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. SQL Server's -- comment needs no trailing space, so admin'-- closes the string and drops the rest of the line:

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

The password test is commented out and the admin row is returned. An always-true condition returns the first row when no account is targeted:

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. Since MSSQL permits stacked queries, a login injection can sometimes go further (for example ; UPDATE users SET ...), but the bypass itself needs only the row to be returned.

References#

• Microsoft SQL Server Documentation: comments, SELECT, operator precedence
• OWASP Testing Guide: Testing for SQL Injection