INT31-J. Do not rely on write() method to output integers

The write() method that is defined in java.io.OutputStream, takes an integer argument intended to be between 0 and 255. Because an int is otherwise designed to store 4 byte numbers, failure to validate user input may lead to unexpected results.

The general contract for write is that one byte is written to the output stream. The byte to be written is the eight low-order bits of the argument b. The 24 high-order bits of b are ignored. Java Specification

Noncompliant Code Example

The noncompliant code example accepts a value from the user without validating it. If this value is greater than 255, it will result in a wrap around. For instance, write(305) will print '1' since the lower order bits of 305 are preserved while the top 24 order bits are lost. That is, the result is remainder modulo 256 of the absolute value of the input.

class console_write {
  public static void main(String[] args) { 
    //Any input value > 255 will result in unexpected output
    System.out.write(args[0]);
  }
}

Compliant Solution

Use alternate means to output integers such as the System.out.print* methods. Again, performing input validation is extremely critical.

class console_write {
  public static void main(String[] args) { 
    //Perform input validation
    if(args[0] <= 255) {
      System.out.print(args[0]);
    }
    else {
      //handle error 
    } 
  }
}

Risk Assessment

TODO

Automated Detection

TODO

Related Vulnerabilities

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

References

Java I/O, Elliotte Rusty Harold