SQL Injection Attacks by Example

The database isn't readonly

The database isn't readonly

So far, we have done nothing but query the database, and even though a SELECT is readonly, that doesn't mean that SQL is. SQL uses the semicolon for statement termination, and if the input is not sanitized properly, there may be nothing that prevents us from stringing our own unrelated command at the end of the query. The most drastic example is:

SELECT email, passwd, login_id, full_name FROM members
    WHERE email = 'x'; DROP TABLE members; --'; -- Boom!

The first part provides a dummy email address -- 'x' -- and we don't care what this query returns: we're just getting it out of the way so we can introduce an unrelated SQL command. This one attempts to drop (delete) the entire members table, which really doesn't seem too sporting.

This shows that not only can we run separate SQL commands, but we can also modify the database. This is promising.

Adding a new member

Given that we know the partial structure of the members table, it seems like a plausible approach to attempt adding a new record to that table: if this works, we'll simply be able to login directly with our newly-inserted credentials.

This, not surprisingly, takes a bit more SQL, and we've wrapped it over several lines for ease of presentation, but our part is still one contiguous string:

SELECT email, passwd, login_id, full_name FROM members
    WHERE email = 'x';
     INSERT INTO members ('email','passwd','login_id','full_name')
        VALUES ('steve@unixwiz.net','hello','steve','Steve Friedl');--
';

Even if we have actually gotten our field and table names right, several things could get in our way of a successful attack:

  1. We might not have enough room in the web form to enter this much text directly (though this can be worked around via scripting, it's much less convenient).
  2. The web application user might not have INSERT permission on the members table.
  3. There are undoubtedly other fields in the members table, and some may require initial values, causing the INSERT to fail.
  4. Even if we manage to insert a new record, the application itself might not behave well due to the auto-inserted NULL fields that we didn't provide values for.
  5. A valid "member" might require not only a record in the members table, but associated information in other tables (say, "accessrights"), so adding to one table alone might not be sufficient.
In the case at hand, we hit a roadblock on either #4 or #5 - we can't really be sure -- because when going to the main login page and entering in the above username + password, a server error was returned. This suggests that fields we did not populate were vital, but nevertheless not handled properly.

A possible approach here is attempting to guess the other fields, but this promises to be a long and laborious process: though we may be able to guess other "obvious" fields, it's very hard to imagine the bigger-picture organization of this application.

We ended up going down a different road.

Mail me a password

We then realized that though we are not able to add a new record to the members database, we can modify an existing one, and this proved to be the approach that gained us entry.

From a previous step, we knew that bob@example.com had an account on the system, and we used our SQL injection to update his database record with our email address:

SELECT email, passwd, login_id, full_name
    FROM members WHERE email = 'x';
    UPDATE members SET email = 'steve@unixwiz.net' WHERE email = 'bob@example.com
';

After running this, we of course received the "we didn't know your email address", but this was expected due to the dummy email address provided. The UPDATE wouldn't have registered with the application, so it executed quietly.

We then used the regular "I lost my password" link - with the updated email address - and a minute later received this email:

Now it was now just a matter of following the standard login process to access the system as a high-ranked MIS staffer, and this was far superior to a perhaps-limited user that we might have created with our INSERT approach.

We found the intranet site to be quite comprehensive, and it included - among other things - a list of all the users. It's a fair bet that many Intranet sites also have accounts on the corporate Windows network, and perhaps some of them have used the same password in both places. Since it's clear that we have an easy way to retrieve any Intranet password, and since we had located an open PPTP VPN port on the corporate firewall, it should be straightforward to attempt this kind of access.

We had done a spot check on a few accounts without success, and we can't really know whether it's "bad password" or "the Intranet account name differs from the Windows account name". But we think that automated tools could make some of this easier.

Other Approaches

In this particular engagement, we obtained enough access that we did not feel the need to do much more, but other steps could have been taken. We'll touch on the ones that we can think of now, though we are quite certain that this is not comprehensive.

We are also aware that not all approaches work with all databases, and we can touch on some of them here.

Use xp_cmdshell
Microsoft's SQL Server supports a stored procedure xp_cmdshell that permits what amounts to arbitrary command execution, and if this is permitted to the web user, complete compromise of the webserver is inevitable.

What we had done so far was limited to the web application and the underlying database, but if we can run commands, the webserver itself cannot help but be compromised. Access to xp_cmdshell is usually limited to administrative accounts, but it's possible to grant it to lesser users.

Map out more database structure
Though this particular application provided such a rich post-login environment that it didn't really seem necessary to dig further, in other more limited environments this may not have been sufficient.

Being able to systematically map out the available schema, including tables and their field structure, can't help but provide more avenues for compromise of the application.

One could probably gather more hints about the structure from other aspects of the website (e.g., is there a "leave a comment" page? Are there "support forums"?). Clearly, this is highly dependent on the application and it relies very much on making good guesses.

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About the author

Stephen J. Friedl United States

UNIX Wizard and Microsoft MVP

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