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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, whereas this . This rule applies the same principle to deserialized mutable fields.

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This noncompliant code example fails to defensively copy the mutable Date object date. An attacker might be able to create an instance of MutableSer whose date object contains a nefarious subclass of Date and whose methods can perform actions specified by an attacker. Any code that depends on the immutability of the subobject 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
  }
}

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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.getFieldget("date", epoch);
  // Defensively copy the mutable component
  date = new Date(inDate.getTime());
  // Perform validation if necessary
}

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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 nonfinal 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|AA. Bibliography#Bloch 08]\].

Risk Assessment

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

Automated Detection

Related Guidelines

Bibliography

Bibliography

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Image Removed      13. Serialization (SER)      SER07-J. Do not use the default serialized form for implementation-defined invariantsImage Added Image Added Image Added