Session manipulation can occur when an application allows user-input in session keys. Since sessions are typically considered a source of truth (e.g. to check the logged-in user or to match CSRF tokens), allowing an attacker to manipulate the session may lead to unintended behavior.

For example:

user_id = session[params[:name]]
current_user = User.find(user_id)

In this scenario, the attacker can point the name parameter to some other session value (for example, _csrf_token) that will be interpreted as a user ID. If the ID matches an existing account, the attacker will now have access to that account.

To prevent this type of session manipulation, avoid using user-supplied input as session keys.

(See here for a tiny, self-contained challenge demonstrating this issue.)

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