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11g Release 1 (11.1)

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7 Using Dynamic SQL

Dynamic SQL is a programming methodology for generating and executing SQL statements at run time. It is useful when writing general-purpose and flexible programs like ad hoc query systems, when writing programs that must execute DDL statements, or when you do not know at compilation time the full text of a SQL statement or the number or data types of its input and output variables.

PL/SQL provides two ways to write dynamic SQL:

Native dynamic SQL code is easier to read and write than equivalent code that uses the DBMS_SQL package, and runs noticeably faster (especially when it can be optimized by the compiler). However, to write native dynamic SQL code, you must know at compile time the number and data types of the input and output variables of the dynamic SQL statement. If you do not know this information at compile time, you must use the DBMS_SQL package.

When you need both the DBMS_SQL package and native dynamic SQL, you can switch between them, using the DBMS_SQL.TO_REFCURSOR Function and DBMS_SQL.TO_CURSOR_NUMBER Function.

Topics:

When You Need Dynamic SQL

In PL/SQL, you need dynamic SQL in order to execute the following:

If you do not need dynamic SQL, use static SQL, which has the following advantages:

For information about using static SQL statements with PL/SQL, see Chapter 6, "Using Static SQL."

Using Native Dynamic SQL

Native dynamic SQL processes most dynamic SQL statements by means of the EXECUTE IMMEDIATE statement.

If the dynamic SQL statement is a SELECT statement that returns multiple rows, native dynamic SQL gives you the following choices:

The SQL cursor attributes work the same way after native dynamic SQL INSERT, UPDATE, DELETE, and single-row SELECT statements as they do for their static SQL counterparts. For more information about SQL cursor attributes, see Managing Cursors in PL/SQL.

Topics:

Using the EXECUTE IMMEDIATE Statement

The EXECUTE IMMEDIATE statement is the means by which native dynamic SQL processes most dynamic SQL statements.

If the dynamic SQL statement is self-contained (that is, if it has no placeholders for bind arguments and the only result that it can possibly return is an error), then the EXECUTE IMMEDIATE statement needs no clauses.

If the dynamic SQL statement includes placeholders for bind arguments, each placeholder must have a corresponding bind argument in the appropriate clause of the EXECUTE IMMEDIATE statement, as follows:

  • If the dynamic SQL statement is a SELECT statement that can return at most one row, put out-bind arguments (defines) in the INTO clause and in-bind arguments in the USING clause.

  • If the dynamic SQL statement is a SELECT statement that can return multiple rows, put out-bind arguments (defines) in the BULK COLLECT INTO clause and in-bind arguments in the USING clause.

  • If the dynamic SQL statement is a DML statement other than SELECT, without a RETURNING INTO clause, put all bind arguments in the USING clause.

  • If the dynamic SQL statement is a DML statement with a RETURNING INTO clause, put in-bind arguments in the USING clause and out-bind arguments in the RETURNING INTO clause.

  • If the dynamic SQL statement is an anonymous PL/SQL block or a CALL statement, put all bind arguments in the USING clause.

    If the dynamic SQL statement invokes a subprogram, ensure that:

    • Every bind argument that corresponds to a placeholder for a subprogram parameter has the same parameter mode as that subprogram parameter (as in Example 7-1) and a data type that is compatible with that of the subprogram parameter. (For information about compatible data types, see Formal and Actual Subprogram Parameters.)

    • No bind argument has a data type that SQL does not support (such as BOOLEAN in Example 7-2).

The USING clause cannot contain the literal NULL. To work around this restriction, use an uninitialized variable where you want to use NULL, as in Example 7-3.

For syntax details of the EXECUTE IMMEDIATE statement, see EXECUTE IMMEDIATE Statement.

Example 7-1 Invoking a Subprogram from a Dynamic PL/SQL Block

-- Subprogram that dynamic PL/SQL block invokes:
CREATE PROCEDURE create_dept ( deptid IN OUT NUMBER,
                               dname  IN VARCHAR2,
                               mgrid  IN NUMBER,
                               locid  IN NUMBER
                             ) AS
BEGIN
  deptid := departments_seq.NEXTVAL;
  INSERT INTO departments VALUES (deptid, dname, mgrid, locid);
END;
/
DECLARE
  plsql_block VARCHAR2(500);
  new_deptid  NUMBER(4);
  new_dname   VARCHAR2(30) := 'Advertising';
  new_mgrid   NUMBER(6)    := 200;
  new_locid   NUMBER(4)    := 1700;
BEGIN
 -- Dynamic PL/SQL block invokes subprogram:
  plsql_block := 'BEGIN create_dept(:a, :b, :c, :d); END;';

