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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.

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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 \[[Bloch 05|AA. Java References#Bloch 05]\])

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

\[[JLS 05|AA. Java References#JLS 05]\] [Transient modifier|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#37020]
\[[SCG 07|AA. Java References#SCG 07]\] Guideline 5-1 Guard sensitive data during serialization
\[[Sun 06|AA. Java References#Sun 06]\] "Serialization specification: A.4  Preventing Serialization of Sensitive Data"
\[[Harold 99|AA. Java References#Harold 99]\]
\[[Long 05|AA. Java References#Long 05]\] Section 2.4, Serialization
\[[Greanier 00|AA. Java References#Greanier 00]\] [Discover the secrets of the Java Serialization API|http://java.sun.com/developer/technicalArticles/Programming/serialization/]
\[[Bloch 05|AA. Java References#Bloch 05]\] Puzzle 83: Dyslexic Monotheism
\[[Bloch 01|AA. Java References#Bloch 01]\] Item 1: Enforce the singleton property with a private constructor
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 502|http://cwe.mitre.org/data/definitions/502.html] "Deserialization of Untrusted Data", [CWE ID 499|http://cwe.mitre.org/data/definitions/499.html] "Serializable Class Containing Sensitive Data"

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