SER07-J. Make defensive copies of private mutable components

Every serializable container class that has private mutable instance variables must defensively copy them in the readObject() method. An adversary can append extra references to the variables to generate a new byte stream. When deserialized, this byte stream allows the creation of a container instance whose internal variable references are attacker controllable. Consequently, this allows the instance of the container class to mutate and violate its guarantees.

Noncompliant Code Example

This noncompliant code example lacks defensive copying of the mutable components or sub-objects (the Date object in this case). An attacker may be able to create an instance of MutableSer for which all invariants hold when validation is carried out that later mutates the value of the date sub-object to violate the class's contract. Any code that depends on the immutability of the sub-object is vulnerable.

class MutableSer implements Serializable {
  private static final Date epoch = new Date(0);
  private Date date = null; // Mutable component
  
  public MutableSer(Date d){
    date = new Date(d.getTime()); // Constructor performs defensive copying
  }

  private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
    ois.defaultReadObject();
    // Perform validation if necessary
  }
}

Compliant Solution

This compliant solution creates a defensive copy of the mutable Date object in the readObject() method. Note the use of field-by-field input and validation of incoming fields (see guideline SER04-J. Validate deserialized objects for additional information). Additionally, note that this compliant solution is insufficient to protect sensitive data (see guideline SER03-J. Do not serialize sensitive data for additional information).

private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
  ObjectInputStream.GetField fields = ois.readFields();
  Date inDate = (Date) fields.getField("date", epoch);
  // Defensively copy the mutable component
  date = new Date(inDate.getTime());
  // Perform validation if necessary
}

There is no need to copy immutable sub-objects. Also, avoid using the sub-object's clone() method because it can be overridden when the sub-object's class is non-final. Moreover, it produces only a shallow copy. The sub-objects (date) themselves must be non-final so that defensive copying can occur. It is also inadvisable to use the writeUnshared() and readUnshared() methods as an alternative [[Bloch 2008]].

Risk Assessment

Failure to defensively copy mutable components during deserialization can violate the immutability contract of an object.

Related Vulnerabilities

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

Bibliography

[[API 2006]]
[[Bloch 2008]] Item 76: "Write readObject methods defensively"
[[Sun 2006]] "Serialization specification: A.6 Guarding Unshared Deserialized Objects"

SER06-J. Do not serialize instances of inner classes      16. Serialization (SER)      SER08-J. Do not use the default serialized form for implementation-defined invariants