 /* Specify bind arguments in USING clause.
    Specify mode for first parameter.
    Modes of other parameters are correct by default. */
  EXECUTE IMMEDIATE plsql_block
    USING IN OUT new_deptid, new_dname, new_mgrid, new_locid;
END;
/

Example 7-2 Unsupported Data Type in Native Dynamic SQL

DECLARE
  FUNCTION f (x INTEGER)
    RETURN BOOLEAN
  AS
  BEGIN
    ...
  END f;
  dyn_stmt VARCHAR2(200);
  b1       BOOLEAN;
BEGIN
  dyn_stmt := 'BEGIN :b := f(5); END;';
  -- Fails because SQL does not support BOOLEAN data type:
  EXECUTE IMMEDIATE dyn_stmt USING OUT b1;
END;

Example 7-3 Uninitialized Variable for NULL in USING Clause

CREATE TABLE employees_temp AS
  SELECT * FROM EMPLOYEES
/
DECLARE
  a_null  CHAR(1);  -- Set to NULL automatically at run time
BEGIN
  EXECUTE IMMEDIATE 'UPDATE employees_temp SET commission_pct = :x'
    USING a_null;
END;
/

Using the OPEN-FOR, FETCH, and CLOSE Statements

If the dynamic SQL statement represents a SELECT statement that returns multiple rows, you can process it with native dynamic SQL as follows:

  1. Use an OPEN-FOR statement to associate a cursor variable with the dynamic SQL statement. In the USING clause of the OPEN-FOR statement, specify a bind argument for each placeholder in the dynamic SQL statement.

    The USING clause cannot contain the literal NULL. To work around this restriction, use an uninitialized variable where you want to use NULL, as in Example 7-3.

    For syntax details, see OPEN-FOR Statement.

  2. Use the FETCH statement to retrieve result set rows one at a time, several at a time, or all at once.

    For syntax details, see FETCH Statement.

  3. Use the CLOSE statement to close the cursor variable.

    For syntax details, see CLOSE Statement.

Example 7-4 lists all employees who are managers, retrieving result set rows one at a time.

Example 7-4 Native Dynamic SQL with OPEN-FOR, FETCH, and CLOSE Statements

DECLARE
  TYPE EmpCurTyp  IS REF CURSOR;
  v_emp_cursor    EmpCurTyp;
  emp_record      employees%ROWTYPE;
  v_stmt_str      VARCHAR2(200);
  v_e_job         employees.job%TYPE;
BEGIN
  -- Dynamic SQL statement with placeholder:
  v_stmt_str := 'SELECT * FROM employees WHERE job_id = :j';

  -- Open cursor & specify bind argument in USING clause:
  OPEN v_emp_cursor FOR v_stmt_str USING 'MANAGER';

  -- Fetch rows from result set one at a time:
  LOOP
    FETCH v_emp_cursor INTO emp_record;
    EXIT WHEN v_emp_cursor%NOTFOUND;
  END LOOP;

  -- Close cursor:
  CLOSE v_emp_cursor;
END;
/

Repeating Placeholder Names in Dynamic SQL Statements

If you repeat placeholder names in dynamic SQL statements, be aware that the way placeholders are associated with bind arguments depends on the kind of dynamic SQL statement.

Topics:

Dynamic SQL Statement is Not Anonymous Block or CALL Statement

If the dynamic SQL statement does not represent an anonymous PL/SQL block or a CALL statement, repetition of placeholder names is insignificant. Placeholders are associated with bind arguments in the USING clause by position, not by name.

For example, in the following dynamic SQL statement, the repetition of the name :x is insignificant:

sql_stmt := 'INSERT INTO payroll VALUES (:x, :x, :y, :x)';

In the corresponding USING clause, you must supply four bind arguments. They can be different; for example:

EXECUTE IMMEDIATE sql_stmt USING a, b, c, d;

The preceding EXECUTE IMMEDIATE statement executes the following SQL statement:

INSERT INTO payroll VALUES (a, b, c, d)

To associate the same bind argument with each occurrence of :x, you must repeat that bind argument; for example:

EXECUTE IMMEDIATE sql_stmt USING a, a, b, a;

The preceding EXECUTE IMMEDIATE statement executes the following SQL statement:

INSERT INTO payroll VALUES (a, a, b, a)

Dynamic SQL Statement is Anonymous Block or CALL Statement

If the dynamic SQL statement represents an anonymous PL/SQL block or a CALL statement, repetition of placeholder names is significant. Each unique placeholder name must have a corresponding bind argument in the USING clause. If you repeat a placeholder name, you need not repeat its corresponding bind argument. All references to that placeholder name correspond to one bind argument in the USING clause.

