SEC05-J. Minimize accessibility of classes and their members

Classes and class members (classes, interfaces, fields and methods) are subject to access control in Java. The access is indicated by an access modifier: public, protected, private, or the absence of an access modifier (the default access). A simplified view of the access control rules is summarized in the following table. An 'x' conveys that the particular access is permitted from within that domain.

*For referencing protected members, the accessing class can be a sub-class in either the same or a different package.

Classes and class members should be given the minimum access possible so that malicious code has the least chance of manipulating the system. As far as possible, sensitive classes should avoid implementing interfaces. This is because only public methods are allowed to be declared within interfaces. Additionally, even if a class's visibility is default, it can be susceptible to misuse if it exposes a public method.

Noncompliant Code Example

In this noncompliant example, the class PublicClass is declared public. The member function getPoint as well as the (x, y) coordinates are also declared public. This gives world-access to the class members. A real world vulnerability, for example, can arise when a malicious applet attempts to access the credit card field of another object that is declared public. Note that a non-public class may also be vulnerable if its members are declared public.

public class PublicClass {
  public int x;
  public int y;
	
  public void getPoint() {
     System.out.println("(" + x + "," + y + ")");  
  }	
}

Compliant Solution

Limiting the scope of classes, interfaces, methods and fields as far as possible reduces the chance of malicious manipulation. Limit the accessibility depending on the desired implementation scope. This also helps eliminate the threat of malicious overriding. The most restrictive access conditions are demonstrated in this compliant solution.

final class PrivateClass {
  private int x;
  private int y;
	
  private void getPoint() { 
     System.out.println("(" + x + "," + y + ")");  
  }	
}

Noncompliant Code Example

This noncompliant code example overrides the finalize() method of the superclass and changes its accessibility from protected to public.

According to Sun's Secure Coding Guidelines [[SCG 07]]:

In addition, refrain from increasing the accessibility of an inherited method, as doing so may break assumptions made by the superclass. A class that overrides the protected java.lang.Object.finalize method and declares that method public, for example, enables hostile callers to finalize an instance of that class, and to call methods on that instance after it has been finalized. A superclass implementation unprepared to handle such a call sequence could throw runtime exceptions that leak private information, or that leave the object in an invalid state that compromises security. One noteworthy exception to this guideline pertains to classes that implement the java.lang.Cloneable interface. In these cases, the accessibility of the Object.clone method should be increased from protected to public.

final class SubClass extends Base {
  public void finalize() {
    // ...
  }
}

Compliant SOlution

This compliant solution correctly declares the finalize() method protected.

final class SubClass extends Base {
  protected void finalize() {
    // ... 
  }
}

Risk Assessment

Granting unnecessary access weakens the security of Java applications.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[JLS 05]] Section 6.6, Access Control
[[SCG 07]] Guideline 1-1 Limit the accessibility of classes, interfaces, methods, and fields
[[Campione 96]] Access Control
[[McGraw 00]] Chapter 3, Java Language Security Constructs
[[Bloch 08]] Item 13: Minimize the accessibility of classes and members

SEC06-J. Assume that all Java clients can be reverse engineered, monitored, and modified      01. Platform Security (SEC)      ENV30-J. Create a secure sandbox using a Security Manager