Mass assignment is a feature of Rails which allows an application to create a record from the values of a hash.

Example:

User.new(params[:user])

Unfortunately, if there is a user field called admin which controls administrator access, now any user can make themselves an administrator with a query like

?user[admin]=true

Rails With Strong Parameters

In Rails 4 and newer, protection for mass assignment is on by default.

Query parameters must be explicitly whitelisted via permit in order to be used in mass assignment:

User.new(params.permit(:name, :password))

Care should be taken to only whitelist values that are safe for a user (or attacker) to set. Foreign keys such as account_id are likely unsafe, allowing an attacker to manipulate records belonging to other accounts.

Brakeman will warn on potentially dangerous attributes that are whitelisted.

Brakeman will also warn about uses of params.permit!, since that allows everything.

Rails Without Strong Parameters

In older versions of Rails, attr_accessible and attr_protected can be used to limit mass assignment. However, Brakeman will warn unless attr_accessible is used, or mass assignment is completely disabled.

There are two different mass assignment warnings which can arise. The first is when mass assignment actually occurs, such as the example above. This results in a warning like

Unprotected mass assignment near line 61: User.new(params[:user])

The other warning is raised whenever a model is found which does not use attr_accessible. This produces generic warnings like

Mass assignment is not restricted using attr_accessible

with a list of affected models.

In Rails 3.1 and newer, mass assignment can easily be disabled:

config.active_record.whitelist_attributes = true

Unfortunately, it can also easily be bypassed:

User.new(params[:user], :without_protection => true)

Brakeman will warn on uses of without_protection.

More Information

Strong Parameters in Rails Security Guide Mass Assignment in Rails Security Guide

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