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A mutable input has the characteristic that its value may vary; that is, multiple accesses may see differing values. This characteristic enables potential attacks that exploit race conditions. For example, a time-of-check, time-of-use (TOCTOU) vulnerability may result when a field contains a value that passes validation and security checks but changes before use.

Returning references to an object's internal mutable components provides an attacker with the opportunity to corrupt the state of the object. Consequently, accessor methods must return defensive copies of internal mutable objects (see rule OBJ05-J. Defensively copy Do not return references to private mutable class members before returning their references for more information).

Noncompliant Code Example

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public final class MutableDemo {
  // java.net.HttpCookie is mutable
  public void useMutableInput(HttpCookie cookie) {
    if (cookie == null) {
       throw new NullPointerException();
    }

    // Check whether cookie has expired
    if (cookie.hasExpired()) {
      // Cookie is no longer valid,; handle condition by throwing an exception
    }

    // Cookie may have expired since time of check 
    doLogic(cookie);
  }
}

...

This compliant solution avoids the TOCTOU vulnerability by copying the mutable input and performing all operations on the copy. Consequently, an attacker's changes to the mutable input cannot affect the copy. Acceptable techniques include using a copy constructor or implementing the java.lang.Cloneable interface and declaring a public clone method (for classes not declared final). In cases like such as HttpCookie where the mutable class is declared final — that final—that is, it cannot provide an accessible copy method — perform a manual copy of the object state within the caller . See rule (see OBJ04-J. Provide mutable classes with copy functionality to safely allow passing instances to untrusted code for more information). Note that any input validation must be performed on the copy rather than on the original object.

public final class MutableDemo {
  // java.net.HttpCookie is mutable
  public void useMutableInput(HttpCookie cookie) {
    if (cookie == null) {
      throw new NullPointerException();
    }

    // Create copy
    cookie = (HttpCookie)cookie.clone();

    // Check whether cookie has expired
    if (cookie.hasExpired()) {
      // Cookie is no longer valid,; handle condition by throwing an exception
    }

    doLogic(cookie);
  }
}

...

Some copy constructors and clone() methods perform a shallow copy of the original instance. For example, invocation of clone() on an array results in creation of an array instance whose elements have the same values as the original instance. This shallow copy is sufficient for arrays of primitive types but fails to protect against TOCTOU vulnerabilities when the elements are references to mutable objects, such as an array of cookies. Such cases require a deep copy that also duplicates the reference objects.

This compliant solution demonstrates correct use both of a shallow copy (for the array of int) and of a deep copy (for the array of cookies).:

public void deepCopy(int[] ints, HttpCookie[] cookies) {
  if (ints == null || cookies == null) {
    throw new NullPointerException();
  }

  // Shallow copy
  int[] intsCopy = ints.clone();

  // Deep copy
  HttpCookie[] cookiesCopy = new HttpCookie[cookies.length];
  for (int i = 0; i < cookies.length; i++) {
    // Manually create copy of each element in array
    cookiesCopy[i] = (HttpCookie)cookies[i].clone();
  }
 
  doLogic(intsCopy, cookiesCopy);
}

...

When the class of a mutable input is nonfinal or is an interface, an attacker can write a subclass that maliciously overrides the parent class's clone() method. The attacker's clone() method can subsequently subvert defensive copying. This noncompliant code example demonstrates this weakness.:

// java.util.Collection is an interface
public void copyInterfaceInput(Collection<String> collection) {
  doLogic(collection.clone());
}

...

Some objects appear to be immutable because they have no mutator methods. For example, the java.lang.CharSequence interface describes an immutable sequence of characters. Note, however, that a variable of type CharSequence is a reference to an underlying object of some other class that implements the CharSequence interface; that other class may be mutable. When the underlying object changes, the CharSequence changes. Essentially, the java.lang.CharSequence interface omits methods that would permit object mutation through that interface but lacks any guarantee of true immutability. Such objects must still be defensively copied before use. For the case of the java.lang.CharSequence interface, one permissible approach is to obtain an immutable copy of the characters by using the toString() method. Mutable fields should not be stored in static variables. When there is no other alternative, create defensive copies of the fields to avoid exposing them to untrusted code.

Risk Assessment

Failing to create a copy of a mutable input may result in a TOCTOU TOCTOU vulnerability or expose internal mutable components to untrusted code.

Related Vulnerabilities

CVE-2012-0507 describes an exploit that managed to bypass Java's applet security sandbox and run malicious code on a remote user's machine. The exploit created a data structure that is normally impossible to create in Java , but was built using deserialization, and the deserialization process did not perform defensive copies of the deserialized data. See the code examples in SER07-J. Do not use the default serialized form for classes with implementation-defined invariants for more information.

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Bibliography

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