I'm doing some webpentesting exercises and there is this one task saying that I need to make a new account with this name and this password by using SQL injection. On the web page there is a text input username and another text input 'password'. I can enter whatever characters I want inside the username text-input but inside the password text input I need to type a specific injection.
I know I need to inject insert query but the thing is I don't know the table name. In order to insert a new user I need to know the table name so I'm wondering how can I make the web showing me an error with tablename revealed.
The error showing should be like: Unexpected end of command in statement [SELECT * FROM (tablename) WHERE...]
I tried to enter with just one character (maybe >1 chars are required) inside the password field to make the error show or even delete the parameter text input password. But it just won't show the SQL syntax error.
So the question is: How can I make use of the exploit to make the web page view the SQL syntax error with table name revealed?
And why doesn't injection work on username field but it does on password field?
The first thing is understand the difference between "application" errors (for example, "the user doesn't exist") and execution errors, in which the application fails itself, like the "Unexpected end of command" you mention. The first case is not usually a problem from the security point of view (unless the application is very badly programmed), the second one is what could allow hacking.
A well programmed web application should ideally only have application errors, but the more realistic approach is cope with unexpected execution errors in way that don't make it vulnerable. Also, application should process any user input in a way that don't cause an execution error.
It seems that they are teaching you the most flagrant case that allows SQL injection, apps than don't do any processing to user input (so it's very easy put text in an input field that cause an execution error), and that don't cope with execution errors (in this case, showing the internal execution message to the user).
A common mistake in web applications is constructing the SQL query with string concatenation, so the simplest way of make it fail is using the string delimiter (
') in a field, causing the string value to end prematurely. In an sloppy web app it would cause an execution error that shows the full error message, usually including the table name.From there you craft a SQL query in the input field that insert the user in the table, you could find examples online (note that you need at least basic knowledge on SQL and PHP (or ASP, Java, etc.), in order to do SQL injection, since you need to know how the database access works in order to make it fail).
Finally, SQL injection could work in any input field that is not properly processed, but it depends on how the application is programmed. I suppose that both fields would work but it will be easy do that with the password field because probably is the last one in the SQL query.