable usage.
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As you know by now, a bind variable is a parameter used in a SQL statement, the value of which is bound at runtime. So, for example, you could have the following statement, in which the values to be inserted into the emp table are bound at runtime: PreparedStatement pstmt = conn.prepareStatement( "insert into emp values ( , , )"); However, can you run the following statement, in which the table name is bound at runtime PreparedStatement pstmt = conn.prepareStatement( "insert into values ( , , )"); The answer is no. If you try to run such code, you will get the exception java.sql.SQLException: ORA-00903: invalid table name. Recall that the concept of bind variables exists so that Oracle can reuse the generated execution plans for a statement by substituting placeholders with literal values. Also, the parsing and query plan generation of a statement occur before the bind variables are evaluated. In the preceding case, for example, the parsing cannot be done because the optimizer needs to know the table name to generate a plan, to carry out the semantic checks (e.g., whether the user has the privilege to insert into the table), and so on. In other words, the optimizer does not have enough information to generate a query plan. A simple test to find out if something can be used as a bind variable is to ask, Can I substitute a literal value (a string, an integer whatever is appropriate) in its place and have SQL*Plus run it legally If the answer is yes, then you can use a bind variable there; otherwise, you cannot. Table 5-2 gives some examples (with explanations) of correct and incorrect uses of as a bind variable placeholder.
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