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

Likewise, an output stream must not have more than one buffered wrapper because multiple wrappers can cause multiple output strings to be output in an unexpected order. For example, the javax.servlet.ServletResponse allows for the creation of a PrintWriter or an OutputStream to hold the response generated by a web servlet. But only one or the other should be used, not both.

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 is 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 it. Consequently, data that remains in the buffer at the end of a particular execution of getChar() is 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)

When compiled under Java 1.6.0 and run from the command line, this program successfully takes two characters as input and prints them out. However, when run with a file redirected to standard input, the program throws 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)

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

Compliant Solution (Accessible Class Variable)

This compliant solution uses both System.in and the InputLibrary class, which creates a buffered wrapper around System.in. Because the InputLibrary class and the remainder of the program must share a single buffered wrapper, the InputLibrary class must export a reference to that wrapper. Code outside the InputLibrary class must use the exported wrapper rather than create and use its own additional buffered wrapper around System.in.

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 compliant solution may be inappropriate 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

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Creating multiple scanners on System.in upsets the predictability of program behavior, especially when System.in has been re-directed.

Noncompliant Code Example

import java.util.Scanner;

public class InputLibrary{

   public static int getInt(){       System.out.println("Please enter an int:");       Scanner in = new Scanner(System.in);       return in.nextInt();    }

   public static int getDouble(){       System.out.println("Please enter a double:");       Scanner in = new Scanner(System.in);       return in.nextDouble();    }

}

Compliant Code Example

import java.util.Scanner;

public class InputLibrary{

   private static Scanner in = new Scanner(System.in);

   public static int getInt(){       System.out.println("Please enter an int:");       return in.nextInt();    }

   public static int getDouble(){       System.out.println("Please enter a double:");       return in.nextDouble();    }

}