In Example 7-5, all references to the first unique placeholder name, :x, are associated with the first bind argument in the USING clause, a, and the second unique placeholder name, :y, is associated with the second bind argument in the USING clause, b.

Example 7-5 Repeated Placeholder Names in Dynamic PL/SQL Block

CREATE PROCEDURE calc_stats (
  w NUMBER,
  x NUMBER,
  y NUMBER,
  z NUMBER )
IS
BEGIN
  DBMS_OUTPUT.PUT_LINE(w + x + y + z);
END;
/
DECLARE
  a NUMBER := 4;
  b NUMBER := 7;
  plsql_block VARCHAR2(100);
BEGIN
  plsql_block := 'BEGIN calc_stats(:x, :x, :y, :x); END;';
  EXECUTE IMMEDIATE plsql_block USING a, b;  -- calc_stats(a, a, b, a)
END;
/

Using DBMS_SQL Package

The DBMS_SQL package defines an entity called a SQL cursor number. Because the SQL cursor number is a PL/SQL integer, you can pass it across call boundaries and store it. You can also use the SQL cursor number to obtain information about the SQL statement that you are executing.

You must use the DBMS_SQL package to execute a dynamic SQL statement when you don't know either of the following until run-time:

In the following situations, you must use native dynamic SQL instead of the DBMS_SQL package:

For information about native dynamic SQL, see Using Native Dynamic SQL.

When you need both the DBMS_SQL package and native dynamic SQL, you can switch between them, using the following:

Note:

You can invoke DBMS_SQL subprograms remotely.

See Also:

Oracle Database PL/SQL Packages and Types Reference for more information about the DBMS_SQL package, including instructions for executing a dynamic SQL statement that has an unknown number of input or output variables ("Method 4")

DBMS_SQL.TO_REFCURSOR Function

The DBMS_SQL.TO_REFCURSOR function converts a SQL cursor number to a weakly-typed variable of the PL/SQL data type REF CURSOR, which you can use in native dynamic SQL statements.

Before passing a SQL cursor number to the DBMS_SQL.TO_REFCURSOR function, you must OPEN, PARSE, and EXECUTE it (otherwise an error occurs).

After you convert a SQL cursor number to a REF CURSOR variable, DBMS_SQL operations can access it only as the REF CURSOR variable, not as the SQL cursor number. For example, using the DBMS_SQL.IS_OPEN function to see if a converted SQL cursor number is still open causes an error.

Example 7-6 uses the DBMS_SQL.TO_REFCURSOR function to switch from the DBMS_SQL package to native dynamic SQL.

Example 7-6 Switching from DBMS_SQL Package to Native Dynamic SQL

CREATE OR REPLACE TYPE vc_array IS TABLE OF VARCHAR2(200);
/
CREATE OR REPLACE TYPE numlist IS TABLE OF NUMBER;
/
CREATE OR REPLACE PROCEDURE do_query_1 (
  placeholder vc_array,
  bindvars vc_array,
  sql_stmt VARCHAR2                    )
IS
  TYPE curtype IS REF CURSOR;
  src_cur      curtype;
  curid        NUMBER;
  bindnames    vc_array;
  empnos       numlist;
  depts        numlist;
  ret          NUMBER;
  isopen       BOOLEAN;
BEGIN
  -- Open SQL cursor number:
  curid := DBMS_SQL.OPEN_CURSOR;

  -- Parse SQL cursor number:
  DBMS_SQL.PARSE(curid, sql_stmt, DBMS_SQL.NATIVE);

  bindnames := placeholder;

  -- Bind arguments:
  FOR i IN 1 .. bindnames.COUNT LOOP
    DBMS_SQL.BIND_VARIABLE(curid, bindnames(i), bindvars(i));
  END LOOP;

  -- Execute SQL cursor number:
  ret := DBMS_SQL.EXECUTE(curid);

  -- Switch from DBMS_SQL to native dynamic SQL:
  src_cur := DBMS_SQL.TO_REFCURSOR(curid);
  FETCH src_cur BULK COLLECT INTO empnos, depts;

  -- This would cause an error because curid was converted to a REF CURSOR:
  -- isopen := DBMS_SQL.IS_OPEN(curid);

  CLOSE src_cur;
END;
/

DBMS_SQL.TO_CURSOR_NUMBER Function

The DBMS_SQL.TO_CURSOR function converts a REF CURSOR variable (either strongly or weakly typed) to a SQL cursor number, which you can pass to DBMS_SQL subprograms.

