SEC00-J. Do not allow privileged blocks to leak sensitive information outside a trust boundary

The java.security.AccessController class is part of Java's security mechanism; it is responsible for enforcing whatever security policy is applied to code. This class's static method doPrivileged() can be used to execute a block of code while relaxing a security policy. So the block of code effectively runs with elevated privileges.

Consequently, any method that contains a doPrivileged() method call must assume responsibility for enforcing its own security on the code block supplied to doPrivileged(). Likewise any code in the doPrivileged() method must take care to prevent sensitive information from leaking out of a trust boundary.

For example, suppose that a web application must maintain a sensitive file of passwords for a web service, and further that it must also run untrusted code. The application could then enforce a security policy preventing the majority of its own code, and all untrusted code, from accessing the sensitive file. Because it must also provide mechanisms for adding and changing passwords, it can use the doPrivileged() method to temporarily bypass its own security policy for the purpose of managing passwords. In this case, any privileged block must prevent any 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 to its caller. 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
  }

  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 a 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)

Both 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 must also not be allowed to leak outside the trust boundary.

This compliant solution suppresses the exception, using a null return 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.

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() {
        public Object 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. 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 and prove that nothing sensitive leaks out of 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

Bibliography

14. Platform Security (SEC)      14. Platform Security (SEC)