php - Prepared statements and second order SQL injections

Question

I have read somewhere here that using prepared statements in PDO makes your app only immune to first order SQL injections, but not totally immune to second order injections.

My question is: if we used prepared statements in all queries inlcuding SELECT queries and not only in INSERT query, then how can a second order sql injection be possible?

For example in the following queries there is no chance for a 2nd order injection:

write:

INSERT INTO posts (userID,text,date) VALUES(?,?,?)

read:

SELECT * FROM posts WEHRE userID=?

delete:

DELETE FROM posts WHERE userID=?

Answer 1

What you have read is a plain rubbish. Someone who wrote it just have no clue.

You should use prepared statements not for the query but for the data. Every time you have to add a variable into query, you have to make it via placeholder only. So, your query separation theory makes no sense: it doesn't matter if it SELECT or ALTER or GRANT or whatever. The only thing that matters - if any variable goes into query or not.