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Every serializable class that has private mutable instance variables must defensively copy them in the readObject() method. An attacker can tamper with the serialized form of such a class, appending extra references to the byte stream. When deserialized, this byte stream could allow the creation of a class instance whose internal variable references are controlled by the attacker. Consequently, the class instance can mutate and violate its class invariants.

This rule is an instance of rule OBJ06-J. Defensively copy mutable inputs and mutable internal components, which applies to constructors and to other methods that accept untrusted mutable arguments. This rule applies the same principle to deserialized mutable fields.

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

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

There is no need to copy immutable subobjects. Also, avoid using the subobject's clone() method because it can be overridden when the subobject's class is not final and produces only a shallow copy. The references to the subobjects themselves must be nonfinal 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.

Automated Detection

Related Guidelines

Bibliography

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