SER04-J. Do not allow serialization and deserialization to bypass the Security Manager

The serialization and deserialization features can be exploited to bypass security manager checks. A serializable class may install security manager checks in its constructors for various reasons including, for example, preventing untrusted code from modifying the internal state of the class. Such security manager checks must be replicated at all points where a class instance can be constructed. Because deserialization acts like a constructor, all the relevant methods must contain all relevant security checks.

If the class enables a caller to retrieve sensitive internal state contingent upon security checks, the same checks must be replicated during deserialization. This ensures that an attacker cannot glean sensitive information by deserializing the object.

Noncompliant Code Example

In this noncompliant code example, security manager checks are used within the constructor but are omitted from the writeObject() and readObject() methods that are used in the serialization-deserialization process. This allows untrusted code to maliciously create instances of the class.

public final class Hometown implements Serializable {
  // Private internal state
  private String town;
  private static final String UNKNOWN = "UNKNOWN";

  void performSecurityManagerCheck() throws AccessDeniedException {
    // ...
  }

  void validateInput(String newCC) throws InvalidInputException {
    // ...
  }

  public Hometown() {
    performSecurityManagerCheck();

    // Initialize town to default value
    town = UNKNOWN;
  }

  // Allows callers to retrieve internal state
  String getValue() {
    performSecurityManagerCheck();
    return town;
  }

  // Allows callers to modify (private) internal state
  public void changeTown(String newTown) {
    if (town.equals(newTown)) {
      // No change
      return;
    } else {  
      performSecurityManagerCheck();
      validateInput(newTown);
      town = newTown;
    }
  }

  // writeObject() correctly enforces checks during serialization
  private void writeObject(ObjectOutputStream out) throws IOException {
    out.writeObject(town);
  }

  // readObject() correctly enforces checks during deserialization
  private void readObject(ObjectInputStream in) throws IOException {
    in.defaultReadObject();
    // If the deserialized name does not match the default value normally
    // created at construction time, duplicate the checks
    if (!UNKNOWN.equals(town)) {
      validateInput(town);
    }
  }
}

Despite the security manager checks, the data is not considered sensitive, as a sensitive serializable class would violate SER03-J. Prevent serialization of unencrypted, sensitive data.

We assume that AccessDeniedException and InvalidInputException are both security exceptions, and, for that reason, are not checked. So any method can throw them without requiring a throws declaration.

Compliant Solution

This compliant solution implements the required security manager checks in all constructors and methods that can either modify or retrieve internal state. Consequently, an attacker cannot create a modified instance of the object (using deserialization) or read the serialized byte stream to reveal serialized data.

public final class Hometown implements Serializable {
  // ... all methods the same except the following:

  // writeObject() correctly enforces checks during serialization
  private void writeObject(ObjectOutputStream out) throws IOException {
    performSecurityManagerCheck();
    out.writeObject(town);
  }

  // readObject() correctly enforces checks during deserialization
  private void readObject(ObjectInputStream in) throws IOException {
    in.defaultReadObject();
    // If the deserialized name does not match the default value normally
    // created at construction time, duplicate the checks
    if (!UNKNOWN.equals(town)) {
      performSecurityManagerCheck();
      validateInput(town);
    }
  }
}

Refer to rule SEC03-J. Protect sensitive operations with security manager checks to learn about implementing the performSecurityManagerCheck() method. As with rule void SER04-J. Validate deserialized objects, it is important to protect against the finalizer attack.

Risk Assessment

Allowing serialization or deserialization to bypass the Security Manager may result in classes being constructed without requird security checks.

Related Vulnerabilities

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

Bibliography

SER03-J. Prevent serialization of unencrypted, sensitive data      13. Serialization (SER)