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Suppose a system authenticates users by issuing the following query to a SQL database. If the query returns any results, authentication succeeds; otherwise, authentication fails.

SELECT * FROM db_user WHERE username='<USERNAME>' AND 
                            password='<PASSWORD>'

Suppose an attacker can substitute arbitrary strings for <USERNAME> and <PASSWORD>. In that case, the authentication mechanism can be bypassed by supplying the following <USERNAME> with an arbitrary password:

validuser' OR '1'='1

The authentication routine dynamically constructs the following query:

SELECT * FROM db_user WHERE username='validuser' OR '1'='1' AND password='<PASSWORD>'

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Similarly, an attacker could supply the following string for <PASSWORD> with an arbitrary username:

' OR '1'='1

producing the following query:

SELECT * FROM db_user WHERE username='<USERNAME>' AND password='' OR '1'='1'

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Related Vulnerabilities

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