SER12-J. Prevent deserialization of untrusted classes

Deserializing untrusted data can cause Java to create an object of an arbitrary attacker-specified class, provided that the class is available on the classpath specified for the JVM.  Some classes have triggers that execute additional code when they are created in this manner.  If such classes are poorly designed, such code could even call Runtime.exec() with an attacker-supplied argument.  Therefore, untrusted input to be deserialized should be validated to ensure that the serialized data contains only classes from a whitelist of expected classes.  This can be done by overloading the resolveClass() method of the ObjectInputStream class.  As an alternative to validation of the serialized data, a SecurityManager can be used to perform deserialization in a less-privileged context.

Non-Compliant Code Example

This non-compliant code deserializes a byte array without first validating what classes will be created and without using a SecurityManager.

class DeserializeExample {
  public static Object deserialize(byte[] buffer) throws IOException, ClassNotFoundException {
    Object ret = null;
    try (ByteArrayInputStream bais = new ByteArrayInputStream(buffer)) {
      try (ObjectInputStream ois = new ObjectInputStream(bais)) {
        ret = ois.readObject();
      }
    }
    return ret;
  }
} 

Compliant Solution

This compliant solution is based on http://www.ibm.com/developerworks/library/se-lookahead/ :

class LookAheadObjectInputStream extends ObjectInputStream {
  public LookAheadObjectInputStream(InputStream inputStream) throws IOException {
    super(inputStream);
  }

  @Override
  protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException {
    switch (desc.getName()) {
      case "GoodClass1": break;
      case "GoodClass2": break;
      default:	
        throw new InvalidClassException("Unexpected serialized class", desc.getName());
    }
    return super.resolveClass(desc);
  }
}

class DeserializeExample {
  private static Object deserialize(byte[] buffer) throws IOException, ClassNotFoundException {
    Object ret = null;
    try (ByteArrayInputStream bais = new ByteArrayInputStream(buffer)) {
      try (LookAheadObjectInputStream ois = new LookAheadObjectInputStream(bais)) {
        ret = ois.readObject();
      }
    }
    return ret;
  } 
}

Exceptions

SER12-EX0: Serialized data from a trusted input source does not require validation, provided that the code clearly documents that it relies on the input source being trustworthy.  For example, if a library is being audited, a routine of that library may have a documented precondition that its callers pre-validate any passed-in serialized data.

Risk Assessment

Whether a violation of this rule is exploitable depends on what classes are on the JVM's classpath.  (Note that is a property of the execution environment, not of the code being audited.) In the worst case, it could lead to remote execution of arbitrary code.

Automated Detection

It should not be difficult to write a static analysis to check for deserialization that fails to overload resolveClass() to compare against a whitelist.

 Related Guidelines

Bibliography

http://www.ibm.com/developerworks/library/se-lookahead/

http://foxglovesecurity.com/2015/11/06/what-do-weblogic-websphere-jboss-jenkins-opennms-and-your-application-have-in-common-this-vulnerability/