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Do not create multiple wrappers that buffer input from an InputStream. Instead, create and use only one wrapper, either by passing it as an argument to the methods that need it or declaring it as a class variable.

Noncompliant Code Example

Despite just one declaration, this noncompliant code example creates multiple BufferedInputStream wrappers on System.in because each time getChar() is called, it conceives a new BufferedInputStream. Due to the inherent channeling and buffering mechanism, the data that is read from the underlying stream once cannot be replaced so that a second call can read the same data again. While this code uses a BufferedInputStream to illustrate that any buffered wrapper is unsafe, this condition is also exploitable if a Scanner is used instead.

import java.io.BufferedInputStream;
import java.io.EOFException;

public final class InputLibrary {
  public static char getChar() throws EOFException {
    BufferedInputStream in = new BufferedInputStream(System.in); // wrapper
    int input = in.read();
    if (input == -1) {
    	throw new EOFException();
    }
    return (char)input; //okay because InputStream guarantees read() in range 0..255 if it is not -1
  }

  public static void main(String[] args) {
    try {
      // Either redirect input from the console or use System.setIn(new FileInputStream("input.dat")); 
      System.out.print("Enter first initial: ");
      char first = getChar();
      System.out.println("Your first initial is " + first);
      System.out.print("Enter last initial: ");
      char last = getChar();
      System.out.println("Your last initial is " + last);
    } catch(EOFException e) {
      System.out.println("ERROR");
    }
  }
}

Implementation Details

This program was compiled with the command javac InputLibrary.java on a system with Java 1.5.0. When run from the command line with java InputLibrary, the program will successfully take two characters as input and print them out. However, when run with java InputLibrary < input, where input is a file that contains the exact same input, the program prints "ERROR" because the second call to getChar() finds no characters to read upon encountering the end of the stream.

Compliant Solution

Create and use only a single BufferedInputStream on System.in. This compliant code example declares the BufferedInputStream as a class variable so that all methods can access it. However, if a program were to use this library in conjunction with other input from a user that also needs some buffered wrapper on System.in, the library would need to be modified so that all code uses the same buffered wrapper instead of creating additional ones.

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It may appear that the mark() and reset() methods of BufferedInputStream would "replace" the read bytes but this idea is deceptive since these methods provide look-ahead by operating on the internal buffers and not directly on the underlying stream.

Implementation Details

This program was compiled with the command javac InputLibrary.java on a system with Java 1.5.0. When run from the command line with java InputLibrary, the program will successfully take two characters as input and print them out. Also, when run with java InputLibrary < input, where input is a file that contains the exact same input, the program will successfully take two characters as input and print them out.

Risk Assessment

Creating multiple buffered wrappers on an InputStream can cause unexpected program behavior when the InputStream is re-directed.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[API 06|AA. Java References#API 06]\] [method read|http://java.sun.com/javase/6/docs/api/java/io/InputStream.html#read()]
\[[API 06|AA. Java References#API 06]\] [class BufferedInputStream|http://java.sun.com/javase/6/docs/api/java/io/BufferedInputStream.html]