CVE-2017-20189 in Clojure
Summary
by MITRE • 01/22/2024
In Clojure before 1.9.0, classes can be used to construct a serialized object that executes arbitrary code upon deserialization. This is relevant if a server deserializes untrusted objects.
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Analysis
by VulDB Data Team • 05/31/2025
This vulnerability exists in Clojure versions prior to 1.9.0 and represents a critical deserialization flaw that enables remote code execution through malicious serialized objects. The issue stems from the language's handling of serialized data structures where untrusted input can be manipulated to trigger arbitrary code execution during the deserialization process. Attackers can craft specially constructed serialized objects that, when processed by a vulnerable server, will execute malicious code on the target system. This vulnerability directly relates to CWE-502 which categorizes deserialization of untrusted data as a dangerous practice that can lead to remote code execution. The flaw is particularly dangerous in server environments where applications deserialize data from external sources without proper validation or sanitization.
The technical implementation of this vulnerability leverages Clojure's serialization mechanism to construct malicious objects that contain executable code within their serialized representation. When the server attempts to deserialize these objects, the runtime environment executes the embedded code as part of the normal deserialization process. This attack vector aligns with ATT&CK technique T1059.007 which describes the execution of code through deserialization attacks. The vulnerability affects any Clojure application that processes untrusted serialized data, particularly web applications, backend services, and any system that accepts serialized input from external sources. The exploitation requires minimal privileges and can be executed remotely, making it particularly attractive to attackers who seek to gain persistent access to target systems.
The operational impact of CVE-2017-20189 is severe and can result in complete system compromise, data exfiltration, and persistence mechanisms being established. Organizations running vulnerable Clojure applications face significant risk of unauthorized access, especially in cloud environments where deserialization occurs across network boundaries. The vulnerability can be exploited through various attack vectors including web applications, API endpoints, and file upload mechanisms that accept serialized data. Security teams must consider this vulnerability as a high-priority threat requiring immediate remediation, particularly in environments where applications process serialized data from untrusted sources. The attack surface is broad since many applications use serialization for data persistence, communication, or configuration management, creating multiple potential entry points for exploitation.
Mitigation strategies for this vulnerability require immediate version upgrades to Clojure 1.9.0 or later where the deserialization security issues have been addressed. Organizations should implement strict input validation and sanitization for all serialized data, particularly when processing external or untrusted inputs. Network segmentation and firewall rules can help limit exposure by restricting access to deserialization endpoints. Additionally, implementing application-level protections such as custom deserialization filters, using secure serialization formats, and employing sandboxing techniques can provide additional layers of defense. Security monitoring should be enhanced to detect unusual deserialization activities and potential exploitation attempts. Regular security assessments and penetration testing should verify that the mitigation measures are effective and that no other deserialization vulnerabilities exist within the application stack. The remediation process should include comprehensive testing to ensure that the upgrade does not introduce compatibility issues with existing application functionality while maintaining robust security posture against similar deserialization attacks.