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Wednesday, April 7, 2010

How SQL Injection works

The primary form of SQL injection consists of direct insertion of code into user-input variables that are concatenated with SQL commands and executed.
A less direct attack injects malicious code into strings that are destined for storage in a table or as metadata.
When the stored strings are subsequently concatenated into a dynamic SQL command, the malicious code is executed.The injection process works by terminating a text string and appending a new command. Because the inserted command may have additional strings appended to it before it is executed, the crook terminates the injected string with a comment mark "--". Subsequent text is ignored at execution time.
Example:-

SELECT * FROM Shipments where tracking_id = '@tracking'

Where @tracking is a variable passed . Under normal circumstances, this may function perfectly normal. For example, if a user enters the tracking number 1A2123ZC2, the corresponding query would be:

SELECT * FROM Shipments where tracking_id = '1A2123ZC2'

Our assumption is that the user will only enter a valid tracking number. Malicious individuals are not likely to be so cooperative. Suppose that the user instead enters the string shown below in the tracking number field:

1A2123ZC2' or true

The corresponding query will now be:

SELECT *
FROM Shipments WHERE tracking_id = '1A2123ZC2' OR TRUE

This will have the unintended consequence of retrieving all of the tracking information stored in the database. If the user enters the following string:

'1A2123ZC2'; DELETE FROM Shipments

This would cause the database to execute the following query:

SELECT *
FROM Shipments WHERE tracking_id = '1A2123ZC2';
DELETE FROM Shipments

This will have the clearly undesirable result of deleting all of the tracking information from the database.

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