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A character encoding or charset specifies the binary representation of the coded character set.   Every instance of the Java virtual machine Virtual Machine (JVM) has a default charset, which may or may not be one of the standard charsets. The default charset is determined during virtual-machine startup and typically depends upon on the locale and charset being used by the underlying operating system [API 2014]. The default character encoding that can be set at startup time, for example:

java -Dfile.encoding=UTF-8 … ... com.x.Main

The available encodings are listed in the Supported Encodings document [Encodings 2014].   In the absence of an explicitly specified encoding, conversions use the system default encoding. Compatible encodings must be used when characters are output as an array of bytes then input by another JVM and subsequently converted back to characters.

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

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

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This noncompliant code example reads a byte array and converts it into a String using the platform's default character encoding. If the byte array does not represent a string, or if it represents a string that was encoded using an encoding other than the default encoding, the resulting String is likely to be incorrect. The behavior resulting from malformed-input and unmappable-character errors is unspecified.

FileInputStream fis = null;
try {
  fis = new FileInputStream("SomeFile");
  DataInputStream dis = new DataInputStream(fis);
  byte[] data = new byte[1024];
  dis.readFully(data);
  String result = new String(data);
} catch (IOException x) {
  // handleHandle error
} finally {
  if (fis != null) {
    try {
      fis.close();
    } catch (IOException x) {
      // Forward to handler
    }
  }
}

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This compliant solution explicitly specifies the character encoding used to create the output the string (in this example, UTF-16LE) as the second argument to the String constructor.   The LE form of UTF-16 uses little-endian byte serialization (least significant byte first). Provided that the character data was encoded in UTF-16LE, it will decode correctly.

FileInputStream fis = null;
try {
  fis = new FileInputStream("SomeFile");
  DataInputStream dis = new DataInputStream(fis);
  byte[] data = new byte[1024];
  dis.readFully(data);
  String result = new String(data, "UTF-16LE");
} catch (IOException x) {
  // handleHandle error
} finally {
  if (fis != null) {
    try {
      fis.close();
    } catch (IOException x) {
      // Forward to handler
    }
  }
}

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Using incompatible encodings when communicating string data between JVMs can result in corrupted data.

Automated Detection

Sound automated detection of this vulnerability is not feasible.

Bibliography

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