SEC51-JG. Minimize privileged code

Programs must comply with the principle of least privilege not only by providing privileged blocks with the minimum permissions required for correct operation (see SEC50-JG. Avoid granting excess privileges), but also by ensuring that privileged code contains only those operations that require the increased privileges. Superfluous code contained within a privileged block necessarily operates with the privileges of that block; increasing the attack surface.

Noncompliant Code Example

This noncompliant code example shows a changePassword() method that attempts to open a password file within a doPrivileged block. The doPrivileged block also contains logic that operates on the file and a superfluous System.loadLibrary() call.

public void changePassword() {
  final FileInputStream f[] = { null };

  AccessController.doPrivileged(new PrivilegedAction() {
    public Object run() {
      try {
        String passwordFile = System.getProperty("user.dir") + File.separator + "PasswordFileName";
        f[0] = new FileInputStream(passwordFile);                                                     
        // Operate on the file ...
        System.loadLibrary("LibName");
      } catch (FileNotFoundException cnf) {
        // Forward to handler
      }
      return null;
    }
  }); // end of doPrivileged()
}

This example violates the principle of least privilege because an unprivileged caller will also cause the specified library to be loaded.

Compliant Solution

This compliant solution moves the call to System.loadLibrary() outside the doPrivileged() block.

public void changePassword() {
  final FileInputStream f[] = { null };

  AccessController.doPrivileged(new PrivilegedAction() {
    public Object run() {
      try {
        String passwordFile = System.getProperty("user.dir") + File.separator + "PasswordFileName";
        f[0] = new FileInputStream(passwordFile);                                                     
      } catch (FileNotFoundException cnf) {
        // Forward to handler
      }
      return null;
    }
  });  // end of doPrivileged()
  // Operations on the file using handle f[0]
  // while ensuring that the f[0] reference     
  // remains contained within changePassword()
  System.loadLibrary("LibName");
}

The open FileInputStream f[0] must not be allowed to escape out of changePassword() (see SEC00-J. Do not allow privileged blocks to leak sensitive information across a trust boundary).

Minimizing the amount of code that requires elevated privileges eases the necessary task of auditing privileged code.

Applicability

Failure to follow the principle of least privilege can lead to privilege escalation.

Related Guidelines

Bibliography