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Noncompliant Code Example

The untrustedCode method of class Untrusted invokes loadLib method of class NativeCode in this noncompliant example. This is insecure as the library gets loaded on behalf of the untrusted code. In essence, the untrusted code's class loader may be able to load the intended library even if it does not have sufficient permissions.

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Non-native library code can also be susceptible to related security flaws. Loading a non-native safe library, by itself may not expose a vulnerability but after loading an unsafe library, an attacker can easily exploit it if it contains other vulnerabilities. Moreover, non-native libraries often make use of doPrivileged blocks, making them a lucrative target.

Compliant Solution

Ensure that untrusted code cannot invoke the affected APIs directly or indirectly (that is, via a call to an invoking method). In this case, the loadLib() method must be declared private so that it is only available to a more restrictive method within the class. The restrictive method can ensure that the caller has sufficient permissions to load the library.

private final native void loadLib();

Noncompliant Code Example

Accepting tainted inputs from untrusted code can further exacerbate the issue. The single argument Class.forname() method is another example of an API that uses its immediate caller's class loader to load a desired class. Untrusted code can indirectly misuse this API to manufacture classes with the same privileges as those of the immediate caller.

// className is Foo
Class c = Class.forName(className);

Compliant Solution

Again, limit the visibility of the method that uses this API. Do not operate on tainted inputs. The preceding noncompliant code example can be fixed by hard-coding the class's name.

Class c = Class.forName("Foo"); // explicitly hardcode

Noncompliant Code Example

This noncompliant code example returns an instance of the immediate caller's class loader to any invoker. A malicious invoker can therefore, obtain the associated class loader using standard APIs such as java.lang.Class.getClassLoader(). Once this is achieved, it is trivial to use Class.forname() to load the malicious class from attacker space or exploit a trusted method that calls Class.newInstance() on an arbitrarily supplied object. Class.newInstance() does not throw any security exception when the class loader is either the same or the delegation ancestor of its immediate caller. (SEC02-J. Do not expose standard APIs that may bypass Security Manager checks to untrusted code)

private Class doLogic() {
  ClassLoader myLoader = new myClassLoader();
  Class myClass = myLoader.loadClass("MyClass");
  return myClass; // returns Class instance to untrusted code
}

Compliant Solution

Always make sure that any internal Class, ClassLoader and Thread instances are not returned to untrusted code.

private void doLogic() {
  ClassLoader myLoader = new myClassLoader();
  Class myClass = myLoader.loadClass("MyClass");
  // do what is is required here itself; do not return myClass
}

Exceptions

EX1: It is permissible to use APIs that do not use the immediate caller's class loader instance. For example, the three-argument java.lang.Class.forName() method requires an explicit argument that specifies the class loader instance to use.

public static Class forName(String name,
                            boolean initialize,
                            ClassLoader loader) // explicitly specify the class loader to use
                     throws ClassNotFoundException

Risk Assessment

Allowing untrusted code to carry out actions using the immediate caller's class loader may allow it to execute with the same privileges as the immediate caller.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[SCG 07|AA. Java References#SCG 07]\] Guideline 6-3 Safely invoke standard APIs that perform tasks using the immediate caller's class loader instance

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