FIO03-J. Specify the character encoding while performing file or network IO

Every Java platform has a default character encoding. The available encodings are listed in the Supported Encodings document [[Encodings 06]]. The default encoding is used when a character is converted to a sequence of bytes and vice versa. If characters are converted into an array of bytes to be sent as output, transmitted across some medium, input and converted back into characters, then the same encoding must be used on both sides of the conversation.

According to the Java API [API 06] for the String class:

The length of the new String is a function of the charset, and hence may not be equal to the length of the byte array. The behavior of this constructor when the given bytes are not valid in the given charset is unspecified.

Also, see the related guideline FIO02-J. Keep track of bytes read and account for character encoding while reading data.

Noncompliant Code Example

This noncompliant code example reads a byte array and converts it into a String using the default character encoding for the platform. If this is not the same encoding as the one that was used to produce the byte array, the resulting String is likely to be incomprehensible because some of the bytes may not have valid character representations in the default encoding.

FileInputStream fis = new FileInputStream("SomeFile");
DataInputStream dis = new DataInputStream(fis);
int bytesRead = 0;
byte[] data = new byte[1024];

bytesRead = dis.readFully(data);

if (bytesRead > 0) {
  String result = new String(data);
}

Compliant Solution

This compliant solution explicitly specifies the encoding by passing the string encoding as the second argument to the String constructor.

String encoding = "SomeEncoding" // for example, "UTF-16LE"

FileInputStream fis = new FileInputStream("SomeFile");
DataInputStream dis = new DataInputStream(fis);
int bytesRead = 0;
byte[] data = new byte[1024];

bytesRead = dis.readFully(data);

if (bytesRead > 0) {
   String result = new String(data, encoding);
}

Exceptions

EX1: If the data is coming from another Java application that uses the same platform and it is known that the application is using the default character encoding, an explicit character encoding is not required to be specified on the receiving side.

Risk Assessment

Failure to specify the character encoding while performing file or network IO can corrupt the data.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[Encodings 06]]

FIO02-J. Keep track of bytes read and account for character encoding while reading data      09. Input Output (FIO)      FIO30-J. Do not log sensitive information