Before passing a REF CURSOR variable to the DBMS_SQL.TO_CURSOR function, you must OPEN it.

After you convert a REF CURSOR variable to a SQL cursor number, native dynamic SQL operations cannot access it.

After a FETCH operation begins, passing the DBMS_SQL cursor number to the DBMS_SQL.TO_REFCURSOR or DBMS_SQL.TO_CURSOR function causes an error.

Example 7-7 uses the DBMS_SQL.TO_CURSOR function to switch from native dynamic SQL to the DBMS_SQL package.

Example 7-7 Switching from Native Dynamic SQL to DBMS_SQL Package

CREATE OR REPLACE PROCEDURE do_query_2 (sql_stmt VARCHAR2) IS
  TYPE curtype IS REF CURSOR;
  src_cur  curtype;
  curid    NUMBER;
  desctab  DBMS_SQL.DESC_TAB;
  colcnt   NUMBER;
  namevar  VARCHAR2(50);
  numvar   NUMBER;
  datevar  DATE;
  empno    NUMBER := 100;
BEGIN
  -- sql_stmt := SELECT ... FROM employees WHERE employee_id = :b1';

  -- Open REF CURSOR variable:
  OPEN src_cur FOR sql_stmt USING empno;

  -- Switch from native dynamic SQL to DBMS_SQL package:
  curid := DBMS_SQL.TO_CURSOR_NUMBER(src_cur);
  DBMS_SQL.DESCRIBE_COLUMNS(curid, colcnt, desctab);

  -- Define columns:
  FOR i IN 1 .. colcnt LOOP
    IF desctab(i).col_type = 2 THEN
      DBMS_SQL.DEFINE_COLUMN(curid, i, numvar);
    ELSIF desctab(i).col_type = 12 THEN
      DBMS_SQL.DEFINE_COLUMN(curid, i, datevar);
      -- statements
    ELSE
      DBMS_SQL.DEFINE_COLUMN(curid, i, namevar, 50);
    END IF;
  END LOOP;

  -- Fetch rows with DBMS_SQL package:
  WHILE DBMS_SQL.FETCH_ROWS(curid) > 0 LOOP
    FOR i IN 1 .. colcnt LOOP
      IF (desctab(i).col_type = 1) THEN
        DBMS_SQL.COLUMN_VALUE(curid, i, namevar);
      ELSIF (desctab(i).col_type = 2) THEN
        DBMS_SQL.COLUMN_VALUE(curid, i, numvar);
      ELSIF (desctab(i).col_type = 12) THEN
        DBMS_SQL.COLUMN_VALUE(curid, i, datevar);
        -- statements
      END IF;
    END LOOP;
  END LOOP;

  DBMS_SQL.CLOSE_CURSOR(curid);
END;
/

Avoiding SQL Injection in PL/SQL

SQL injection is a technique for maliciously exploiting applications that use client-supplied data in SQL statements, thereby gaining unauthorized access to a database in order to view or manipulate restricted data. This section describes SQL injection vulnerabilities in PL/SQL and explains how to guard against them.

To try the examples in this topic, connect to the HR schema and execute the statements in Example 7-8.

Example 7-8 Setup for SQL Injection Examples

CREATE TABLE secret_records (
  user_name    VARCHAR2(9),
  service_type VARCHAR2(12),
  value        VARCHAR2(30),
  date_created DATE);

INSERT INTO secret_records
  VALUES ('Andy', 'Waiter', 'Serve dinner at Cafe Pete', SYSDATE);

INSERT INTO secret_records
  VALUES ('Chuck', 'Merger', 'Buy company XYZ', SYSDATE);

Topics:

Overview of SQL Injection Techniques

SQL injection techniques differ, but they all exploit a single vulnerability: string input is not correctly validated and is concatenated into a dynamic SQL statement. This topic classifies SQL injection attacks as follows:

Statement Modification

Statement modification means deliberately altering a dynamic SQL statement so that it executes in a way unintended by the application developer. Typically, the user retrieves unauthorized data by changing the WHERE clause of a SELECT statement or by inserting a UNION ALL clause. The classic example of this technique is bypassing password authentication by making a WHERE clause always TRUE.

