FIO06-J. Do not create multiple buffered wrappers on a single InputStream

Java input classes such as Scanner and BufferedInputStream facilitate fast, non-blocking I/O by buffering an underlying input stream. Programs can create multiple wrappers on an InputStream. Programs that use multiple wrappers around a single stream, however, can behave unpredictably depending on whether the wrappers allow look-ahead. An attacker can exploit this difference in behavior by, for example, redirecting System.in (from a file) or by using the System.setIn() method to redirect System.in. In general, any input stream that supports non-blocking buffered I/O is susceptible to this form of misuse.

An input stream must not have more than one buffered wrapper. Instead, create and use only one wrapper per input stream, either by passing it as an argument to the methods that need it or by declaring it as a class variable.

Noncompliant Code Example

This noncompliant code example creates multiple BufferedInputStream wrappers on System.in, even though there is only one declaration of a BufferedInputStream. The getChar() method creates a new BufferedInputStream each time it is called. Data that are read from the underlying stream and placed in the buffer during execution of one call cannot be replaced in the underlying stream so that a second call has access to them. Consequently, data that remain in the buffer at the end of a particular execution of getChar() are lost. Although this noncompliant code example uses a BufferedInputStream, any buffered wrapper is unsafe; this condition is also exploitable when using a Scanner, for example.

public final class InputLibrary {
  public static char getChar() throws EOFException, IOException {
    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.err.println("ERROR");
      // Forward to handler
    } catch (IOException e) {
      System.err.println("ERROR");
      // Forward to handler
    }
  }
}

Implementation Details (POSIX)

This program was compiled under Java 1.6.0. When run from the command line, the program successfully takes two characters as input and prints them out. However, when run with a file redirected to standard input, the program throws an EOFException because the second call to getChar() finds no characters to read upon encountering the end of the stream.

It may appear that the mark() and reset() methods of BufferedInputStream could be used to replace the read bytes. However, these methods provide look-ahead by operating on the internal buffers of the BufferedInputStream rather than by operating directly on the underlying stream. Because the example code creates a new BufferedInputStream on each call to getchar(), the internal buffers of the previous BufferedInputStream are lost.

Compliant Solution (Class Variable)

Create and use only a single BufferedInputStream on System.in. This compliant solution ensures that all methods can access the BufferedInputStream by declaring it as a class variable.

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

  public static char getChar() throws EOFException, IOException {
    int input = in.read();
    if (input == -1) {
      throw new EOFException();
    }
    in.skip(1); // This statement is to advance to the next line
                // The noncompliant code example deceptively
                // appeared to work without it (in some cases)
    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.err.println("ERROR");
      // Forward to handler
    } catch (IOException e) {
       System.err.println("ERROR");
       // Forward to handler
    }
  }
}

Implementation Details (POSIX)

This program was compiled under Java 1.6.0. When run from the command line, the program successfully takes two characters as input and prints them out. Unlike the NCCE, this program also produces correct output when run with a file redirected to standard input.

Compliant Solution (Accessible Class Variable)

If another class needs to use System.in as well as the InputLibrary class, the class must use the same buffered wrapper that is used by InputLibrary, rather than creating and using its own additional buffered wrapper. Consequently InputLibrary must provide a reference to its buffered wrapper for such classes.

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

  static BufferedInputStream getBufferedWrapper() {
    return in;
  }

  // ...other methods
}


// Some code that requires user input from System.in
class AppCode {
  private static BufferedInputStream in;

  AppCode() {
    in = InputLibrary.getBufferedWrapper();
  }

  // ...other methods
}

Note that reading from a stream is not a thread-safe operation by default; consequently, this scheme may not work well in multithreaded environments. In such cases, explicit synchronization is required.

Risk Assessment

Creating multiple buffered wrappers around an InputStream can cause unexpected program behavior when the InputStream is redirected.

Automated Detection

Sound automated detection of this vulnerability is not feasible in the general case. Heuristic approaches may be useful.

Bibliography

FIO05-J. Do not expose buffers created using the wrap() or duplicate() methods to untrusted code      12. Input Output (FIO)