INT01-J. Provide mechanisms to handle unsigned data when required

Java does not support the use of unsigned types, except for the 16 bit char datatype. Sometimes, it is necessary to interoperate with native languages such as C/C++ that use unsigned types extensively. The standard practice to deal with unsigned types is to read their values into Java's larger signed primitives. For example, a signed long is used to hold an unsigned integer.

Noncompliant Code Example

This example incorrectly uses a generic method for reading in integer data irrespective of the signedness. It assumes that the value is always signed and thus treats the most significant bit (MSB) as the sign bit causing misinterpretations about the actual magnitude of the integer.

public static int getInteger(DataInputStream is) throws IOException {
  return is.readInt();	
}

Compliant Solution

This compliant solution reads-in an unsigned integer value into an array of four bytes. The bytes are left shifted the appropriate amount and OR'ed together. However, when the bytes are shifted they are promoted to int. If the byte is negative, then all the left-most bits of the resulting int will be set, and these have to be masked off. Finally, the whole int expression may be negative when it is promoted to the long result and, again, the left-most bits have to be masked off. (cf. [[Harold 97]] but the code there is wrong.)

public static long readInt(InputStream is) throws IOException {
  byte[] buffer = new byte[4];
  int check = is.read(buffer);
  if (check != 4) throw new IOException("Unexpected End of Stream!");
  long result = (buffer[0] << 24) | (0x00FFFFFF&(buffer[1] << 16)) |
       (0x0000FFFF&(buffer[2] << 8)) | (0x000000FF&buffer[3]);
  result &= 0xFFFFFFFFL;
  return result;
}

Risk Assessment

Treating an unsigned type as signed can result in misinterpretations and can lead to erroneous calculations.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 06]] Class DataInputStream: method readInt
[[Harold 97]] Chapter 2: Primitive Data Types, Cross Platform Issues, Unsigned Integers

INT00-J. Provide methods to read and write Little-Endian data      04. Integers (INT)      04. Integers (INT)