02. Limit the lifetime of sensitive data

Sensitive data in memory can be vulnerable to compromise. An adversary who can execute code on the same system as an application may be able to access such data if the application

• Uses objects to store sensitive data whose contents are not cleared or garbage-collected after use.
• Has memory pages that can be swapped out to disk as required by the operating system (for example, to perform memory management tasks or to support hibernation).
• Holds sensitive data in a buffer (such as BufferedReader) that retains copies of the data in the OS cache or in memory.
• Bases its control flow on reflection that allows circumventing countermeasures to limit the lifetime of sensitive variables.
• Reveals sensitive data in debugging messages, log files, environment variables, or through thread and core dumps.

Using such attacks to compromise sensitive data becomes harder if the memory containing the data has been cleared. Sensitive data that remains live beyond the minimum period required for its use has an unnecessarily large window of vulnerability. Consequently, programs must minimize the lifetime of sensitive data.

Currently, complete mitigation (that is, complete protection of data in memory) requires support from the underlying operating system and Java Virtual Machine. For instance, if swapping sensitive data out to disk is an issue, a secure operating system that disables swapping and hibernation is required.

Noncompliant Code Example

This noncompliant code example reads login information from the console and stores the password as a String object. The credentials remain exposed until the garbage collector reclaims the memory associated with the String.

class Password {
  public static void main (String args[]) throws IOException {
    Console c = System.console();
    if (c == null) {
      System.err.println("No console.");
      System.exit(1);
    }

    String login = c.readLine("Enter your user name: ");
    String password = c.readLine("Enter your password: ");

    if (!verify(login, password)) {
      throw new SecurityException("Invalid Credentials");     
    }
    // ...
  }

  // Dummy verify method, always returns true   
  private static final boolean verify(String login, String password) {
    return true;
  }
}

Compliant Solution

This compliant solution uses the Console.readPassword() method to obtain the password from the console. This method allows the password to be returned as a sequence of characters rather than as a String object. Consequently, the programmer can clear the password from the array immediately after use. The method also disables echoing of the password to the console.

class Password {
  public static void main (String args[]) throws IOException {
    Console c = System.console();
    
    if (c == null) {
      System.err.println("No console.");
      System.exit(1);
    }

    String login = c.readLine("Enter your user name: ");
    char[] password = c.readPassword("Enter your password: ");
  
    if (!verify(login, password)) {
      throw new SecurityException("Invalid Credentials");     
    }
  
    // Clear the password
    Arrays.fill(password, ' ');
  }

  // Dummy verify method, always returns true   
  private static final boolean verify(String login, char[] password) {
    return true;
  }
}

Noncompliant Code Example

This noncompliant code example uses a BufferedReader to wrap an InputStreamReader object so that sensitive data can be read from a file:

void readData() throws IOException{
  BufferedReader br = new BufferedReader(new InputStreamReader(
  new FileInputStream("file")));
  // Read from the file

}

Compliant Solution

This compliant solution uses a directly allocated NIO (new I/O) buffer to read sensitive data from the file. The data can be cleared immediately after use and is not cached or buffered at multiple locations. It exists only in the system memory.

void readData() throws IOException{
  ByteBuffer buffer = ByteBuffer.allocateDirect(16 * 1024);
  try(FileChannel rdr = (new FileInputStream("file")).getChannel()){
   	while (rdr.read(buffer) > 0) {
      // Do something with the buffer
	  buffer.clear();
	}
  } 
  catch (Exception e) {
    // Handle error
  }
} 

Note that manual clearing of the buffer data is mandatory because direct buffers are exempt from garbage collection.

Applicability

Failure to limit the lifetime of sensitive data can lead to information leaks.

This rule may be violated when both of the following are true:

1. It can be proved that the code is free from other errors that can expose the sensitive data.
2. Attackers lack physical access to the target machine.

Bibliography