SEC18-J. Define wrappers around native methods

Native methods are defined in Java and written in traditional languages such as C and C++ [[JNI 2006]]. The added extensibility comes at the cost of flexibility and portability as the code no longer conforms to the policies enforced by Java. In the past, native methods were used for performing platform specific operations, interfacing with legacy library code and improving program performance [[Bloch 2008]]. Although this is no longer completely true — because of poor portability, safety and (quite ironically) performance issues — native code is still used to interface with legacy code.

Defining a wrapper method facilitates installing appropriate security manager checks, performing input validation before passing the arguments to the native code or when obtaining return values, defensively copying mutable inputs and sanitizing user input. Therefore every native method must be private, and must be invoked only by a wrapper method.

Noncompliant Code Example

In this noncompliant code example, the nativeOperation is both native and public; hence untrusted callers may invoke it. Native method invocations bypass security manager checks.

The example does include a wrapper; the doOperation() method invokes nativeOperation() native method but fails to provide input validation or security checks.

public final class NativeMethod {

  // public native method
  public native void nativeOperation(byte[] data, int offset, int len);

  // wrapper method that lacks security checks and input validation
  public void doOperation(byte[] data, int offset, int len) {
    nativeOperation(data, offset, len);
  }
  
  static {
    // load native library in static initializer of class
    System.loadLibrary("NativeMethodLib"); 
  }
}

Compliant Solution

This compliant solution declares the native method private. Furthermore, the doOperation() wrapper method performs routine permission checking to determine whether the succeeding operations are permitted to continue. This is followed by the creation of a defensive copy of the mutable input array data as well as by range checking of the parameters. The nativeOperation() method is thus called with safe inputs. Note that the validation checks must produce outputs that conform to the input requirements of the native implementations/libraries.

public final class NativeMethodWrapper {

  // private native method
  private native void nativeOperation(byte[] data, int offset, int len);

  // wrapper method performs SecurityManager and input validation checks
  public void doOperation(byte[] data, int offset, int len) {
    // permission needed to invoke native method
    securityManagerCheck();

    if (data == null) {
      throw new NullPointerException();
    }

    // copy mutable input
    data = data.clone();

    // validate input
    if ((offset < 0) || (len < 0) || (offset > (data.length - len))) {
      throw new IllegalArgumentException();
    }

    nativeOperation(data, offset, len);
  }

  static {
    // load native library in static initializer of class
    System.loadLibrary("NativeMethodLib"); 
  }
}

Risk Assessment

Failure to define wrappers around native methods can allow unprivileged callers to invoke them and thus exploit inherent vulnerabilities such as those resulting from invalid inputs.

Automated Detection

Automated detection is not feasible in the fully general case. However, an approach similar to Design Fragments [[Fairbanks 07]] could assist both programmers and static analysis tools.

Related Vulnerabilities

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

Related Guidelines

Bibliography

SEC11-J. Call the superclass's getPermissions method when writing a custom class loader      14. Platform Security (SEC)      SEC19-J. Do not rely on the default automatic signature verification provided by URLClassLoader and java.util.jar