FIO05-J. Do not create multiple buffered wrappers on an InputStream

As the java.io.InputStream class is abstract, a wrapper such as BufferedInputStream is required to provide a concrete implementation that overrides its methods. Java input classes, for example Scanner and BufferedInputStream, often buffer the underlying input stream to facilitate fast, non-blocking I/O.

It is permissible to create multiple wrappers on an InputStream. Programs that encourage multiple wrappers around the same stream, however, behave significantly different depending on whether the InputStream allows look-ahead or not. An adversary can exploit this difference in behavior by, for example, redirecting System.in (from a file). This is also possible when a program uses the System.setIn() method to redirect System.in. That said, redirecting input from the console is a standard practice in UNIX based platforms but finds limited application in others such as Windows, where console programs are largely considered outmoded. In general, any input stream that supports non-blocking buffered I/O is susceptible to misuse.

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. Because of 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.

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();
    }
    // Down casting is permitted because InputStream guarantees read() in range  
    // 0..255 if it is not -1
    return (char)input; 
  }

  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");
        // foward to handler
    }
  }
}

Implementation Details

This program was compiled with the command javac InputLibrary.java on a system with Java 1.6.0. When run from the command line with java InputLibrary, the program successfully takes two characters as input and prints them out. However, when run with java InputLibrary < input, where input is a file that contains the exact same input, the program throws an EOFException 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 solution declares the BufferedInputStream as a class variable so that all methods can access it.

public final class InputLibrary {
  private static BufferedInputStream in = new BufferedInputStream(System.in);

  public static char getChar() throws EOFException {
    int input = in.read();
    if (input == -1) {
      throw new EOFException();
    }
    in.skip(1); // This statement is now necessary to go to the next line
                // The noncompliant code example deceptively worked without it
    return (char)input; 
  }

  public static void main(String[] args) {
    try {
      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");
    }
  }
}

It may appear that the mark() and reset() methods of BufferedInputStream would replace the read bytes but this idea is deceptive, for, 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 1.6.0. When run from the command line with java InputLibrary, the program successfully takes two characters as input and prints them out. Also, when run with java InputLibrary < input, where input is a file that contains the exact same input, the program successfully takes two characters as input and print them out.

Compliant Solution

If a program intends to use the library InputLibrary in conjunction with other code that requires user input and consequently needs another buffered wrapper around System.in, its code must use the same buffered wrapper as the one used by the library, instead of creating and using an additional buffered wrapper. The library InputLibrary must return an instance of the buffered wrapper to support this functionality.

public final class InputLibrary {
  private static BufferedInputStream in = new BufferedInputStream(System.in);

  // Other methods

  static BufferedInputStream getBufferedWrapper() {
    return in;
  }
}

// Some code that requires user input from System.in
class AppCode {
  private static BufferedInputStream in;
  
  AppCode() {
    in =  InputLibrary.getBufferedWrapper();
  }
  // Other methods
}

Note that this scheme may not work very well in multi-threaded environments because by default, reading from a stream is not a thread-safe operation. Explicit synchronization is required in such cases.

Risk Assessment

Creating multiple buffered wrappers around 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.

Bibliography

[[API 2006]] method read
[[API 2006]] class BufferedInputStream

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