The SQL*Plus script in Example 7-9 creates a procedure that is vulnerable to statement modification and then invokes that procedure with and without statement modification. With statement modification, the procedure returns a supposedly secret record.

Example 7-9 Procedure Vulnerable to Statement Modification

SQL> REM Create vulnerable procedure
SQL>
SQL> CREATE OR REPLACE PROCEDURE get_record
  (user_name    IN  VARCHAR2,
   service_type IN  VARCHAR2,
   record       OUT VARCHAR2)
IS
  query VARCHAR2(4000);
BEGIN
  -- Following SELECT statement is vulnerable to modification
  -- because it uses concatenation to build WHERE clause.
  query := 'SELECT value FROM secret_records WHERE user_name='''
           || user_name 
           || ''' AND service_type=''' 
           || service_type 
           || '''';
  DBMS_OUTPUT.PUT_LINE('Query: ' || query);
  EXECUTE IMMEDIATE query INTO record;
  DBMS_OUTPUT.PUT_LINE('Record: ' || record);
END;
/

Procedure created.

SQL> REM Demonstrate procedure without SQL injection
SQL> 
SQL> SET SERVEROUTPUT ON;
SQL>
SQL> DECLARE
  2    record_value VARCHAR2(4000);
  3  BEGIN
  4    get_record('Andy', 'Waiter', record_value);
  5  END;
  6  /
Query: SELECT value FROM secret_records WHERE user_name='Andy' AND
service_type='Waiter'
Record: Serve dinner at Cafe Pete
 
PL/SQL procedure successfully completed.
 
SQL>
SQL> REM Example of statement modification
SQL> 
SQL> DECLARE
  2    record_value VARCHAR2(4000);
  3  BEGIN
  4    get_record(
  5      'Anybody '' OR service_type=''Merger''--',
  6      'Anything',
  7      record_value);
  8  END;
  9  /
Query: SELECT value FROM secret_records WHERE user_name='Anybody ' OR
service_type='Merger'--' AND service_type='Anything'
Record: Buy company XYZ

PL/SQL procedure successfully completed.

SQL>

Statement Injection

Statement injection means that a user appends one or more new SQL statements to a dynamic SQL statement. Anonymous PL/SQL blocks are vulnerable to this technique.

The SQL*Plus script in Example 7-10 creates a procedure that is vulnerable to statement injection and then invokes that procedure with and without statement injection. With statement injection, the procedure deletes the supposedly secret record exposed in Example 7-9.

Example 7-10 Procedure Vulnerable to Statement Injection

SQL> REM Create vulnerable procedure
SQL>
SQL> CREATE OR REPLACE PROCEDURE p
  2    (user_name    IN  VARCHAR2,
  3     service_type IN  VARCHAR2)
  4  IS
  5    block VARCHAR2(4000);
  6  BEGIN
  -- Following block is vulnerable to statement injection
  -- because it is built by concatenation.
  7    block :=
  8      'BEGIN
  9         DBMS_OUTPUT.PUT_LINE(''user_name: ' || user_name || ''');'
 10         || 'DBMS_OUTPUT.PUT_LINE(''service_type: ' || service_type || ''');
 11       END;';
 12
 13    DBMS_OUTPUT.PUT_LINE('Block: ' || block);
 14
 15    EXECUTE IMMEDIATE block;
 16  END;
 17  /

Procedure created.

SQL>

SQL> REM Demonstrate procedure without SQL injection
SQL>
SQL> SET SERVEROUTPUT ON;
SQL>
SQL> BEGIN
  2    p('Andy', 'Waiter');
  3  END;
  4  /
Block: BEGIN
         DBMS_OUTPUT.PUT_LINE('user_name: Andy');
         DBMS_OUTPUT.PUT_LINE('service_type: Waiter');
       END;
user_name: Andy
service_type: Waiter

PL/SQL procedure successfully completed.

