XStream is vulnerable to an Arbitrary Code Execution attack

Vulnerability

CVE-2021-39151 is an arbitrary code execution vulnerability in XStream, a Java library for serializing objects to XML and back. All versions up to and including 1.4.17 are affected when used without a whitelist-based security framework. The vulnerability exists because XStream deserializes type information from the input stream, allowing an attacker to inject or replace objects that trigger the execution of arbitrary code loaded from a remote server [1][2].

Exploitation

An attacker can exploit this vulnerability by crafting a malicious XML payload that includes type references to specific Java classes, such as javax.swing.event.EventListenerList, com.sun.xml.internal.ws.api.message.Packet, and others. The attacker must be able to send the manipulated XML stream to the XStream unmarshalling process. No authentication or special network position is required; the attack is remote and can be triggered by supplying the malicious input to a vulnerable application [2].

Impact

Successful exploitation allows a remote attacker to execute arbitrary code on the target system. This leads to a complete compromise of confidentiality, integrity, and availability (CIA). The attacker gains the ability to run arbitrary commands, load malicious classes, or exfiltrate sensitive data from the affected host [1][2].

Mitigation

Users of XStream should upgrade to version 1.4.18, which no longer uses a blacklist by default and instead relies on a whitelist that users must configure. The official recommendation is to set up XStream's security framework with a whitelist limited to the minimal required types. No user who has followed this guidance is affected. As of the publication date (2021-08-23), version 1.4.18 is the fixed release [1][2].