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String objects in Java are encoded in UTF-16. Java Platform is required to support other character encodings or charsets such as US-ASCII, ISO-8859-1, and UTF-8.  Errors may occur when converting between differently coded character data.  There are two general types of encoding errors. If the byte sequence is not valid for the specified charset then the input is considered malformed. If the byte sequence cannot be mapped to an equivalent character sequence then an unmappable character has been encountered.

According to the Java API  [API 2014] for the String constructors:

The behavior of this constructor when the given bytes are not valid in the given charset is unspecified.

Similarly, the description of the String.getBytes(Charset) method states:

This method always replaces malformed-input and unmappable-character sequences with this charset's default replacement byte array.

The CharsetEncoder class is used to transform character data into a sequence of bytes in a specific charset.   The input character sequence is provided in a character buffer or a series of such buffers. The output byte sequence is written to a byte buffer or a series of such buffers.  The CharsetDecoder class reverses this process by transforming a sequence of bytes in a specific charset into character data.  The input byte sequence is provided in a byte buffer or a series of such buffers, while the output character sequence is written to a character buffer or a series of such buffers.

Special care should be taken when decoding untrusted byte data to ensure that malformed input or unmappable character errors do not result in defects and vulnerabilities.  Encoding errors can also occur, for example, encoding a cryptographic key containing malformed input for transmission will result in an error. Encoding and decoding errors typically result in data corruption. 

In Java, byte arrays are often used to transmit raw binary data as well as character encoded data. Attempts to read raw binary data as if it were character encoded data often fail, because some of the bytes fall outside the default or specified encoding scheme and hence fail to denote valid characters. For example, converting a cryptographic key containing non-representable bytes to character encoded data for transmission may result in an error.

Also see guidelines FIO02-J. Keep track of bytes read and account for character encoding while reading data and FIO03-J. Specify the character encoding while performing file or network IO.

Noncompliant Code Example

This noncompliant code example is similar to the one used in STR03-J. Do not represent numeric data as strings in that it attempts to convert the a byte array representing a BigInteger into containing the two's-complement representation of this BigInteger value to a String. Because some of the bytes do not denote valid characters, the resulting String representation loses information. Converting the String back to a BigInteger produces a different valuethe byte array contains malformed-input sequences, the behavior of the String constructor is unspecified.

import java.math.BigInteger;
import java.nio.CharBuffer;

public class CharsetConversion {
  public static void main(String[] args) {
    BigInteger x = new BigInteger ("530500452766");
    byte [] byteArray = x.toByteArray();
 // convert to byte array
String s = new String(byteArray);
    // s prints as "{„J?ž" -
                                     // the fourth character is invalid

// convert s back to a BigInteger
byteArray = s.getBytes();       // convert to bytes
x = new BigInteger(byteArray);  // now x = 530500435870

Compliant Solution

This compliant solution first produces a String representation of the BigInteger, then converts the String to a byte array. This process is reversed on input. Because the textual representation in the String was generated by the BigInteger class, it contains valid characters.

BigInteger x = new BigInteger ("530500452766");
String s = x.toString();  // valid character data

byte [] byteArray = s.getBytes("UTF8");
String ns = new String(byteArray, "UTF8");  // ns prints as "530500452766"

BigInteger x1 = new BigInteger(ns); // construct the original BigInteger

Do not try to convert the String object to a byte array to obtain the original BigInteger. Character encoded data may yield a byte array which when converted to a BigInteger, results in a completely different value.

Risk Assessment

Attempting to read a byte array containing raw character data as if it were character data may produce erroneous results.

 System.out.println(s);
  }
}

Compliant Solution

The java.nio.charset.CharsetEncoder and java.nio.charset.CharacterDecoder provide greater control over the process.  In this compliant solution, the CharsetDecode.decode() method is used to convert the byte array containing the two's-complement representation of this BigInteger value to a CharBuffer.  Because the bytes do not represent a valid UTF-16, the input is considered malformed, and a MalformedInputException is thrown.

import java.math.BigInteger;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.CharacterCodingException;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.MalformedInputException;
import java.nio.charset.StandardCharsets;
import java.nio.charset.UnmappableCharacterException;

public class CharsetConversion {
  public static void main(String[] args) {
    CharBuffer charBuffer;
    CharsetDecoder decoder = StandardCharsets.UTF_16.newDecoder();
    BigInteger x = new BigInteger("530500452766");
    byte[] byteArray = x.toByteArray();
    ByteBuffer byteBuffer = ByteBuffer.wrap(byteArray);
    try {
      charBuffer = decoder.decode(byteBuffer);
      s = charBuffer.toString();
      System.out.println(s);
    } catch (IllegalStateException e) {
      e.printStackTrace();
    } catch (MalformedInputException e) {
      e.printStackTrace();
    } catch (UnmappableCharacterException e) {
      e.printStackTrace();
    } catch (CharacterCodingException e) {
      e.printStackTrace();
    }
  }
}

Risk Assessment

Malformed input or unmappable character errors can result in a loss of data integrity.

Related

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

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] class [String|http://java.sun.com/javase/6/docs/api/java/lang/String.html]

Guidelines

Bibliography

Image Added Image Added Image Added FIO10-J. Do not let Runtime.exec() fail or block indefinitely      Input Output (FIO)      Input Validation and Data Sanitization (IDS)