what's the meaning of 'admin' OR 1=1 -- '

Question

The following query return all the passwords in the table tbl_user but I can not understand why this is happening.

SELECT password FROM tbl_users WHERE name = 'admin' OR 1=1 -- '

Please help me to understand this part of the query: 'admin' OR 1=1 -- '

Can you introduce other threats like this (website, book, etc)?

Answer 1

This is a classic SQL injection.

See this fiddle while I explain it: SQLfiddle

In this example, there are 5 users being added to the table. Your query is then run. The expected result would be to return the password value for the admin user only.

However, by adding 1=1, which is a true statement, all passwords are returned.

Another way to help visualize this, is to add parenthesis so that you can see how everything is evaluated.

SELECT pass FROM users WHERE (user_name = 'admin')              OR (1=1) -- '
                                 ^ Pulls only the admin user        ^ Pulls everything because 1=1

So, we are selecting the password from the table where the user name is admin. We are also pulling the password from the table where ever 1=1 - which is always true. Each row is evaluated to true, thus all passwords are returned.

The final -- ' is used to comment out the rest of your query.

SELECT pass from users WHERE user_name = 'admin' or (1=1) -- 'and permission='superadmin'

Normally, (if the 1=1 hadn't been injected), you'd pull the password for the user with user_name of admin and superadmin permissions. You've now commented that out, and it isn't executed. This is how the entire table of passwords can be returned.