OBJ05-J. Defensively copy private mutable class members before returning their references

Returning references to internal mutable members of a class can compromise an application's security, both by breaking encapsulation and by providing the opportunity to corrupt the internal state of the class (whether accidentally or maliciously). As a result, programs must not return references to internal mutable classes.

Returning a reference to a defensive copy of a mutable internal state ensures that the caller cannot modify the original internal state, although the copy remains mutable.

Noncompliant Code Example

This noncompliant code example shows a getDate() accessor method that returns the sole instance of the private Date object.

class MutableClass {
  private Date d;

  public MutableClass() {
    d = new Date();
  }

  public Date getDate() {
    return d;
  }
}

An untrusted caller can manipulate a private Date object because returning the reference exposes the internal mutable component beyond the trust boundaries of MutableClass.

Compliant Solution (clone())

This compliant solution returns a clone of the Date object from the getDate() accessor method. While Date can be extended by an attacker, this is safe because the Date object returned by getDate() is controlled by MutableClass and is known to be a nonmalicious subclass.

public Date getDate() {
  return (Date)d.clone();
}

Note that defensive copies performed during execution of a constructor must avoid use of the clone() method when the class could be subclassed by untrusted code. This restriction prevents execution of a maliciously crafted overriding of the clone() method. For more details, see rule OBJ07-J. Sensitive classes must not let themselves be copied.

Classes that have public setter methods, that is, methods whose purpose is to change class fields, must follow the related advice found in rule OBJ06-J. Defensively copy mutable inputs and mutable internal components. Note that setter methods can (and usually should) perform input validation and sanitization before setting internal fields.

Noncompliant Code Example (Mutable Member Array)

In this noncompliant code example, the getDate() accessor method returns an array of Date objects. The method fails to make a defensive copy of the array before returning it. Because the array contains references to Date objects that are mutable, a shallow copy of the array is insufficient because an attacker can modify the Date objects in the array.

class MutableClass {
  private Date[] date;

  public MutableClass() {
    date = new Date[20];
    for (int i = 0; i < date.length; i++) {
      date[i] = new Date();
    }
  }

  public Date[] getDate() {
    return date; // or return date.clone()
  }
}

Compliant Solution (Deep Copy)

This compliant solution creates a deep copy of the date array and returns the copy, thereby protecting both the date array and the individual Date objects.

class MutableClass {
  private Date[] date;

  public MutableClass() {
    date = new Date[20];
    for(int i = 0; i < date.length; i++) {
      date[i] = new Date();
    }
  }

  public Date[] getDate() {
    Date[] dates = new Date[date.length];
    for (int i = 0; i < date.length; i++) {
      dates[i] = (Date) date[i].clone();
    }
    return dates;
  }
}

Noncompliant Code Example (Mutable Member Containing Immutable Objects)

In this noncompliant code example, class ReturnRef contains a private Hashtable instance field. The hash table stores immutable but sensitive data (for example, social security numbers SSNs). The getValues() method gives the caller access to the hash table by returning a reference to it. An untrusted caller can use this method to gain access to the hash table; as a result, hash table entries can be maliciously added, removed, or replaced. Furthermore, multiple threads can perform these modifications, providing ample opportunities for race conditions.

class ReturnRef {
  // Internal state, may contain sensitive data
  private Hashtable<Integer,String> ht = new Hashtable<Integer,String>();

  private ReturnRef() {
    ht.put(1, "123-45-6666");
  }

  public Hashtable<Integer,String> getValues(){
    return ht;
  }

  public static void main(String[] args) {
    ReturnRef rr = new ReturnRef();
    Hashtable<Integer, String> ht1 = rr.getValues(); // Prints sensitive data 123-45-6666
    ht1.remove(1);                                   // Untrusted caller can remove entries
    Hashtable<Integer, String> ht2 = rr.getValues(); // Now prints null, original entry is removed
  }
}

In returning a reference to the ht hash table, this example also hinders efficient garbage collection.

Compliant Solution (Shallow Copy)

Make defensive copies of private internal mutable object state. For mutable fields that contain immutable data, a shallow copy is sufficient. Fields that refer to mutable data generally require a deep copy.

This compliant solution creates and returns a shallow copy of the hash table containing immutable SSNs. Consequently, the original hash table remains private, and any attempts to modify it are ineffective.

class ReturnRef {
  // ...
  private Hashtable<Integer,String> getValues(){
    return (Hashtable<Integer, String>) ht.clone(); // shallow copy
  }

  public static void main(String[] args) {
    ReturnRef rr = new ReturnRef();
    // Prints non-sensitive data
    Hashtable<Integer,String> ht1 = rr.getValues(); 
    // Untrusted caller can only modify copy
    ht1.remove(1); 
    // Prints non-sensitive data
    Hashtable<Integer,String> ht2 = rr.getValues(); 
  }
}

When a hash table contains references to mutable data such as Date objects, each of those objects must also be copied by using a copy constructor or method. For further details, refer to rules OBJ06-J. Defensively copy mutable inputs and mutable internal components and OBJ04-J. Provide mutable classes with copy functionality to safely allow passing instances to untrusted code.

Note that making deep copies of the keys of a hash table is unncessary; shallow copying of the references suffices because a hash table's contract dictates that its keys must produce consistent results to the equals() and hashCode() methods. Mutable objects whose equals() or hashCode() method results may be modified are not suitable keys.

Exceptions

OBJ05-EX0: When a method is called with only an unmodifiable view of an object, that method may freely use the unmodifiable view without defensive copying. This decision should be made early in the design of the API. Note that new callers of such methods must also expose only unmodifiable views.

Risk Assessment

Returning references to internal object state (mutable or immutable) can render an application susceptible to information leaks and corruption of its objects' states, which consequently violates class invariants. Control flow can also be affected in some cases.

Automated Detection

Sound automated detection is infeasible; heuristic checks could be useful.

Related Vulnerabilities

Pugh [Pugh 2009] cites a vulnerability discovered by the Findbugs static analysis tool in the early betas of JDK 1.7 where the sun.security.x509.InvalidityDateExtension class returned a Date instance through a public accessor without creating defensive copies.

Related Guidelines

Bibliography

      04. Object Orientation (OBJ)