![]() ![]() You do not need to test each and every type supported by the database engine. When doing manual testing, keep in mind that we can split data types in two groups: numeric values and the rest (considered as strings since they are enclosed between quotes). Fortunately, some tools like sqlmap can automate this process. However when the number of columns is large the number of possible combinations exponentially grows. With just 3 columns it is relatively easy to test the different cases (only 9 combinations). No error message is returned and data is listed. SELECT name, description, price FROM products WHERE category= 1 UNION SELECT 'A', 'B', 3 FROM all_tables However, after some tests the correct combination can be determined and the structure of the query is discovered.ġ UNION SELECT 'A', 'B', 3 FROM all_tables In our example, the system uses Oracle which provide none of those "hits" for the attacker. Also, in some cases comprehensive error messages can be returned by the database engine to indicate which column has a data type mismatch. Some DBMS like MySQL and SQL Server are not strict on data types and will allow implicit numeric conversion. The last step is to determine the data type of each column of the original query. Even if this approach is perfectly valid, the first one is more popular. The number of columns in the injected select is increased until the database engine does not return an error related to the number of columns. ![]() The alternative technique to determine the number of columns is to directly inject a new statement with UNION. We can now conclude that the original query has 3 columns. ORA-01785: ORDER BY item must be the number of a SELECT-list expression. SELECT name, description, price FROM products WHERE id= 1 ORDER BY 4 Query generated (selects only 3 columns). Let’s now see how to get rid of a table name error with an example (this list of system tables was used): Notice that at this step, it is not even necessary to specify column names since a minimal SELECT statement can be used. Even if database systems have different naming convention, the number of popular DBMS is really limited and a valid system table name can be found quickly. Guessing may be an option to find a table name that exists in the database (a good one in some cases), but let’s consider an approach that will guarantee successful results even if luck is not on your side. The best way to find such information is to use system tables instead of user tables. For more information refer to the last section of the article. However, the same principle would apply if it was not the case. To simplify learning, this article explains how it can be done when error reporting is enabled. ![]() To do this, a valid table name must be known but it is also necessary to determine the number of columns in the first query and their data type. Because the UNION operator can only be used if both queries have the exact same structure, the attacker must craft a SELECT statement similar to the original query. ![]() UNION-based attacks allow the tester to easily extract information from the database. Understanding how to create a valid UNION-based attack to extract information ![]()
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