SQL Injection

SQL injection maliciously exploits applications that use client-supplied data in SQL statements, thereby gaining unauthorized access to a database to view or manipulate restricted data.

This section describes SQL injection vulnerabilities in PL/SQL and explains how to guard against them.

Topics

• SQL Injection Techniques

• Guards Against SQL Injection

Example 8-15 Setup for SQL Injection Examples

To try the examples, run these statements.

Live SQL:

You can view and run this example on Oracle Live SQL at SQL Injection Demo

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

INSERT INTO secret_records (
  user_name, service_type, value, date_created
)
VALUES ('Andy', 'Waiter', 'Serve dinner at Cafe Pete', SYSDATE);
 
INSERT INTO secret_records (
  user_name, service_type, value, date_created
)
VALUES ('Chuck', 'Merger', 'Buy company XYZ', SYSDATE);

SQL Injection Techniques

All SQL injection techniques exploit a single vulnerability: String input is not correctly validated and is concatenated into a dynamic SQL statement.

Topics

• Statement Modification

• Statement Injection

• Data Type Conversion

Statement Modification

Statement modification means deliberately altering a dynamic SQL statement so that it runs 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.

Example 8-16 Procedure Vulnerable to Statement Modification

This example 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.

Live SQL:

You can view and run this example on Oracle Live SQL at SQL Injection Demo

Create vulnerable procedure:

CREATE OR REPLACE PROCEDURE get_record (
  user_name    IN  VARCHAR2,
  service_type IN  VARCHAR2,
  rec          OUT VARCHAR2
) AUTHID DEFINER
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 rec ;
  DBMS_OUTPUT.PUT_LINE('Rec: ' || rec );
END;
/

Demonstrate procedure without SQL injection:

SET SERVEROUTPUT ON;

DECLARE
  record_value VARCHAR2(4000);
BEGIN
  get_record('Andy', 'Waiter', record_value);
END;
/

Result:

Query: SELECT value FROM secret_records WHERE user_name='Andy' AND
service_type='Waiter'
Rec: Serve dinner at Cafe Pete
 

Example of statement modification:

DECLARE
  record_value VARCHAR2(4000);
BEGIN
  get_record(
  'Anybody '' OR service_type=''Merger''--',
  'Anything',
  record_value);
END;
/

Result:

Query: SELECT value FROM secret_records WHERE user_name='Anybody ' OR
service_type='Merger'--' AND service_type='Anything'
Rec: Buy company XYZ

PL/SQL procedure successfully completed.

Statement Injection

Statement injection means that a user appends one or more SQL statements to a dynamic SQL statement.

Anonymous PL/SQL blocks are vulnerable to this technique.

Example 8-17 Procedure Vulnerable to Statement Injection

This example 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 8-16.

Live SQL:

You can view and run this example on Oracle Live SQL at SQL Injection Demo

Create vulnerable procedure:

CREATE OR REPLACE PROCEDURE p (
  user_name    IN  VARCHAR2,
  service_type IN  VARCHAR2
) AUTHID DEFINER
IS
  block1 VARCHAR2(4000);
BEGIN
  -- Following block is vulnerable to statement injection
  -- because it is built by concatenation.
  block1 :=
    'BEGIN
    DBMS_OUTPUT.PUT_LINE(''user_name: ' || user_name || ''');'
    || 'DBMS_OUTPUT.PUT_LINE(''service_type: ' || service_type || ''');
    END;';

  DBMS_OUTPUT.PUT_LINE('Block1: ' || block1);
  
  EXECUTE IMMEDIATE block1;
END;
/

Demonstrate procedure without SQL injection:

SET SERVEROUTPUT ON;

BEGIN
  p('Andy', 'Waiter');
END;
/

Result:

Block1: BEGIN
         DBMS_OUTPUT.PUT_LINE('user_name: Andy');
         DBMS_OUTPUT.PUT_LINE('service_type: Waiter');
       END;
user_name: Andy
service_type: Waiter

SQL*Plus formatting command:

COLUMN date_created FORMAT A12;

Query:

SELECT * FROM secret_records ORDER BY user_name;

Result:

USER_NAME SERVICE_TYPE VALUE                          DATE_CREATED
--------- ------------ ------------------------------ ------------
Andy      Waiter       Serve dinner at Cafe Pete      28-APR-10
Chuck     Merger       Buy company XYZ                28-APR-10

Example of statement modification:

BEGIN
  p('Anybody', 'Anything'');
  DELETE FROM secret_records WHERE service_type=INITCAP(''Merger');
END;
/

Result:

Block1: 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.

Query:

SELECT * FROM secret_records;

Result:

USER_NAME SERVICE_TYPE VALUE                          DATE_CREATED
--------- ------------ ------------------------------ ------------
Andy      Waiter       Serve dinner at Cafe Pete      18-MAR-09
 
1 row selected.

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 concatenation 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 runs 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 8-18.

