SER08-J. Do not use the default serialized form for implementation-defined invariants

Serialization can be used maliciously. Examples include using serialization to maliciously violate the intended invariants of a class. Deserialization is equivalent to object construction; consequently all invariants enforced during object construction must also be enforced during deserialization. The default serialized form lacks any enforcement of class invariants; consequently, it is forbidden to use the default serialized form for any class with implementation-defined invariants.

The deserialization process creates a new instance of the class without invoking any of the class's constructors. Consequently, any input validation checks present within the constructors are bypassed. Moreover, transient and static fields may fail to reflect their true values because such fields are bypassed during the serialization procedure and consequently cannot be restored from the object stream. Therefore any class with transient fields, and any class that performs validation checks in its constructors, must also perform similar valiation checks when being deserialized.

Validating deserialized objects helps confirm that the object state is within defined limits and ensures that all transient and static fields have their default safe values. Fields that are declared final and contain a constant value will contain the proper value after deserialization, rather than the default value. For example, the value of the field private transient final n = 42 after deserialization will be 42, rather than 0. Deserialization produces default values for all other cases.

Noncompliant Code Example (Singleton)

In this noncompliant code example (based on [[Bloch 2005]]), a class with singleton semantics uses the default serialized form, which fails to enforce any implementation-defined invariants. Consequently, the malicious code can create a second instance even though the class should have only a single instance. For purposes of this example, we assume that the class contains only nonsensitive data.

public class SingletonClass extends Exception {
  public static final SingletonClass INSTANCE = new SingletonClass();
  private SingletonClass() {
    // Perform security checks and parameter validation
  }

  protected int printData() {
    int data = 1000;
    return data;
  }
}

class Malicious {
  public static void main(String[] args) {
    SingletonClass sc = (SingletonClass) deepCopy(SingletonClass.INSTANCE);
    System.out.println(sc == SingletonClass.INSTANCE);  // Prints false; indicates new instance
    System.out.println("Balance = " + sc.printData());
  }

  // This method should not be used in production quality code
  static public Object deepCopy(Object obj) {
    try {
      ByteArrayOutputStream bos = new ByteArrayOutputStream();
       new ObjectOutputStream(bos).writeObject(obj);
      ByteArrayInputStream bin = new ByteArrayInputStream(bos.toByteArray());
      return new ObjectInputStream(bin).readObject();
    } catch (Exception e) { 
      throw new IllegalArgumentException(e);
    }
  }
}

Compliant Solution

This compliant solution adds a custom readResolve() method that replaces the deserialized instance with a reference to the appropriate singleton from the current execution. More complicated cases may also require custom writeObject() or readObject() methods in addition to (or instead of) the custom readResolve() method. Note that the custom serialization methods must be declared final to prevent a malicious subclass from overriding them.

More information on correctly handling singleton classes is available in the guideline MSC16-J. Address the shortcomings of the Singleton design pattern.

class SingletonClass extends Exception {
  // ...
  private final Object readResolve() throws NotSerializableException {
    return INSTANCE;
  }
}

Noncompliant Code Example

This noncompliant code example uses a custom defined readObject() method, but fails to perform input validation after deserialization. The design of the system requires the maximum value of any lottery ticket to be 20,000; however, an attacker can manipulate the serialized array to generate a different number on deserialization.

public class Lottery implements Serializable {	
  private int ticket = 1;
  private SecureRandom draw = new SecureRandom();

  public Lottery(int ticket) {
    this.ticket = (int) (Math.abs(ticket % 20000) + 1);
  }

  public int getTicket() {
    return this.ticket;	
  }

  public int roll() {
    this.ticket = (int) ((Math.abs(draw.nextInt()) % 20000) + 1);
    return this.ticket;
  }

  public static void main(String[] args) {
    Lottery l = new Lottery(2);
    for(int i = 0; i < 10; i++) {
      l.roll();
      System.out.println(l.getTicket());
    }
  }

  private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
    in.defaultReadObject();
  }
}

Compliant Solution

Any input validation performed in the constructors must also be implemented at all places where an object can be deserialized. This compliant solution performs field-by-field validation by reading all fields of the object using the readFields() and getField() methods. The value for each field must be fully validated before it is assigned to the object under construction. For more complicated invariants, this may require reading multiple field values into local variables to enable checks that depend on combinations of field values.

public final class Lottery implements Serializable { 
  // ...
  private synchronized void readObject(java.io.ObjectInputStream s)
    throws IOException, ClassNotFoundException {
    ObjectInputStream.GetField fields = s.readFields();
    int ticket = fields.get("ticket", 0);
    if (ticket > 20000 || ticket <= 0) {
      throw new InvalidObjectException("Not in range!");
    }
    // Validate draw
    this.ticket = ticket;
  }
}

Note that the class must also be declared final to prevent a malicious subclass from carrying out a finalizer attack. (See guideline OBJ05-J. Prevent access to partially initialized objects.) For extendable classes, an acceptable alternative is use of a flag that indicates whether the instance is safe for use. The flag can be set after validation and must be checked in every method before any operation is performed.

Additionally, any transient or static fields must be explicitly set to an appropriate value within readObject().

This compliant solution must also validate that draw is a secure random number generator. If an attacker made draw generate insufficiently random numbers, the attacker could control the future tickets generated by roll().

Note that this compliant solution is insufficient to protect sensitive data. See guideline SER03-J. Do not serialize sensitive data for additional information.

Compliant Solution (transient)

This compliant solution marks the fields as transient, so they are not serialized. The readObject() initializes them using the roll() method. This class need not be final, as its fields are private and cannot be tampered with by subclasses.

public class Lottery implements Serializable {	
  private transient int ticket = 1;
  private transient SecureRandom draw = new SecureRandom();

  public Lottery(int ticket) {
    this.ticket = (int) (Math.abs(ticket % 20000) + 1);
  }

  public int getTicket() {
    return this.ticket;	
  }

  public int roll() {
    this.ticket = (int) ((Math.abs(draw.nextInt()) % 20000) + 1);
    return this.ticket;
  }

  public static void main(String[] args) {
    Lottery l = new Lottery(2);
    for(int i = 0; i < 10; i++) {
      l.roll();
      System.out.println(l.getTicket());
    }
  }

  private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
    in.defaultReadObject();
    this.draw = new SecureRandom();
    roll();
  }
}

Compliant Solution (non-serializable)

This compliant solution simply does not mark the Lottery class serializable.

public final class Lottery {	
  // ...
}

Risk Assessment

Serializing objects with implementation defined characteristics can corrupt the state of the object.

Related Vulnerabilities

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

Bibliography

[[API 2006]] Class Object, Class Hashtable
[[Bloch 2008]] Item 75: "Consider using a custom serialized form"
[[Greanier 2000]]
[[Harold 1999]] Chapter 11: Object Serialization, Validation
[[Hawtin 2008]] Antipattern 8: Believing deserialisation is unrelated to construction
[[MITRE 2009]] CWE ID 502 "Deserialization of Untrusted Data"
[[SCG 2007]] Guideline 5-2 View deserialization the same as object construction

SER07-J. Make defensive copies of private mutable components during deserialization      16. Serialization (SER)      SER09-J. Minimize privileges before deserializing from a privileged context