The java.security.AccessController class is part of Java's security mechanism; it is responsible for enforcing the applicable security policy. This class's static doPrivileged() method executes a code block with a relaxed security policy. The doPrivileged() method stops permissions from being checked further down the call chain.

Consequently, any method that invokes doPrivileged() must assume responsibility for enforcing its own security on the code block supplied to doPrivileged(). Likewise, code in the doPrivileged() method must not leak sensitive information or capabilities.

For example, suppose that a web application must maintain a sensitive password file for a web service and also must run untrusted code. The application could then enforce a security policy preventing the majority of its own code—as well as all untrusted code—from accessing the sensitive file. Because it must also provide mechanisms for adding and changing passwords, it can call the doPrivileged() method to temporarily allow untrusted code to access the sensitive file. In this case, any privileged block must prevent all information about passwords from being accessible to untrusted code.

Noncompliant Code Example

In this noncompliant code example, the doPrivileged() method is called from the openPasswordFile() method. The openPasswordFile() method is privileged and returns a FileInputStream for the sensitive password file. Because the method is public, it could be invoked by an untrusted caller.

public class PasswordManager {

  public static void changePassword() throws FileNotFoundException {
    FileInputStream fin = openPasswordFile();

    // Test old password with password in file contents; change password,
    // then close the password file
  }

  public static FileInputStream openPasswordFile()
      throws FileNotFoundException {
    final String password_file = "password";
    FileInputStream fin = null;
    try {
      fin = AccessController.doPrivileged(
        new PrivilegedExceptionAction<FileInputStream>() {
          public FileInputStream run() throws FileNotFoundException {
            // Sensitive action; can't be done outside privileged block
            FileInputStream in = new FileInputStream(password_file);
            return in;
          }
      });
    } catch (PrivilegedActionException x) {
      Exception cause = x.getException();
      if (cause instanceof FileNotFoundException) {
        throw (FileNotFoundException) cause;
      } else {
        throw new Error("Unexpected exception type", cause); 
      }
    }
    return fin;
  }
}

Compliant Solution

In general, when any method containing a privileged block exposes a field (such as an object reference) beyond its own boundary, it becomes trivial for untrusted callers to exploit the program. This compliant solution mitigates the vulnerability by declaring openPasswordFile() to be private. Consequently, an untrusted caller can call changePassword() but cannot directly invoke the openPasswordFile() method.

public class PasswordManager {
  public static void changePassword() throws FileNotFoundException {
    // ...
  }

  private static FileInputStream openPasswordFile() 
     throws FileNotFoundException {
    // ...
  }
}

Compliant Solution (Hiding Exceptions)

The previous noncompliant code example and the previous compliant solution throw a FileNotFoundException when the password file is missing. If the existence of the password file is itself considered sensitive information, this exception also must be prevented from leaking outside the trusted code.

This compliant solution suppresses the exception, leaving the array to contain a single null value to indicate that the file does not exist. It uses the simpler PrivilegedAction class rather than PrivilegedExceptionAction to prevent exceptions from propagating out of the doPrivileged() block. The Void return type is recommended for privileged actions that do not return any value.

class PasswordManager {

  public static void changePassword() {
    FileInputStream fin = openPasswordFile();
    if (fin == null) {
      // No password file; handle error
    }

    // Test old password with password in file contents; change password
  }

  private static FileInputStream openPasswordFile() {
    final String password_file = "password";
    final FileInputStream fin[] = { null };
    AccessController.doPrivileged(new PrivilegedAction<Void>() {
        public Void run() {
          try {
            // Sensitive action; can't be done outside
            // doPrivileged() block
            fin[0] = new FileInputStream(password_file);
          } catch (FileNotFoundException x) {
            // Report to handler
          }
          return null;
        }
    });
    return fin[0];
  }
}

Risk Assessment

Returning references to sensitive resources from within a doPrivileged() block can break encapsulation and confinement and can leak capabilities. Any caller who can invoke the privileged code directly and obtain a reference to a sensitive resource or field can maliciously modify its elements.

Automated Detection

Identifying sensitive information requires assistance from the programmer; fully automated identification of sensitive information is beyond the current state of the art.

Assuming user-provided tagging of sensitive information, escape analysis could be performed on the doPrivileged() blocks to prove that nothing sensitive leaks out from them. Methods similar to those used in thread-role analysis could be used to identify the methods that must, or must not, be called from doPrivileged() blocks.

Related Guidelines

Android Implementation Details

The java.security package exists on Android for compatibility purposes only, and it should not be used.

Bibliography