SQL> REM Example of statement modification
SQL>
SQL> SELECT * FROM secret_records;

USER_NAME SERVICE_TYPE VALUE
--------- ------------ ------------------------------
Andy      Waiter       Serve dinner at Cafe Pete
Chuck     Merger       Buy company XYZ
 
2 rows selected.

SQL>
SQL> BEGIN
  2    p('Anybody', 'Anything'');
  3      DELETE FROM secret_records WHERE service_type=INITCAP(''Merger');
  4  END;
  5  /
Block: BEGIN
       DBMS_OUTPUT.PUT_LINE('user_name: Anybody');
       DBMS_OUTPUT.PUT_LINE('service_type: Anything');
       DELETE FROM secret_records WHERE service_type=INITCAP('Merger');
     END;
user_name: Anybody
service_type: Anything

PL/SQL procedure successfully completed.

SQL> SELECT * FROM secret_records;

USER_NAME SERVICE_TYPE VALUE
--------- ------------ ------------------------------
Andy      Waiter       Serve dinner at Cafe Pete
 
1 row selected.

SQL>

Data Type Conversion

A less known SQL injection technique uses NLS session parameters to modify or inject SQL statements.

A datetime or numeric value that is concatenated into the text of a dynamic SQL statement must be converted to the VARCHAR2 data type. The conversion can be either implicit (when the value is an operand of the concatentation operator) or explicit (when the value is the argument of the TO_CHAR function). This data type conversion depends on the NLS settings of the database session that executes the dynamic SQL statement. The conversion of datetime values uses format models specified in the parameters NLS_DATE_FORMAT, NLS_TIMESTAMP_FORMAT, or NLS_TIMESTAMP_TZ_FORMAT, depending on the particular datetime data type. The conversion of numeric values applies decimal and group separators specified in the parameter NLS_NUMERIC_CHARACTERS.

One datetime format model is "text". The text is copied into the conversion result. For example, if the value of NLS_DATE_FORMAT is '"Month:" Month', then in June, TO_CHAR(SYSDATE) returns 'Month: June'. The datetime format model can be abused as shown in Example 7-11.

Example 7-11 Procedure Vulnerable to SQL Injection Through Data Type Conversion

SQL> REM Create vulnerable procedure
SQL> REM Return records not older than a month
SQL>
SQL> CREATE OR REPLACE PROCEDURE get_recent_record
  (user_name    IN  VARCHAR2,
   service_type IN  VARCHAR2,
   record       OUT VARCHAR2)
IS
  query VARCHAR2(4000);
BEGIN
  -- Following SELECT statement is vulnerable to modification
  -- because it uses concatenation to build WHERE clause
  -- and because SYSDATE depends on the value of NLS_DATE_FORMAT.
  query := 'SELECT value FROM secret_records WHERE user_name='''
           || user_name
           || ''' AND service_type='''
           || service_type
           || ''' AND date_created>'''
           || (SYSDATE - 30)
           || '''';
  DBMS_OUTPUT.PUT_LINE('Query: ' || query);
  EXECUTE IMMEDIATE query INTO record;
  DBMS_OUTPUT.PUT_LINE('Record: ' || record);
END;
/ 
. 
Procedure created.
. 
SQL> REM Demonstrate procedure without SQL injection
SQL>
SQL> SET SERVEROUTPUT ON;
SQL>
SQL> ALTER SESSION SET NLS_DATE_FORMAT='DD-MON-YYYY';
. 
Session altered.
. 
SQL> DECLARE
  2    record_value VARCHAR2(4000);
  3  BEGIN
  4    get_recent_record('Andy', 'Waiter', record_value);
  5  END;
  6  /
Query: SELECT value FROM secret_records WHERE user_name='Andy' AND
service_type='Waiter' AND date_created>'27-MAY-2008'
Record: Serve dinner at Cafe Pete
  
PL/SQL procedure successfully completed.
  
SQL>
SQL> REM Example of statement modification
SQL>
SQL> ALTER SESSION SET NLS_DATE_FORMAT='"'' OR service_type=''Merger"';
. 
Session altered.
. 
SQL> DECLARE
  2    record_value VARCHAR2(4000);
  3  BEGIN
  4    get_recent_record('Anybody', 'Anything', record_value);
  5  END;
  6  /
Query: SELECT value FROM secret_records WHERE user_name='Anybody' AND
service_type='Anything' AND date_created>'' OR service_type='Merger'
Record: Buy company XYZ
. 
PL/SQL procedure successfully completed.
. 
SQL> 

Guarding Against SQL Injection

If you use dynamic SQL in your PL/SQL applications, you must check the input text to ensure that it is exactly what you expected. You can use the following techniques:

Using Bind Arguments to Guard Against SQL Injection

The most effective way to make your PL/SQL code invulnerable to SQL injection attacks is to use bind arguments. The database uses the values of bind arguments exclusively and does not interpret their contents in any way. (Bind arguments also improve performance.)

