The product has a method that is declared public, but returns a reference to a private data structure, which could then be modified in unexpected ways.
Declare the method private.
Clone the member data and keep an unmodified version of the data private to the object.
Use public setter methods that govern how a private member can be modified.
The contents of the data structure can be modified from outside the intended scope.
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
Effectiveness: High