SER03-J. Do not serialize sensitive data

Object serialization allows an object's state to be saved as a sequence of bytes and then reconstituted at a later time. The primary application of serialization is in Java Remote Method Invocation (RMI) wherein objects must be packed (marshalled), exchanged between distributed virtual machines and subsequently unpacked (unmarshalled). It also finds extensive use in Java Beans.

Java language's access control mechanisms are ineffective after a class is serialized. Consequently, any sensitive data that was originally protected using access qualifiers (such as the private keyword) are exposed. Moreover, the security manager does not provide any checks to guarantee integrity of the serialized data.

Examples of sensitive data that should not be serialized are cryptographic keys, digital certificates and classes that may hold references to sensitive data at the time of serialization.

Noncompliant Code Example

The data members of class Point are declared as private. The saveState() and readState() methods are used for serialization and de-serialization respectively. The coordinates (x,y) that are written to the data stream are susceptible to malicious tampering.

public class Point {
  private double x;
  private double y;

  public Point(double x, double y) {
    this.x = x;
    this.y = y;
  }

  public Point() {
    // No argument constructor
  }
}

public class Coordinates extends Point implements Serializable {
  public static void main(String[] args) {
    try {
      Point p = new Point(5, 2);
      FileOutputStream fout = new FileOutputStream("point.ser");
      ObjectOutputStream oout = new ObjectOutputStream(fout);
      oout.writeObject(p);
      oout.close();
  }
  catch (Throwable t) { /* Forward to handler */ }
 }
}

Compliant Solution

In the absence of sensitive data, a class can be serialized by implementing the java.io.Serializable interface. By doing so, the class indicates that no security issues may result from the object's serialization. Note that any derived sub classes also inherit this interface and are consequently serializable.

When serialization is unavoidable, it is still possible to have classes that cannot implement serializable effectively. This condition is common when there are references to non-serializable objects within the contained methods. The following compliant solution avoids this issue and also protects sensitive data members from getting serialized accidentally. The basic idea is to declare the target member as transient so that it is not included in the list of fields to be serialized, whenever default serialization is being used.

public class Point {
 private transient double x;
 private transient double y;

 public Point(double x, double y) {
  this.x = x;
  this.y = y;
 }

 public Point()
 {
  //no argument constructor
 }

}

public class Coordinates extends Point implements Serializable {
  public static void main(String[] args) {
    try {
      Point p = new Point(5,2);
      FileOutputStream fout = new FileOutputStream("point.ser");
      ObjectOutputStream oout = new ObjectOutputStream(fout);
      oout.writeObject(p);
      oout.close();
    } catch (Exception e) {
      // Forward to handler
    }
  }
}

Other solutions include using custom implementation of writeObject(), writeReplace() and writeExternal() methods so that sensitive fields are not written to the serialized stream or alternatively, conducting proper validation checks while deserializing. Yet another remedy is to define the serialPersistentFields array field and ensuring that sensitive fields are not added to the array (SER00-J. Maintain serialization compatibility during class evolution). Sometimes it is necessary to prevent a serializable object (whose superclass implements serializable) from getting serialized. This is the focus of the second noncompliant code example.

Noncompliant Code Example

Serialization can also be used maliciously to return multiple instances of a singleton-like class. In this noncompliant code example, a subclass SensitiveClass inadvertently becomes serializable as it extends the Exception class that implements Serializable. (Based on [[Bloch 05]])

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

  protected int printBalance() {
    int balance = 1000;
    return balance;
  }
}

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

  // 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

Undue serialization of the subclass can be prohibited by throwing a NotSerializableException from a custom writeObject() method or the readResolve() method, defined in the subclass SensitiveClass. Ideally, extending a class or interface that implements Serializable should be avoided. It is also required to declare the methods final to prevent a malicious subclass from overriding them.

private final Object readResolve() throws NotSerializableException {
  throw new NotSerializableException();
}

Risk Assessment

If sensitive data can be serialized then it may be transmitted over an insecure link, or stored in an insecure medium, and thereby released inappropriately.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[JLS 05]] Transient modifier
[[SCG 07]] Guideline 5-1 Guard sensitive data during serialization
[[Sun 06]] "Serialization specification: A.4 Preventing Serialization of Sensitive Data"
[[Harold 99]]
[[Long 05]] Section 2.4, Serialization
[[Greanier 00]] Discover the secrets of the Java Serialization API
[[Bloch 05]] Puzzle 83: Dyslexic Monotheism
[[Bloch 01]] Item 1: Enforce the singleton property with a private constructor
[[MITRE 09]] CWE ID 502 "Deserialization of Untrusted Data", CWE ID 499 "Serializable Class Containing Sensitive Data"

SER02-J. Extendable classes should not declare readResolve() and writeReplace() private or static      14. Serialization (SER)      SER04-J. Validate deserialized objects