When an API (see table below) is invoked on a {{Class}} object, a comparison is run between the immediate caller's class loader and that of the {{Class}} object. The {{Class}} object is the object on which an API is invoked. According to \[[JLS 05|AA. Java References#JLS 05]\]:

The method getClass returns the Class object that represents the class of the object.

Security manager checks may get bypassed depending on the immediate caller's class loader.
For instance, in the presence of a security manager, the getSystemClassLoader and getParent methods succeed only if the caller's class loader is the delegation ancestor of the current class loader or if the caller's class loader is the same as the current one or if the code in the current execution context has the RunTimePermission, namely "getClassLoader".

Noncompliant Code Example

The createInstance method is the immediate caller of java.lang.Class.newInstance in this noncompliant example. The newInstance method is being invoked on the dateClass Class object. The issue is that the untrustedCode method can trigger the instantiation of a new class even though it should not have the permission to do so. This behavior is not caught by the security manager.

public class ExceptionExample {
   public static void untrustedCode() {
     Date now = new Date();
     Class<?> dateClass = now.getClass();
     createInstance(dateClass);
    }

    public static void createInstance(Class<?> dateClass) {
      try { // Create another Date object using the Date Class
            Object o = dateClass.newInstance();
            if (o instanceof Date) {
              Date d = (Date)o;
              System.out.println("The time is: " + d.toString());
             }
       }
       catch (InstantiationException ie) { System.out.println(ie.toString()); }
       catch (IllegalAccessException iae) { System.out.println(iae.toString()); }    	
    }
}

A related issue is described in SEC04-J. Beware of standard APIs that perform access checks against the immediate caller.

Compliant Solution

Do not accept Class, ClassLoader or Thread instances from untrusted code. If inevitable, safely acquire these instances by ensuring they come from trusted sources. Additionally, make sure to discard tainted inputs from untrusted code. Likewise, objects returned by the affected methods should not be propagated back to the untrusted code.

Note that the Class.newInstance()} method requires the class to contain a no-argument constructor. If this requirement is not satisfied, a runtime exception results, which indirectly prevents a security breach.

Risk Assessment

Bypassing Securitymanager checks may seriously compromise the security of a Java application.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[Gong 03|AA. Java References#Gong 03]\] Section 4.3.2, Class Loader Delegation Hierarchy
\[[SCG 07|AA. Java References#SCG 07]\] Guideline 6-2 Safely invoke standard APIs that bypass SecurityManager checks depending on the immediate caller's class loader

SEC36-J. Guard doPrivileged blocks against untrusted invocations      00. Security (SEC)      SEC03-J. Do not expose standard APIs that use the immediate caller's class loader instance to untrusted code