Example 8-18 Procedure Vulnerable to SQL Injection Through Data Type Conversion

SELECT * FROM secret_records;
 

Result:

USER_NAME SERVICE_TYPE VALUE                          DATE_CREATE
--------- ------------ ------------------------------ -----------
Andy      Waiter       Serve dinner at Cafe Pete      28-APR-2010
Chuck     Merger       Buy company XYZ                28-APR-2010

Create vulnerable procedure:

-- Return records not older than a month

CREATE OR REPLACE PROCEDURE get_recent_record (
  user_name    IN  VARCHAR2,
  service_type IN  VARCHAR2,
  rec          OUT VARCHAR2
) AUTHID DEFINER
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 rec;
  DBMS_OUTPUT.PUT_LINE('Rec: ' || rec);
END;
/

Demonstrate procedure without SQL injection:

SET SERVEROUTPUT ON;
ALTER SESSION SET NLS_DATE_FORMAT='DD-MON-YYYY';

DECLARE
  record_value VARCHAR2(4000);
BEGIN
  get_recent_record('Andy', 'Waiter', record_value);
END;
/

Result:

Query: SELECT value FROM secret_records WHERE user_name='Andy' AND
service_type='Waiter' AND date_created>'29-MAR-2010'
Rec: Serve dinner at Cafe Pete
  

Example of statement modification:

ALTER SESSION SET NLS_DATE_FORMAT='"'' OR service_type=''Merger"';

DECLARE
  record_value VARCHAR2(4000);
BEGIN
  get_recent_record('Anybody', 'Anything', record_value);
END;
/

Result:

Query: SELECT value FROM secret_records WHERE user_name='Anybody' AND
service_type='Anything' AND date_created>'' OR service_type='Merger'
Rec: Buy company XYZ
 
PL/SQL procedure successfully completed.

Guards 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:

• Bind Variables

• Validation Checks

• Explicit Format Models

Bind Variables

The most effective way to make your PL/SQL code invulnerable to SQL injection attacks is to use bind variables.

The database uses the values of bind variables exclusively and does not interpret their contents in any way. (Bind variables also improve performance.)

Example 8-19 Bind Variables Guarding Against SQL Injection

The procedure in this example is invulnerable to SQL injection because it builds the dynamic SQL statement with bind variables (not by concatenation as in the vulnerable procedure in Example 8-16). The same binding technique fixes the vulnerable procedure shown in Example 8-17.

Create invulnerable procedure:

CREATE OR REPLACE PROCEDURE get_record_2 (
  user_name    IN  VARCHAR2,
  service_type IN  VARCHAR2,
  rec          OUT VARCHAR2
) AUTHID DEFINER
IS
  query VARCHAR2(4000);
BEGIN
  query := 'SELECT value FROM secret_records
            WHERE user_name=:a
            AND service_type=:b';
 
  DBMS_OUTPUT.PUT_LINE('Query: ' || query);
 
  EXECUTE IMMEDIATE query INTO rec USING user_name, service_type;
 
  DBMS_OUTPUT.PUT_LINE('Rec: ' || rec);
END;
/
 

Demonstrate procedure without SQL injection:

SET SERVEROUTPUT ON;
DECLARE
  record_value VARCHAR2(4000);
BEGIN
  get_record_2('Andy', 'Waiter', record_value);
END;
/

Result:

Query: SELECT value FROM secret_records
            WHERE user_name=:a
            AND service_type=:b
Rec: Serve dinner at Cafe Pete
 
PL/SQL procedure successfully completed.
 

Try statement modification:

DECLARE
  record_value VARCHAR2(4000);
BEGIN
  get_record_2('Anybody '' OR service_type=''Merger''--',
               'Anything',
               record_value);
END;
/

Result:

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 15
ORA-06512: at line 4

Validation Checks

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) can realize that they have guessed one of these correctly.

In validation-checking code, the subprograms in the DBMS_ASSERT package are often useful. For example, you can use the DBMS_ASSERT.ENQUOTE_LITERAL function to enclose a string literal in quotation marks, as Example 8-20 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

Example 8-20 Validation Checks Guarding Against SQL Injection

In this example, 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 block invokes the procedure from both a dynamic PL/SQL block and a dynamic SQL statement.

CREATE OR REPLACE PROCEDURE raise_emp_salary (
  column_value  NUMBER,
  emp_column    VARCHAR2,
  amount NUMBER ) AUTHID DEFINER
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;
/

Result:

Salaries were updated for: DEPARTMENT_ID = 110
Salaries were updated for: EMPLOYEE_ID = 112

Explicit Format Models

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.

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 running session. Ensure that the converted values have the format of SQL datetime or numeric literals.

Example 8-21 Explicit Format Models Guarding Against SQL Injection

This procedure 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 8-18).

Create invulnerable procedure:

-- Return records not older than a month

CREATE OR REPLACE PROCEDURE get_recent_record (
  user_name     IN  VARCHAR2,
  service_type  IN  VARCHAR2,
  rec           OUT VARCHAR2
) AUTHID DEFINER
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 rec;
  DBMS_OUTPUT.PUT_LINE('Rec: ' || rec);
END;
/

Try statement modification:

ALTER SESSION SET NLS_DATE_FORMAT='"'' OR service_type=''Merger"'; 

DECLARE
  record_value VARCHAR2(4000);
BEGIN
  get_recent_record('Anybody', 'Anything', record_value);
END;
/

Result:

Query: SELECT value FROM secret_records WHERE user_name='Anybody' AND 
service_type='Anything' AND date_created> DATE '2010-03-29' 
DECLARE 
* 
ERROR at line 1: 
ORA-01403: no data found 
ORA-06512: at "SYS.GET_RECENT_RECORD", line 21 
ORA-06512: at line 4