Is an SQL injection actually possible by adding a second query? - php

Question

There's a lot of warnings about SQL injections here on SO, but no one I've found doesn't really answer, how does it happen? In this question, I'm assuming it's MySQL and PHP.
The basic mysql_ doesn't accept a second query inside a query, right?
So, basically, this
$unsafe = "');DROP TABLE table;--";
mysqli_query($con,"INSERT INTO table (Column) VALUES ('$unsafe'");
doesn't actually do anything harmful? Correct me on this.
I've no experience working with mysqli_, so I'll skip to PDO, and "Prepared statements".
When I started working with PDO, I had a lack of information on it, and basically used it like this, thinking it's safe.
$stm = $pdo->prepare("INSERT INTO table (Column) VALUES ('$unsafe');
$stm->execute();
However, same thing with PDO as with mysql_. It doesn't support multiple queries out of the box, right? Again, correct me on this.
Then, this is consired safe, if I'm not wrong?
$stm = $pdo->prepare("INSERT INTO table (Column) VALUES (?);
$stm->execute(array($unsafe);
How does a malicious user with no access to the database inject malicious data, if multiple queries aren't even supported?
Or are they?

Answer 1

How does a malicious user with no access to the database inject malicious data, if multiple queries aren't even supported?
"SQL injection" is not equal to "second query".
Or are they?
Surely they are.
Second query is just an example. While it can be any valid SQL statement. SQl injection is an exploit of improperly formatted query. If a developer don't format SQL properly, there is a possibility to break from limits of literal and add code to the SQL body.
Is an SQL injection actually possible by adding a second query?
Yes, depends on the API you are using.

Answer 2

Two queries with mysql + php is a fallacy
Source: http://xkcd.com/327/
This will not work with mysql and php without deliberate steps to make it possible, since the normal query function will only execute the first query.
That doesn't mean it's not possible - only that it should be very obvious when it is.
SQL injection is very real
But the above means almost nothing in terms of sql injection. There is a huge, huge amount of information out there about sql injection including a large number of questions here on stack overflow. Taking the example in the question, this is an equivalent attack which would work:
$id = "123 OR 1 = 1 --";
mysqli_query($con,"DELETE FROM table WHERE id = $id LIMIT 1");
i.e. finding an interface to delete my own, e.g., comment, if the id is not escaped it would be trivial to delete all comments. But this example is just the very tip of an iceberg.
Executing arbitrary sql statements are exploitable
This code in the question:
$stm = $pdo->prepare("INSERT INTO table (Column) VALUES ('$unsafe')");
$stm->execute();
Has none of the benefits of using PDO - i.e. any exploit (of the truly massive number) that would work with the mysql/mysqli driver (used naively) will work with pdo used in this way.
Parametrized queries protect against sql injection
Using PDO with prepared statements with parameters escapes values appropriately preventing sql injection attacks, so yes this is safe from injection:
$stm = $pdo->prepare("INSERT INTO table (Column) VALUES (?)");
$stm->execute(array($unsafe));
How does a malicious user with no access to the database inject malicious data
Simply by finding a way to execute sql that either does what they want to do, or gives them the information to do it a different way.
For example:
function login() {
$username = "irrelevant' OR is_admin = 1 --";
$password = hash('irrelevant');
$query = "SELECT id from users where username = '$username' AND password = '$password'";
...
}
How did malicious user get access to the admin functionality on a system with no concern for injection? Very easily.
For general information about injection see the previous references.