CVE-2021-39141 in XStream
Summary
by MITRE • 08/24/2021
XStream is a simple library to serialize objects to XML and back again. In affected versions this vulnerability may allow a remote attacker to load and execute arbitrary code from a remote 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.
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Analysis
by VulDB Data Team • 05/23/2025
CVE-2021-39141 represents a critical remote code execution vulnerability within the XStream library, a widely used Java serialization framework that facilitates object serialization to XML format and vice versa. This vulnerability stems from the library's default security configuration that previously relied on blacklists to prevent malicious object deserialization. The flaw allows remote attackers to execute arbitrary code on systems processing untrusted XML input by manipulating the serialized object stream, effectively bypassing traditional security controls. The vulnerability is classified under CWE-502 as deserialization of untrusted data, which directly enables arbitrary code execution through maliciously crafted input streams. This represents a fundamental flaw in the library's security model where the default configuration fails to adequately protect against malicious payloads that can be constructed to exploit the deserialization process.
The technical implementation of this vulnerability occurs through XStream's object deserialization mechanism where the library processes XML input containing serialized Java objects. When the security framework is not properly configured with a whitelist, the system defaults to a vulnerable blacklist approach that cannot be made secure for general use cases. Attackers can construct specially crafted XML payloads that contain malicious serialized objects which, when processed by the vulnerable XStream version, trigger remote code execution. The vulnerability is particularly dangerous because it requires no authentication or user interaction to exploit, making it a severe remote attack vector. The attack chain typically involves crafting a malicious XML document with serialized objects that, when deserialized by XStream, execute arbitrary commands on the target system, potentially allowing full system compromise.
The operational impact of CVE-2021-39141 extends across numerous applications and systems that rely on XStream for object serialization, particularly web applications, enterprise systems, and any software processing external XML input. Organizations using affected versions of XStream without proper security configuration are at risk of complete system compromise, data exfiltration, and unauthorized access to network resources. The vulnerability affects systems where XStream processes untrusted input from external sources such as web services, API endpoints, file uploads, and network communications. Given that the default configuration is vulnerable, even properly configured applications can be at risk if they fail to implement proper security measures such as type whitelisting. This vulnerability directly maps to ATT&CK technique T1059.007 for command and scripting interpreter, where adversaries use serialized objects to execute commands on compromised systems.
Mitigation strategies for CVE-2021-39141 require immediate implementation of proper security configurations within XStream applications. Organizations must ensure that XStream's security framework is properly configured with a whitelist of allowed types, restricting deserialization to only the minimal required classes. This approach directly addresses the vulnerability by preventing deserialization of potentially malicious objects through the whitelist mechanism. The recommended solution involves implementing XStream's security framework with explicit type restrictions and avoiding the default blacklist configuration that cannot be made secure for general use. Additionally, applications should implement input validation, network segmentation, and monitoring to detect potential exploitation attempts. Regular updates to XStream to version 1.4.18 or later are essential, as this version removes the default blacklist approach and requires explicit security configuration. Organizations should also consider implementing web application firewalls, input sanitization, and runtime application self-protection measures to provide defense-in-depth against similar vulnerabilities. The vulnerability highlights the importance of proper security framework implementation and the need for organizations to follow security best practices such as those outlined in the OWASP Top Ten and NIST cybersecurity frameworks.