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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:

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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())

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

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Classes that have public setter methods, that is, methods whose purpose is to change class fields, must follow the related advice found in 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.

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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.

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Note that making deep copies of the keys of a hash table is unnecessary; 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 in which the sun.security.x509.InvalidityDateExtension class returned a Date instance through a public accessor without creating defensive copies.

Related Guidelines

Bibliography

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