CVE-2021-39152 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 request data from internal resources that are not publicly available only by manipulating the processed input stream with a Java runtime version 14 to 8. No user is affected, who followed the recommendation to setup XStream's security framework with a whitelist limited to the minimal required types. If you rely on XStream's default blacklist of the [Security Framework](https://x-stream.github.io/security.html#framework), you will have to use at least version 1.4.18.
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Analysis
by VulDB Data Team • 05/23/2025
CVE-2021-39152 represents a critical deserialization vulnerability within the XStream library that enables remote attackers to access internal system resources through manipulated input streams. This vulnerability specifically affects Java runtime versions 14 through 8 and exploits the library's default security configuration. The flaw resides in XStream's processing of serialized data where attackers can manipulate the input stream to bypass security restrictions and gain access to internal resources that should remain protected from external access. The vulnerability operates at the core of object serialization and deserialization processes, leveraging the library's trust in default configurations to execute unauthorized operations.
The technical implementation of this vulnerability stems from XStream's default security framework which relies on a blacklist approach rather than a whitelist methodology. When users do not explicitly configure XStream with a restricted whitelist of allowed types, the library continues to use its default security settings that may be insufficient to prevent malicious exploitation. This design choice creates a dangerous attack surface where remote adversaries can craft serialized input that traverses the security boundaries of the application. The vulnerability specifically targets the Java runtime environment's handling of deserialization processes, allowing attackers to execute arbitrary code against internal system resources. This represents a classic example of insecure deserialization as classified under CWE-502, where untrusted data is deserialized without proper validation.
The operational impact of CVE-2021-39152 extends beyond simple information disclosure to potentially enable full system compromise through lateral movement and privilege escalation. Attackers can leverage this vulnerability to access internal network resources, databases, and system services that should remain isolated from external networks. The vulnerability affects any application using XStream with default security configurations, making it particularly dangerous in enterprise environments where such libraries are widely deployed. Organizations relying on the default blacklist approach face significant risk as this security model has inherent weaknesses that make it susceptible to bypass techniques. The vulnerability demonstrates the critical importance of following security best practices and implementing proper access controls within serialization frameworks, aligning with ATT&CK technique T1071.004 for application layer protocol manipulation.
The recommended mitigation strategy involves upgrading to XStream version 1.4.18 or later, which addresses the security flaw through improved default configurations. However, the most effective approach requires implementing a whitelist-based security framework that explicitly defines which types are allowed for serialization and deserialization. Organizations should immediately review their XStream configurations to ensure that the security framework is properly initialized with minimal required types. The vulnerability highlights the fundamental principle that default security configurations are often insufficient and must be explicitly tailored to the specific security requirements of each application. Security teams should conduct comprehensive audits of all systems using XStream to identify potential exposure and implement proper security controls. This vulnerability underscores the necessity of adopting defense-in-depth strategies and following security guidelines that emphasize the importance of explicit type restrictions rather than relying on implicit security measures. The attack vector demonstrates how seemingly benign library functionality can become a critical security risk when default configurations are not properly hardened against known exploitation techniques.