FIO05-J. Do not expose buffers or their backing arrays methods to untrusted code

Buffer classes defined in the java.nio package, such as IntBuffer, CharBuffer, and ByteBuffer, define a variety of methods that wrap an array (or a portion of the array) of the corresponding primitive data type into a buffer and return the buffer as a Buffer object. Although these methods create a new Buffer object, the new Buffer is backed by the given input array. According to the Java API for these methods [API 2014],

The new buffer will be backed by the given character array; that is, modifications to the buffer will cause the array to be modified and vice versa.

Exposing these buffers to untrusted code exposes the backing array of the original buffer to malicious modification. Likewise, the duplicate(), array(), slice(), and subsequence() methods create additional buffers that are backed by the original buffer's backing array; exposing such additional buffers to untrusted code affords the same opportunity for malicious modification.

This rule is an instance of OBJ06-J. Defensively copy mutable inputs and mutable internal components.

Noncompliant Code Example (wrap())

This noncompliant code example declares a char array, wraps it within a CharBuffer, and exposes that CharBuffer to untrusted code via the getBufferCopy() method:

final class Wrap {
  private char[] dataArray;

  public Wrap() {
    dataArray = new char[10];
    // Initialize
  }

  public CharBuffer getBufferCopy() {
    return CharBuffer.wrap(dataArray);
  }
}

Compliant Solution (asReadOnlyBuffer())

This compliant solution returns a read-only view of the char array in the form of a read-only CharBuffer. The standard library implementation of CharBuffer guarantees that attempts to modify the elements of a read-only CharBuffer will result in a java.nio.ReadOnlyBufferException.

final class Wrap {
  private char[] dataArray;

  public Wrap() {
    dataArray = new char[10];
    // Initialize
  }

  public CharBuffer getBufferCopy() {
    return CharBuffer.wrap(dataArray).asReadOnlyBuffer();
  }
}

Compliant Solution (Copy)

This compliant solution allocates a new CharBuffer and explicitly copies the contents of the char array into it before returning the copy. Consequently, malicious callers can modify the copy of the array but cannot modify the original.

final class Wrap {
  private char[] dataArray;

  public Wrap() {
    dataArray = new char[10];
    // Initialize
  }

  public CharBuffer getBufferCopy() {
    CharBuffer cb = CharBuffer.allocate(dataArray.length);
    cb.put(dataArray);
    return cb;
  }
}

Noncompliant Code Example (duplicate())

This noncompliant code example invokes the duplicate() method to create and return a copy of the CharBuffer. As stated in the contract for the duplicate() method, the returned buffer is backed by the same array as is the original buffer. Consequently, if a caller were to modify the elements of the backing array, these modifications would also affect the original buffer.

final class Dup {
  CharBuffer cb;

  public Dup() {
    cb = CharBuffer.allocate(10);
    // Initialize
  }

  public CharBuffer getBufferCopy() {
    return cb.duplicate();
  }
}

Compliant Solution (asReadOnlyBuffer())

This compliant solution exposes a read-only view of the CharBuffer to untrusted code:

final class Dup {
  CharBuffer cb;

  public Dup() {
    cb = CharBuffer.allocate(10);
    // Initialize
  }

  public CharBuffer getBufferCopy() {
    return cb.asReadOnlyBuffer();
  }
}

Risk Assessment

Exposing buffers created using the wrap(), duplicate(), array(), slice(), or subsequence() methods may allow an untrusted caller to alter the contents of the original data.

Automated Detection

Sound automated detection of this vulnerability is not feasible. Heuristic approaches may be useful.

Bibliography