The procedure in Example 7-12 is invulnerable to SQL injection because it builds the dynamic SQL statement with bind arguments (not by concatenation as in the vulnerable procedure in Example 7-9). The same binding technique fixes the vulnerable procedure shown in Example 7-10.

Example 7-12 Using Bind Arguments to Guard Against SQL Injection

SQL> REM Create invulnerable procedure
SQL> 
SQL> CREATE OR REPLACE PROCEDURE get_record_2
  2    (user_name    IN  VARCHAR2,
  3     service_type IN  VARCHAR2,
  4     record       OUT VARCHAR2)
  5  IS
  6    query VARCHAR2(4000);
  7  BEGIN
  8    query := 'SELECT value FROM secret_records
  9              WHERE user_name=:a
 10              AND service_type=:b';
 11  
 12    DBMS_OUTPUT.PUT_LINE('Query: ' || query);
 13  
 14    EXECUTE IMMEDIATE query INTO record USING user_name, service_type;
 15  
 16    DBMS_OUTPUT.PUT_LINE('Record: ' || record);
 17  END;
 18  /
 
Procedure created.
 
SQL> REM Demonstrate procedure without SQL injection
SQL> 
SQL> SET SERVEROUTPUT ON;
SQL> 
SQL> DECLARE
  2    record_value VARCHAR2(4000);
  3  BEGIN
  4    get_record_2('Andy', 'Waiter', record_value);
  5  END;
  6  /
Query: SELECT value FROM secret_records
            WHERE user_name=:a
            AND service_type=:b
Record: Serve dinner at Cafe Pete
 
PL/SQL procedure successfully completed.
 
SQL> 
SQL> REM Attempt statement modification
SQL> 
SQL> DECLARE
  2    record_value VARCHAR2(4000);
  3  BEGIN
  4    get_record_2('Anybody '' OR service_type=''Merger''--',
  5                 'Anything',
  6                 record_value);
  7  END;
  8  /
Query: SELECT value FROM secret_records
            WHERE user_name=:a
            AND service_type=:b
DECLARE
*
ERROR at line 1:
ORA-01403: no data found
ORA-06512: at "HR.GET_RECORD_2", line 14
ORA-06512: at line 4
 
SQL>

Using Validation Checks to Guard Against SQL Injection

Always have your program validate user input to ensure that it is what is intended. For example, if the user is passing a department number for a DELETE statement, check the validity of this department number by selecting from the departments table. Similarly, if a user enters the name of a table to be deleted, check that this table exists by selecting from the static data dictionary view ALL_TABLES.

Caution:

When checking the validity of a user name and its password, always return the same error regardless of which item is invalid. Otherwise, a malicious user who receives the error message "invalid password" but not "invalid user name" (or the reverse) will realize that he or she has guessed one of these correctly.

In validation-checking code, the subprograms in the package DBMS_ASSERT are often useful. For example, you can use the DBMS_ASSERT.ENQUOTE_LITERAL function to enclose a string literal in quotation marks, as Example 7-13 does. This prevents a malicious user from injecting text between an opening quotation mark and its corresponding closing quotation mark.

Caution:

Although the DBMS_ASSERT subprograms are useful in validation code, they do not replace it. For example, an input string can be a qualified SQL name (verified by DBMS_ASSERT.QUALIFIED_SQL_NAME) and still be a fraudulent password.

See Also:

Oracle Database PL/SQL Packages and Types Reference for information about DBMS_ASSERT subprograms

In Example 7-13, the procedure raise_emp_salary checks the validity of the column name that was passed to it before it updates the employees table, and then the anonymous PL/SQL block invokes the procedure from both a dynamic PL/SQL block and a dynamic SQL statement.

