Does anyone know what something like OR 1# means in the context of MySQL injection?
It is MySQL's version of the line comment delimiter. In standard SQL, the line comment delimiter is --.
-- This is a standard SQL comment.
# This is a MySQL comment.
So in the context of SQL injection, if the attacker knows you're using MySQL he may use it to abruptly terminate the malicious SQL statement, causing MySQL to ignore whatever is behind the # and execute only the stuff that comes before it. This is only effective against single-line SQL statements, however. Here's an example:
Input:
Username:
fake' OR 1#
Password:pass
Resultant SQL:
SELECT * FROM users WHERE username = 'fake' OR 1#' AND password = 'pass'
Which is executed as this, which returns every row:
SELECT * FROM users WHERE username = 'fake' OR 1