INT07-J. Provide methods to read and write Little-Endian data

In Java, data is stored in big-endian format (also called network order). That is, all data is represented sequentially starting from the most significant bit (MSB) to the least significant. Prior to JDK 1.4, custom methods had to be defined to be compatible with little-endian systems that use the reverse byte order. Handling byte order related issues is critical when data is to be exchanged in a networked environment that consists of machines varying in endianness. Failure to handle data ordering issues can cause misinterpretations and unexpected program behavior.

Noncompliant Code Example

The class java.io.DataInputStream defines read methods (readByte, readShort, readInt, readLong, readFloat and readDouble) and the corresponding write methods. All these methods work with big-endian data only. The use of these methods can be unsafe while interfacing with traditional languages such as C or C++, that do not provide any guarantees on endianness. This noncompliant code example shows such a discrepancy.

DataInputStream dis = new DataInputStream(
  new FileInputStream("data"));  // Little-endian data might be read as big-endian
int serialNumber = dis.readInt();

Compliant Solution

This compliant solution wraps the byte array containing the integer bytes read-in into a ByteBuffer and sets the byte order to little-endian. The result is stored in the integer serialNumber.

	 
DataInputStream dis = new DataInputStream(
  new FileInputStream("data"));

byte[] buffer= new byte[4];
int bytesRead = dis.read(buffer);  // Bytes are read into buffer
int serialNumber = ByteBuffer.wrap(buffer).order(ByteOrder.LITTLE_ENDIAN).getInt();

Compliant Solution

Assuming that an integer value is to be read from the file, read and write methods can be defined for handling little-endian data. The readLittleEndianInteger() method reads data into a byte buffer and then pieces together the integer in the right order. The writeLittleEndianInteger() method obtains bytes by repeatedly casting the integer so that the most significant byte is extracted on successive right shifts. Long values can be handled by defining a byte buffer of size eight.

	 
// read method
public static int readLittleEndianInteger(InputStream ips) throws IOException {
  byte[] buffer = new byte[4];
  int check = ips.read(buffer);

  if (check != 4) {
    throw new IOException("Unexpected End of Stream");
  }
 
  int result = (buffer[3] << 24) | (buffer[2] << 16) | (buffer[1] << 8) | buffer[0];
  return result;
}

// write method
public static void writeLittleEndianInteger(int i, OutputStream ops) throws IOException {
  byte[] buffer = new byte[4];
  buffer[0] = (byte) i;
  buffer[1] = (byte) (i >> 8);
  buffer[2] = (byte) (i >> 16);
  buffer[3] = (byte) (i >> 24);
  ops.write(buffer);
}

Compliant Solution

In JDK 1.5+, the reverseBytes() method defined in the classes Character, Short, Integer and Long can be used to reverse the order of the bytes constituting the integer. Note that there is no such method for float and double values.

	 
public static int reverse(int i){
  return Integer.reverseBytes(i);
}

Risk Assessment

Reading and writing data without considering endianness may lead to serious misinterpretations about magnitude and sign, alike.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 2006]] Class ByteBuffer: Methods wrap and order. Class Integer: method reverseBytes
[[Harold 1997]] Chapter 2: Primitive Data Types, Cross Platform issues
[[MITRE 2009]] CWE ID 198 "Use of Incorrect Byte Ordering"

INT06-J. Do not use bitwise operators on integers incorrectly      06. Integers (INT)      INT08-J. Provide mechanisms to handle unsigned data when required