Example 7-13 Using Validation Checks to Guard Against SQL Injection

CREATE OR REPLACE PROCEDURE raise_emp_salary (
  column_value  NUMBER,
  emp_column    VARCHAR2,
  amount NUMBER                              )
IS
  v_column  VARCHAR2(30);
  sql_stmt  VARCHAR2(200);
BEGIN
  -- Check validity of column name that was given as input:
  SELECT COLUMN_NAME INTO v_column
    FROM USER_TAB_COLS
      WHERE TABLE_NAME = 'EMPLOYEES'
        AND COLUMN_NAME = emp_column;
  sql_stmt := 'UPDATE employees SET salary = salary + :1 WHERE '
    || DBMS_ASSERT.ENQUOTE_NAME(v_column,FALSE) || ' = :2';
  EXECUTE IMMEDIATE sql_stmt USING amount, column_value;
  -- If column name is valid:
  IF SQL%ROWCOUNT > 0 THEN
    DBMS_OUTPUT.PUT_LINE('Salaries were updated for: '
      || emp_column || ' = ' || column_value);
  END IF;
  -- If column name is not valid:
  EXCEPTION
    WHEN NO_DATA_FOUND THEN
      DBMS_OUTPUT.PUT_LINE ('Invalid Column: ' || emp_column);
END raise_emp_salary;
/

DECLARE
  plsql_block  VARCHAR2(500);
BEGIN
  -- Invoke raise_emp_salary from a dynamic PL/SQL block:
  plsql_block :=
    'BEGIN raise_emp_salary(:cvalue, :cname, :amt); END;';
  EXECUTE IMMEDIATE plsql_block
    USING 110, 'DEPARTMENT_ID', 10;

  -- Invoke raise_emp_salary from a dynamic SQL statement:
  EXECUTE IMMEDIATE 'BEGIN raise_emp_salary(:cvalue, :cname, :amt); END;'
    USING 112, 'EMPLOYEE_ID', 10;
END;
/

Using Explicit Format Models to Guard Against SQL Injection

If you use datetime and numeric values that are concatenated into the text of a SQL or PL/SQL statement, and you cannot pass them as bind variables, convert them to text using explicit format models that are independent from the values of the NLS parameters of the executing session. Ensure that the converted values have the format of SQL datetime or numeric literals. Using explicit locale-independent format models to construct SQL is recommended not only from a security perspective, but also to ensure that the dynamic SQL statement runs correctly in any globalization environment.

The procedure in Example 7-14 is invulnerable to SQL injection because it converts the datetime parameter value, SYSDATE - 30, to a VARCHAR2 value explicitly, using the TO_CHAR function and a locale-independent format model (not implicitly, as in the vulnerable procedure in Example 7-11).

Example 7-14 Using Explicit Format Models to Guard Against SQL Injection

SQL> REM Create invulnerable procedure 
SQL> REM Return records not older than a month 
SQL> 
SQL> CREATE OR REPLACE PROCEDURE get_recent_record 
  (user_name    IN  VARCHAR2, 
   service_type IN  VARCHAR2, 
   record       OUT VARCHAR2) 
IS 
  query VARCHAR2(4000); 
BEGIN 
  -- Following SELECT statement is vulnerable to modification 
  -- because it uses concatenation to build WHERE clause. 
  query := 'SELECT value FROM secret_records WHERE user_name=''' 
           || user_name 
           || ''' AND service_type=''' 
           || service_type 
           || ''' AND date_created> DATE ''' 
           || TO_CHAR(SYSDATE - 30,'YYYY-MM-DD') 
           || ''''; 
  DBMS_OUTPUT.PUT_LINE('Query: ' || query); 
  EXECUTE IMMEDIATE query INTO record; 
  DBMS_OUTPUT.PUT_LINE('Record: ' || record); 
END; 
/ 
. 
Procedure created. 
. 
SQL> 
SQL> REM Attempt statement modification 
SQL> 
SQL> ALTER SESSION SET NLS_DATE_FORMAT='"'' OR service_type=''Merger"'; 
. 
Session altered. 
. 
SQL> DECLARE 
  2    record_value VARCHAR2(4000); 
  3  BEGIN 
  4    get_recent_record('Anybody', 'Anything', record_value); 
  5  END; 
  6  / 
Query: SELECT value FROM secret_records WHERE user_name='Anybody' AND 
service_type='Anything' AND date_created> DATE '2008-05-27' 
DECLARE 
* 
ERROR at line 1: 
ORA-01403: no data found 
ORA-06512: at "SYS.GET_RECENT_RECORD", line 18 
ORA-06512: at line 4 
. 
SQL>