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. JDK versions prior to JDK 1.4 required definition of custom methods that manage reversing byte order to maintain compatibility with little-endian systems. Correct handling of byte order related issues is critical when exchanging data in a networked environment that includes both big-endian and little-endian machines. Failure to handle byte ordering issues can cause unexpected program behavior.
Noncompliant Code Example
The read methods (readByte, readShort, readInt, readLong, readFloat and readDouble
) and the corresponding write methods defined by class java.io.DataInputStream
operate only on big-endian data. Use of these methods while interoperating with traditional languages, such as C or C++, is unsafe because such languages lack any guarantees about 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 (Use ByteBuffer
)
This compliant solution uses methods provided by class ByteBuffer
(see [[API 2006]] ByteBuffer) to correctly extract an int
from the original input value. It wraps the input byte array with a ByteBuffer
, sets the byte order to little-endian, and extracts the int
. 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();
Class ByteBuffer
provides analogous get and put methods for the other numeric types.
Compliant Solution (Define Special-Purpose Methods)
An alternative compliant solution is to define read and write methods that support the necessary byte-swapping while reading from or writing to the file. In this example, the readLittleEndianInteger()
method reads four bytes 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 least 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 (Use reverseBytes()
)
When programming for JDK 1.5+, use the reverseBytes()
method defined in the classes Character
, Short
, Integer
, and Long
to reverse the order of the integral value's bytes. Note that classes Float
and Double
lack such a method.
public static int reverse(int i){ return Integer.reverseBytes(i); }
Risk Assessment
Reading and writing data without considering endianness can lead to serious misinterpretations about magnitude and sign, alike.
Guideline |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
---|---|---|---|---|---|
INT07-J |
low |
unlikely |
low |
P3 |
L3 |
Automated Detection
Automated detection is infeasible in the general case.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this guideline on the CERT website.
Related Guidelines
MITRE CWE: CWE-198 "Use of Incorrect Byte Ordering"
Bibliography
[[API 2006]] Class ByteBuffer: Methods wrap
and order
. Class Integer: method reverseBytes
[[Harold 1997]] Chapter 2: "Primitive Data Types, Cross Platform issues"
INT06-J. Avoid incorrect mixing of signed integers with bitwise operators 06. Integers (INT) INT08-J. Provide mechanisms to handle unsigned data when required