SER01-J. Do not modify the signatures of serialization methods

The Java API [API 2006] for the Serializable inteface declares:

Classes that require special handling during the serialization and deserialization process must implement special methods with these exact signatures:

private void writeObject(java.io.ObjectOutputStream out)
throws IOException
private void readObject(java.io.ObjectInputStream in)
throws IOException, ClassNotFoundException;
private void readObjectNoData()
throws ObjectStreamException;

Deviating from these method signatures produces a method that will, in fact, not be invoked during object serialization or deserialization. Such a method will clearly not have the expected behavior. Such methods, especially if declared public, might be accessible to untrusted code.

Be aware that, unlike most interfaces, Serializable does not define these method signatures it requires. It cannot, because they are private. Consequently, the Java compiler will not identify a deviant method signature.

Noncompliant Code Example

This noncompliant code example shows a class Ser with a private constructor, indicating that code external to the class should be unable to create instances of it. The class implements java.io.Serializable and defines public readObject() and writeObject() methods. Consequently, untrusted code can obtain the reconstituted objects by using readObject(), and can write to the stream by using writeObject().

public class Ser implements Serializable {
  private final long serialVersionUID = 123456789;
  private Ser() {
    // initialize
  }
  public static void writeObject(final ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
  }
	
  public static void readObject(final ObjectInputStream stream) throws 
      IOException, ClassNotFoundException {
    stream.defaultReadObject();
  }
}

Similarly, omitting the static keyword is insufficient to make this example secure; the JVM will fail to detect the two methods, resulting in failure to use the custom serialized form.

Compliant Solution

This compliant solution declares the readObject() and writeObject() methods private and non-static to limit their accessibility.

private void writeObject(final ObjectOutputStream stream) throws IOException {
  stream.defaultWriteObject();
}
	
private void readObject(final ObjectInputStream stream) throws 
    IOException, ClassNotFoundException {
  stream.defaultReadObject();
}

Reducing the accessibility also prevents malicious overriding of the two methods.

Risk Assessment

Failure to limit the accessibility of the readObject() and writeObject() methods can leave code vulnerable to untrusted invocations.

Related Vulnerabilities

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

Bibliography

[[API 2006]] Serializable
[[Sun 2006]] "Serialization specification"
[[Ware 2008]]

SER00-J. Maintain serialization compatibility during class evolution      16. Serialization (SER)      SER02-J. Extendable classes should not declare readResolve() and writeReplace() private or static