CVE-2020-36366 in MJS
Summary
by MITRE • 05/29/2021
Stack overflow vulnerability in parse_value Cesanta MJS 1.20.1, allows remote attackers to cause a Denial of Service (DoS) via a crafted file.
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Analysis
by VulDB Data Team • 06/03/2021
The CVE-2020-36366 vulnerability represents a critical stack overflow flaw discovered in Cesanta MJS version 1.20.1, a lightweight JavaScript engine designed for embedded systems and IoT devices. This vulnerability resides within the parse_value function, which serves as a core component responsible for interpreting and processing JavaScript code within the MJS runtime environment. The flaw manifests when the engine encounters specially crafted input files that trigger excessive stack consumption during the parsing process, ultimately leading to a system crash or complete denial of service condition. The vulnerability affects the parser's recursive handling of nested data structures, where improper bounds checking allows attackers to manipulate stack memory allocation through malicious input sequences.
The technical exploitation of this vulnerability follows a well-defined pattern that leverages the engine's parsing logic to create deeply nested structures that exceed the available stack space. When the parse_value function processes malformed input, it recursively calls itself to handle complex nested objects or arrays, with each recursive invocation consuming stack memory. The vulnerability stems from insufficient depth validation and stack space management within the parser implementation, creating an exploitable condition where an attacker can craft input files containing excessive nesting levels that cause the stack to overflow. This behavior aligns with CWE-129, which addresses improper validation of buffer access, and specifically manifests as a stack-based buffer overflow scenario that can be triggered through controlled input manipulation.
The operational impact of CVE-2020-36366 extends beyond simple service disruption, particularly affecting embedded systems and IoT devices that rely on Cesanta MJS for JavaScript execution. These devices often operate in environments where continuous availability is critical, such as industrial control systems, network appliances, or smart home ecosystems. When exploited, the vulnerability can cause complete system crashes, requiring manual intervention for recovery, and may even lead to persistent service unavailability that could compromise larger network infrastructure. The remote nature of the attack means that adversaries can trigger the vulnerability without physical access to the target systems, making it particularly dangerous for network-connected devices. This vulnerability directly maps to attack techniques described in the ATT&CK framework under T1499, which covers network denial of service attacks, and T1059, which addresses execution through scripting languages.
Mitigation strategies for CVE-2020-36366 focus primarily on updating to patched versions of Cesanta MJS where the stack overflow protection has been implemented. Organizations should prioritize immediate patching of all affected systems, particularly those running version 1.20.1 or earlier, as the vulnerability has been addressed in subsequent releases through enhanced stack depth validation and improved parsing routines. Additional protective measures include implementing input validation controls at network boundaries, deploying intrusion detection systems that can identify suspicious parsing patterns, and establishing runtime monitoring for unusual memory consumption patterns. Security teams should also consider implementing application sandboxing techniques to limit the potential impact of successful exploitation attempts, while maintaining detailed logging of JavaScript parsing activities to detect anomalous behavior. The vulnerability serves as a reminder of the critical importance of proper stack management in embedded systems and the need for robust input validation in interpreter-based environments.