CVE-2020-7777 in jsen
Summary
by MITRE • 11/23/2020
This affects all versions of package jsen. If an attacker can control the schema file, it could run arbitrary JavaScript code on the victim machine. In the module description and README file there is no mention about the risks of untrusted schema files, so I assume that this is applicable. In particular the required field of the schema is not properly sanitized. The resulting string that is build based on the schema definition is then passed to a Function.apply();, leading to an Arbitrary Code Execution.
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Analysis
by VulDB Data Team • 12/09/2020
The vulnerability identified as CVE-2020-7777 represents a critical arbitrary code execution flaw within the jsen package, a JavaScript schema validation library widely used in Node.js environments. This vulnerability stems from inadequate input sanitization mechanisms within the package's schema processing logic, creating a pathway for malicious actors to execute unauthorized code on affected systems. The flaw specifically manifests when the package processes untrusted schema files, as the module fails to properly validate or sanitize the schema definitions before incorporating them into executable code constructs.
The technical implementation of this vulnerability exploits the package's handling of the "required" field within schema definitions, where user-controlled input flows directly into a Function.apply() execution context without proper sanitization. This design flaw allows attackers to craft malicious schema files that, when processed by the vulnerable jsen package, result in the dynamic execution of arbitrary JavaScript code within the victim's runtime environment. The vulnerability's severity is amplified by the fact that the package's documentation and readme files provide no warnings about the risks associated with processing untrusted schema inputs, leading developers to assume the package operates safely with external schema data.
The operational impact of this vulnerability extends beyond simple code execution, as it enables attackers to perform complete system compromise when the vulnerable package is used in server-side applications. Attackers can leverage this vulnerability to gain unauthorized access to sensitive data, execute malicious commands, or establish persistent backdoors within affected systems. The vulnerability is particularly concerning in environments where schema files are dynamically loaded from external sources or user inputs, as it transforms legitimate validation functionality into a weaponized attack vector. This flaw can be exploited across various deployment scenarios including web applications, API gateways, and backend services that utilize jsen for data validation purposes.
Mitigation strategies for CVE-2020-7777 should prioritize immediate package updates to versions that address the sanitization issues, while organizations should implement strict input validation policies for all schema processing operations. Security teams should conduct comprehensive audits of their codebases to identify all instances where jsen is used with untrusted inputs, and implement proper schema validation mechanisms that prevent dynamic code execution. The vulnerability aligns with CWE-94, which describes "Improper Control of Generation of Code ('Code Injection')" and maps to ATT&CK technique T1059.007 for "Command and Scripting Interpreter: JavaScript,' demonstrating how this flaw enables attackers to execute malicious scripts within the target environment. Organizations should also consider implementing network segmentation and monitoring solutions to detect potential exploitation attempts, as the vulnerability can be leveraged for lateral movement and privilege escalation within compromised systems.