SER33-J. Do not serialize instances of inner classes

Serialization of inner classes (including local and anonymous classes) is error prone. The following postulates have been reproduced from [[Sun 06]], Serialization specification:

• Because inner classes declared in non-static contexts contain implicit non-transient references to enclosing class instances, serializing such an inner class instance will result in serialization of its associated outer class instance as well.

• Synthetic fields generated by javac (or other JavaTM compilers) to implement inner classes are implementation dependent and may vary between compilers; differences in such fields can disrupt compatibility as well as result in conflicting default serialVersionUID values. The names assigned to local and anonymous inner classes are also implementation dependent and may differ between compilers.

• Since inner classes cannot declare static members other than compile-time constant fields, they cannot use the serialPersistentFields mechanism to designate serializable fields.

• Finally, because inner classes associated with outer instances do not have zero-argument constructors (constructors of such inner classes implicitly accept the enclosing instance as a prepended parameter), they cannot implement Externalizable.

None of the issues listed above, however, apply to static member classes.

Noncompliant Code Example

In this non compliant code example, the fields contained within the outer class are also serialized when the inner class is serialized.

public class OuterSer implements Serializable {
  private int ssn;
  class InnerSer implements Serializable {
    protected String name;
    //...
  }
}

Risk Assessment

Attempting to serialize inner classes can cause instances of the outer class to be serialized and also discourage platform independence.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]]
[[Sun 06]] "Serialization specification:
[[Bloch 08]] Item 74: "Implement serialization judiciously"

SER32-J. Do not allow serialization and deserialization to bypass the Security Manager      11. Serialization (SER)      11. Serialization (SER)