Page History

It is not unusual for Java code to deserialize data that comes from an untrusted source.  A serializable class can overload the method A Serializable class can overload the readObject() method, which is called when an object of that class is being deserialized.   This method (as well as similar methods such as readResolve and readObjectNoData) should treat the serialized data as potentially malicious, and it should not perform dangerous operations, nor should it set the stage for such operations to be performed later in the deserialization process.  For example, simply deserializing data should never invoke the method Runtime.exec().

Non-Compliant Code Example

Compliant Solution

 Both this method and the readResolve() method should refrain from performing potentially dangerous operations.  

 A class that performs dangerous operations in the constructor must not be Serializable. This is because SER07-J. Do not use the default serialized form for classes with implementation-defined invariants would require that its readObject() method perform the same dangerous operation as the constructor. As an alternative, such a class  could be Serializable if readObject() always throws an exception.

This guideline is related to rule SER12-J. Prevent deserialization of untrusted data.

Non-Compliant Code Example

In the following non-compliant code example, the class OpenedFile opens a file during deserialization.  Operating systems typically impose a limit on the number of open file handles per process. Usually, this limit is not large (e.g., 1024).  Consequently, deserializing a list of OpenedFile objects can consume all file handles available to the process and consequently cause the program to malfunction if it attempts to open another file before the deserialized OpenedFile objects get garbage-collected.

import java.io.*;

class OpenedFile implements Serializable {
  String filename;
  BufferedReader reader;

  public OpenedFile(String filename) throws FileNotFoundException {
    this.filename = filename;
    init();
  }

  private void init() throws FileNotFoundException {
    reader = new BufferedReader(new FileReader(filename));
  }
    
  private void writeObject(ObjectOutputStream out) throws IOException {
    out.writeUTF(filename);
  }

  private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
    filename = in.readUTF();
    init();
  }
} 

Compliant Solution

In this compliant solution, potentially dangerous operations are moved outside of deserialization, and users of the class are required to make a separate call to init() after deserializing.

import java.io.*;
 
class OpenedFile implements Serializable {
  String filename;
  BufferedReader reader;
  boolean isInitialized;

  public OpenedFile(String filename) {
    this.filename = filename;
    isInitialized = false;
 }

  public void init() throws FileNotFoundException {
    reader = new BufferedReader(new FileReader(filename));
    isInitialized = true;
 }
     
  private void writeObject(ObjectOutputStream out) throws IOException {
    out.writeUTF(filename);
  }

  private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
    filename = in.readUTF();
    isInitialized = false;
 }
}

Compliant Solution

In this compliant solution, we assume that OpenedFile must be Serializable because it inherits from a serializable superclass. Because OpenedFile must perform dangerous operations in the constructor, it intentionally forbids deserialization by throwing an exception in readObject().

import java.io.*;
 
class Unchangeable implements Serializable {
  // ...
}
class OpenedFile extends Unchangeable { // Serializable, unfortunately
  String filename;
  BufferedReader reader;
  boolean isInitialized;

  public OpenedFile(String filename) {
    this.filename = filename;
    isInitialized = false;
 }

  public void init() throws FileNotFoundException {
    reader = new BufferedReader(new FileReader(filename));
    isInitialized = true;
 }
     
  private void writeObject(ObjectOutputStream out) throws IOException {
    out.writeUTF(filename);
  }

  private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
    throw new NotSerializableException(OpenedFile.getClass().getName());
 }
}

Related Vulnerabilities

CERT Vulnerability #576313 describes a family of exploitable vulnerabilities that arise from violating this rule.

Risk Assessment

The severity of violations of this rule depend on the nature of the potentially dangerous operations performed.  If only mildly dangerous operations are performed, the risk might be limited to denial-of-service (DoS) attacks.  At the other extreme, remote code execution is possible if attacker-supplied input is supplied to methods such as Runtime.exec (either directly or via reflection).

...

Automated Detection

Related Guidelines

Bibliography

...

...

Image Added Image Added Image Added