21. Do not let untrusted code misuse privileges of callback methods

Callbacks provide a means to register a method to be invoked (or called back) when an interesting event occurs. Java uses callbacks for applet and servlet life-cycle events, AWT and Swing event notifications such as button clicks, asynchronous reads and writes to storage, and even in Runnable.run() wherein a new thread automatically executes the specified run() method.

In Java, callbacks are typically implemented using interfaces. The general structure of a callback is as follows:

public interface CallBack {
  void callMethod();
}
 
class CallBackImpl implements CallBack {
  public void callMethod() {
    System.out.println("CallBack invoked");
  }
}

class CallBackAction {
  private CallBack callback;

  public CallBackAction(CallBack callback) {
    this.callback = callback;
  }
 
  public void perform() {
    callback.callMethod();
  }
}
class Client {
  public static void main(String[] args) {
    CallBackAction action = new CallBackAction(new CallBackImpl());
    // ...
    action.perform(); // Prints "CallBack invoked"
  }
}

Callback methods are often invoked without changes in privileges, which means that they may be executed in a context that has more privileges than the context in which they are declared. If these callback methods accept data from untrusted code, privilege escalation may occur.

According to Oracle's Secure Coding Guidelines [SCG 2010],

Callback methods are generally invoked from the system with full permissions. It seems reasonable to expect that malicious code needs to be on the stack in order to perform an operation, but that is not the case. Malicious code may set up objects that bridge the callback to a security checked operation. For instance, a file chooser dialog box that can manipulate the filesystem from user actions, may have events posted from malicious code. Alternatively, malicious code can disguise a file chooser as something benign while redirecting user events.

This guideline is an instance of 17. Minimize privileged code and is related to SEC01-J. Do not allow tainted variables in privileged blocks.

Noncompliant Code Example

This noncompliant code example uses a UserLookupCallBack class that implements the CallBack interface to look up a user's name given the user's ID. This lookup code assumes that this information lives in the /etc/passwd file, which requires elevated privileges to open. Consequently, the Client class invokes all callbacks with elevated privileges (within a doPrivileged block).

public interface CallBack {
  void callMethod();
}
 
class UserLookupCallBack implements CallBack {
  private int uid;
  private String name;

  public UserLookupCallBack(int uid) {
    this.uid = uid;
  }

  public String getName() {
    return name;
  }

  public void callMethod() {
    try (InputStream fis = new FileInputStream("/etc/passwd")) {
      // Look up uid & assign to name
    } catch (IOException x) {
      name = null;
    }
  }
}

final class CallBackAction {
  private CallBack callback;

  public CallBackAction(CallBack callback) {
    this.callback = callback;
  }
 
  public void perform() {
    AccessController.doPrivileged(new PrivilegedAction<Void>() {
        public Void run() {
          callback.callMethod();
          return null;
        }
      });
  }
}

This code could be safely used by a client, as follows:

public static void main(String[] args) {
  int uid = Integer.parseInt(args[0]);

  CallBack callBack = new UserLookupCallBack(uid);
  CallBackAction action = new CallBackAction(callBack);

  // ...
  action.perform(); // Looks up user name
  System.out.println("User " + uid + " is named " + callBack.getName());
}

However, an attacker can use CallBackAction to execute malicious code with elevated privileges by registering a MaliciousCallBack instance:

class MaliciousCallBack implements CallBack {
  public void callMethod() {
    // Code here gets executed with elevated privileges
  }
}

// Client code
public static void main(String[] args) {
  CallBack callBack = new MaliciousCallBack();
  CallBackAction action = new CallBackAction(callBack);
  action.perform(); // Executes malicious code
}

Compliant Solution (callback-local doPrivileged block)

According to Oracle's secure coding guidelines [SCG 2010]:

By convention, instances of PrivilegedAction and PrivilegedExceptionAction may be made available to untrusted code, but doPrivileged must not be invoked with caller-provided actions.

This compliant solution moves the invocation of doPrivileged() out of the CallBackAction code and into the callback itself.

public interface CallBack {
  void callMethod();
}
  
class UserLookupCallBack implements CallBack {
  private int uid;
  private String name;
 
  public UserLookupCallBack(int uid) {
    this.uid = uid;
  }
 
  public String getName() {
    return name;
  }
 
  public void callMethod() {
    AccessController.doPrivileged(new PrivilegedAction<Void>() {
        public Void run() {
          try (InputStream fis = 
            new FileInputStream("/etc/passwd")) {
            // Look up userid & assign to 
            // UserLookupCallBack.this.name
          } catch (IOException x) {
            UserLookupCallBack.this.name = null;
          }
          return null;
        }
      });
  }
}
 
final class CallBackAction {
  private CallBack callback;

  public CallBackAction(CallBack callback) {
    this.callback = callback;
  }
 
  public void perform() {
    callback.callMethod();
  }
}

This code behaves the same as before, but an attacker can no longer run malicious callback code with elevated privileges. Even though an attacker can pass a malicious callback instance using the constructor of class CallBackAction, the code is not executed with elevated privileges because the malicious instance must contain a doPrivileged block that cannot have the same privileges as trusted code. Additionally, class CallBackAction cannot be subclassed to override the perform() method as it is declared final.

Compliant Solution (declare callback final)

This compliant solution declares the UserLookupCallBack class final to prevent overriding of callMethod().

final class UserLookupCallBack implements CallBack {
  // ...    
}
 
// Remaining code is unchanged

Applicability

Exposing sensitive methods through callbacks can result in misuse of privileges and arbitrary code execution.

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