SEC06-J. Do not use APIs that perform access checks against the immediate caller

It is possible to reflectively access fields and methods of one object from another. Language access checks are enforced by the JVM to ensure policy compliance, while doing so. For instance, although an object is not normally allowed to access private members or invoke methods of another class, the APIs belonging to the java.lang.reflect package allow an object to do so contingent upon performing the language access checks.

The table below lists the APIs that should be used with care.

Note that the language access checks do not apply to java.lang.reflect.Field.setAccessible/getAccessible methods but to the remaining set* and get* field methods. The former APIs are protected by standard security manager checks.

Noncompliant Code Example

In this noncompliant code snippet, the package-private field i of class C can be accessed from class ReflectionExample. Method makeAccessible accepts fieldName as an input parameter which can be supplied by untrusted code. This is dangerous because despite the untrusted code not having the same capabilities as that of the immediate caller (method makeAccessible), it is allowed to carry out sensitive operations. In this case, the immediate caller has the capability of modifying package-private fields without triggering any language access checks. Hostile code should not be allowed to make such modifications by using it as an oracle.

// Class 'ReflectionExample' and 'C' belong to the same package
public class ReflectionExample {
  public static void makeAccessible(String fieldName) {
    C c = new C();
    try {
      Field f = c.getClass().getDeclaredField(fieldName);
      System.out.println(f.getInt(c)); // prints 10
      f.setInt(c, 1);  // set to 1; bypasses language access checks
      System.out.println(f.getInt(c)); // now prints 1
    }
    catch(NoSuchFieldException nsfa){}
    catch(IllegalAccessException iae) {}
  }
}

class C {
  int i = 10; // package-private
}

Compliant Solution

Do not operate on tainted inputs provided by untrusted code. Likewise, do not return values to an untrusted caller. If you must use Reflection, make sure that the immediate caller (method) is isolated from hostile code by declaring it final, reducing it's scope to private and making it non-static. Also, declare sensitive fields in other classes (Class c) as private.

private final void makeAccessible() { // private final
  String fieldName = "i"; // hardcode
  C c = new C();
  // ...
} 

class C {
  private int i = 10; // private
}

The permission ReflectPermission with action suppressAccessChecks should also not be granted so that the security manager blocks attempts to access private fields of other classes. (See ENV04-J. Do not grant ReflectPermission with target suppressAccessChecks)

Noncompliant Code Example

The class Trusted uses a package-private constructor in this noncompliant code example. It is desired that the code that exists outside the package be not allowed to create a new instance of an arbitrary class. However, since the API is public, it fails to achieve this condition. In this case, despite the package-private constructor, when an attacker passes Trusted.class as a parameter, the create() API returns an instance of the Trusted class.

package Safe;
public class Trusted {
  Trusted() { } // package private constructor
  public static <T> T create(Class<T> c) throws InstantiationException, IllegalAccessException {
    return c.newInstance();
  }
}

package Attacker;
import Safe.Trusted;

public class Attack {
  public static void main(String[] args) throws InstantiationException, IllegalAccessException {
    System.out.println(Trusted.create(Trusted.class)); // succeeds
  }
}

In the presence of a security manager s, the Class.newInstance() method throws a security exception when either:

• invocation of s.checkMemberAccess(this, Member.PUBLIC) denies creation of new instances of this class
• the caller's class loader is not the same as or an ancestor of the class loader for the current class and invocation of s.checkPackageAccess() denies access to the package of this class

For the first condition, "The default policy is to allow access to PUBLIC members, as well as access to classes that have the same class loader as the caller.". This may be unsafe, as demonstrated in this noncompliant code example. For the second condition, sometimes it is inappropriate to rely on the class loader comparison. The checkPackageAccess() method should be independently called.

Compliant Solution

This compliant solution checks whether the Class object has any public constructors. If it does, the java.beans.Beans API is used to explicitly specify the class loader that should be used to instantiate the class object. If no public constructors are present, the security manager's checkPackageAccess() method is invoked to ensure that the caller has sufficient permissions to access members of the package Safe.

package Safe;
import java.beans.Beans;
import java.io.IOException;

public class Trusted  {
  Trusted() { }
  
  public static <T> T create(Class<T> c) throws  IOException, ClassNotFoundException, 
    InstantiationException, IllegalAccessException {
    
    if(c.getConstructors().length == 0) {  // No public constructors  	  
      SecurityManager sm = System.getSecurityManager();    
      if (sm != null) {          
        sm.checkPackageAccess("Safe");          
      }
      return c.newInstance(); // Safe to return 
    } 

    // Executes only if there are public constructors
    ClassLoader cl = new SafeClassLoader();
    Object b = Beans.instantiate(cl, c.getName());
    return c.cast(b);      
  }  
}

// code outside the package
package Attacker;
import Safe.Trusted;

public class Attack {
  public static void main(String[] args) {
    Object o = Trusted.create(Trusted.class); 
  }
}

Risk Assessment

Misuse of APIs that perform language access checks against the immediate caller only, can break data encapsulation.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[Chan 99]] java.lang.reflect AccessibleObject
[[SCG 07]] Guideline 6-4 Be aware of standard APIs that perform Java language access checks against the immediate caller

SEC05-J. Do not expose standard APIs that use the immediate caller's class loader instance to untrusted code      02. Platform Security (SEC)      SEC07-J. Declare classes that derive from a sensitive class or implement a sensitive interface final