SEC04-J. Beware of standard 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 SEC32-J. Do not grant ReflectPermission with action suppressAccessChecks)

Noncompliant Code Example

This noncompliant code example accepts an instance of java.lang.Class from untrusted code. No language access checks are carried out against this instance but instead, with the immediate caller of getInstance(). This allows an attacker to let trusted code interleave with malicious code.

public static void makeAccessible(Class c, String fieldName) {
  try {
   // ... 
   System.out.println(f.getInt(c)); // unsafe!
  }
  // ...
}

Compliant Solution

Avoid invoking affected APIs on Class, Constructor, Field or Method instances obtained from untrusted code. This can be done by explicitly instantiating the class within the makeAccessible method.

public static void makeAccessible() {
  Class c = new C();
  String fieldName = "i";
  try {
   // ... 
   System.out.println(f.getInt(c)); // unsafe!
  }
  // ...
}

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

SEC03-J. Do not expose standard APIs that use the immediate caller's class loader instance to untrusted code      00. Security (SEC)      SEC06-J. Assume that all Java clients can be reverse engineered, monitored, and modified