A Server-Side Forgery Request vulnerability in XStream via HashMap unmarshaling

Vulnerability

XStream versions up to and including 1.4.17, when used with Java runtime versions 8 to 14 and relying on the default blacklist security framework, are vulnerable to a server-side request forgery (SSRF) [1]. The vulnerability exists because XStream reconstructs objects based on type information contained in the XML input during unmarshalling. An attacker can inject manipulated type information, such as jdk.nashorn.internal.runtime.Source_-URLData, causing XStream to create objects that initiate HTTP requests to arbitrary URLs [2].

Exploitation

An attacker with the ability to provide a crafted XML payload to an XStream endpoint can exploit this vulnerability. No authentication is required if the XStream instance is exposed. The attacker marshalls a simple HashMap to XML, then replaces the XML content with a specially crafted snippet that includes a URL pointing to an internal resource (e.g., http://localhost:8080/internal/). Unmarshalling this XML causes XStream to make a request to that URL, effectively performing an SSRF [2].

Impact

A successful exploit allows the attacker to make arbitrary HTTP requests from the server hosting XStream to internal resources (intranet or localhost) that are not publicly accessible. This can lead to disclosure of sensitive data or interaction with internal services. The vulnerability does not directly enable remote code execution but can be a stepping stone for further attacks [1][2].

Mitigation

Users who have set up XStream's security framework with a whitelist limited to minimal required types are not affected. For those relying on the default blacklist, upgrading to XStream version 1.4.18 or later is necessary [1]. The fixed version was released around the time of the CVE publication (2021-08-23) [2]. No workaround is available for versions that cannot be upgraded.