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            Native methods are defined in Java and written in traditional languages such as C and C++ \[[JNI 2006|AA. Bibliography#JNI 06]\]. The added extensibility comes at the cost of flexibility and portability because the code no longer conforms to the policies enforced by Java. InNative themethods past,have nativebeen methods were used for performing platform-specific operations, interfacing with legacy library code, and improving program performance \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. Although this is no longer completely true --- because of poor portability, security, and (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 validating arguments passed to native code, validating return values, defensively copying mutable inputs, and sanitizing untrusted data. Consequently, every native method must be private and must be invoked only by a wrapper method.

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

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This compliant solution declares the native method private. The doOperation() wrapper method performs routine permission checking to determine whether the succeeding operations are permitted to continue. The method also checks permissions, creates a defensive copy of the mutable input array data, as well as performing range checking and checks the ranges of the arguments. The nativeOperation() method is consequently called with safe secure inputs. Note that the validation checks must produce outputs that conform to the input requirements of the native methods.

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"); 
  }
}

Exceptions

SEC08-EX00EX0: Native methods that do not require security manager checks, validation of arguments or return values, or defensive copying of mutable inputs do not need to be wrapped ((for example, the standard C function int rand(void), for example)) do not need to be wrapped.

Risk Assessment

Failure to define wrappers around native methods can allow unprivileged callers to invoke them and consequently exploit inherent vulnerabilities such as those resulting from invalid inputsbuffer overflows in native libraries.

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Related Guidelines

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