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

See rule OBJ13-J. Ensure that references to mutable objects are not exposed for details about leaking references to non-private objects.

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())

Returning a reference to a defensive copy of a mutable internal state ensures that the caller can modify only the copy; he 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.

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 (see OBJ07-J. Sensitive classes must not let themselves be copied for more details).

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 of sensitive nature (SSN numbers). A getter method (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 the getter this method to gain access to the hashtable; consequently, hashtable 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 (see below).

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

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

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

When a hash table contains references to mutable data such as a series of Date objects, each of those objects must also be copied by using a copy constructor or method . For further details, (refer to guidelines FIO00 OBJ06-J. Defensively copy mutable inputs and mutable internal components and OBJ10OBJ04-J. Provide mutable classes with copy functionality to safely allow passing instances to untrusted code safely for further details).

Note that making deep copies of the keys of a hash table need not be deep copiedis unnecessary; shallow copying of the references suffices because a hash table's contract dictates that it cannot hold duplicate keys.

Noncompliant Code Example (Mutable Member)

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 the instance because returning the reference exposes the internal mutable component beyond the trust boundaries of the class.

Compliant Solution (Shallow Copying Using clone() for Final Classes)

Defensive copies performed during execution of a constructor must avoid use of the clone() when the (non-system) class in question can be subclassed by untrusted code. This restriction prevents execution of a maliciously-crafted overriding of the clone() method.

Nevertheless, this compliant solution recommends returning a clone of the Date object from the getDate() accessor method. This is an acceptable solution because a malicious subclass cannot extending the internal mutable Date object; Date is a system class, the internal instance was locally constructed using the system class's constructor, so the operation is consequently safe.

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

This guideline also applies to arrays of primitive types. Because Java arrays are objects in their own right, when a caller receives a reference to an array, he can freely modify its contents. Creating a shallow copy of an arrays of primitive types provides a distinct array instance; although the caller can modify the returned array, he cannot affect the original array.

Classes that have public setter methods must follow the related advice found in guideline FIO00-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. On the other hand, returning references to internal objects may not require the caller to incorporate any of these defensive measures.

Noncompliant Code Example (Mutable Member Array)

In this noncompliant code example, the getDate() accessor method returns an array of Date objects.

class MutableClass {
  private Date[] date;

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

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

It fails to make a defensive copy of the array before returning it. Because the array contains references to Date objects that are themselves mutable, a shallow copy of the array (as discussed at the end of the previous compliant example) would be insufficient.

Compliant Solution (Deep Copying)

This compliant solution creates a deep copy of the date array and returns the copy rather than returning the internal date array.

class MutableClass {
  private Date[] date;

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

  public Date[] getDate() {
    Date[] dates = new Date[20];
    for(int i = 0; i < 20; i++) {
      dates[i] = (Date) date[i].clone();
    }
    return dates;
  }
}

Exceptions

*OBJ11-EX1:* According to Sun's Secure Coding Guidelines document \[[SCG 2007|AA. Bibliography#SCG 07]\]

if a class merely serves as a container for mutable inputs or outputs (the class does not directly operate on them), it may not be necessary to create defensive copies. For example, arrays and the standard collection classes do not create copies of caller-provided values. If a copy is desired so updates to a value do not affect the corresponding value in the collection, the caller must create the copy before inserting it into the collection, or after receiving it from the collection.

OBJ11-EX2: If the performance of the copy method is within reasonable bounds and the class clearly documents its use, this guideline may be violated. (See guideline OBJ10-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely.)

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-J-EX0: When a method is called with only an immutable view of an object, that method may freely use the immutable view without defensive copyingOBJ11-EX3: If the caller exposes an unmodifiable view of the object, it may not be required to defensively program the class to return copies of internal members. This decision should be made early in the design of the API. Note that if some other code wants to use this class in the future, it new callers of such methods must also expose unmodifiable views. (See guideline SEC14-J. Provide sensitive mutable classes with unmodifiable wrappers.)only immutable views.

Risk Assessment

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

Automated Detection

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

Related Vulnerabilities

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

Other Languages

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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

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\[[API 2006|AA. Bibliography#API 06]\] [method clone()|http://java.sun.com/javase/6/docs/api/java/lang/Object.html#clone()]
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 39: Make defensive copies when needed
\[[Goetz 2006|AA. Bibliography#Goetz 06]\] 3.2. Publication and Escape: Allowing Internal Mutable State to Escape
\[[Gong 2003|AA. Bibliography#Gong 03]\] 9.4 Private Object State and Object Immutability
\[[Haggar 2000|AA. Bibliography#Haggar 00]\] [Practical Java Praxis 64: Use clone for Immutable Objects When Passing or Receiving Object References to Mutable Objects|http://www.informit.com/articles/article.aspx?p=20530]
\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 375|http://cwe.mitre.org/data/definitions/375.html] "Passing Mutable Objects to an Untrusted Method"
\[[SCG 2007|AA. Bibliography#SCG 07]\] Guideline 2-1 Create a copy of mutable inputs and outputs
\[[Security 2006|AA. Bibliography#Security 06]\]

Bibliography

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Image Added Image Added Image AddedOBJ10-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely      Object Orientation (OBJ)      OBJ12-J. Use checked collections against external code