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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-J. Avoid granting excess privileges) but also by ensuring that privileged blocks contain code contains only those operations that require the increased privileges. Superfluous code contained within a privileged block necessarily operates must operate with the privileges of that block; this increases , increasing the potential attack surface exposed to an attacker. Consequently, privileged blocks must not contain superfluous code.

Noncompliant Code Example

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

Code Block

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public void changePassword(String currentPassword, String newPassword) {
  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 Check whether oldPassword matches the one in the file ...
        // If not, throw an exception
        System.loadLibrary("LibNameauthentication");
      } catch (FileNotFoundException cnf) {
        // Forward to handler
      }
      return null;
    }
  }); // endEnd of doPrivileged()
}

This example violates the principle of least privilege because a caller who does not have the required privileges may also be able to cause the specified library to be loadedbecause an unprivileged caller could also cause the authentication library to be loaded. An unprivileged caller cannot invoke the System.loadLibrary() method directly because this could expose native methods to the unprivileged code [SCG 2010]. Furthermore, the System.loadLibrary() method checks only the privileges of its immediate caller, so it should be used only with great care. For more information, see SEC52-J. Do not expose methods that use reduced-security checks to untrusted code.

Compliant Solution

This compliant solution moves the call to System.loadLibrary() outside the doPrivileged() block. Doing so allows unprivileged code to perform preliminary password reset checks using the file but prevents it from loading the authentication library.

Code Block

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public void changePassword(String currentPassword, String newPassword) {
  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);                                                    
        // Check whether oldPassword matches the one in the file
        // If not, throw an exception
      } catch (FileNotFoundException cnf) {
        // Forward to handler
      }
      return null;
    }
  });  // endEnd of doPrivileged()
  // Operations on the file using handle f[0]
  // while ensuring that the f[0] reference     
  // remains contained within changePassword()
  System.loadLibrary("LibNameauthentication");
}

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).

The loadLibrary() invocation could also occur before preliminary password reset checks are performed; in this example, it is deferred for performance reasons.

Applicability

Minimizing privileged code reduces the attack surface of an application and simplifies the Minimizing the amount of code that requires elevated privileges eases the necessary task of auditing privileged code.