This warning comes up if a model does not limit what attributes can be set through mass assignment.
In particular, this check looks for attr_accessible inside model definitions. If it is not found, this warning will be issued.
Brakeman also warns on use of attr_protected - especially since it was found to be vulnerable to bypass. Warnings for mass assignment on models using attr_protected will be reported, but at a lower confidence level.
Note that disabling mass assignment globally will suppress these warnings.