CVE-2020-36375 in MJS
Summary
by MITRE • 05/29/2021
Stack overflow vulnerability in parse_equality 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-2020-36375 vulnerability represents a critical stack overflow condition within the Cesanta MJS JavaScript engine version 1.20.1, specifically within the parse_equality function. This flaw manifests as a buffer overflow during the parsing of equality expressions, creating a potential remote code execution vector that can be exploited by malicious actors to disrupt system operations. The vulnerability resides in the engine's handling of malformed input data during the parsing phase, where insufficient bounds checking allows attackers to manipulate stack memory allocation through carefully crafted input files. The issue stems from improper validation of input parameters within the equality parsing logic, which fails to adequately sanitize or limit the size of data structures being processed. This particular vulnerability affects the broader category of software security flaws known as buffer overflows, which are classified under CWE-121 as "Stack-based Buffer Overflow" and are commonly exploited through techniques described in the ATT&CK framework under T1499.1 for network denial of service attacks. The stack overflow occurs when the parser encounters malformed equality expressions that cause the function to allocate excessive memory on the stack without proper bounds verification, leading to memory corruption that can result in program termination or arbitrary code execution. The impact extends beyond simple denial of service to potentially enable more sophisticated attacks depending on the execution environment and memory layout. This vulnerability is particularly concerning because it operates at the parsing layer of the JavaScript engine, meaning that any application utilizing Cesanta MJS for processing external data files becomes susceptible to exploitation. The flaw demonstrates a classic security weakness where input validation occurs too late in the processing pipeline, allowing malformed data to traverse multiple layers before causing the stack corruption. Attackers can leverage this vulnerability by preparing a specially crafted file containing malformed equality expressions that trigger the overflow condition when processed by the vulnerable MJS engine. The vulnerability's exploitation requires minimal privileges and can be executed remotely, making it an attractive target for automated attack tools. The affected environment includes any system running Cesanta MJS 1.20.1 that processes external input through the parse_equality function, particularly web applications, embedded systems, or IoT devices that utilize this JavaScript engine for scripting functionality. The technical complexity of this vulnerability lies in its ability to remain undetected until triggered by specific input patterns, making it difficult to identify through standard security scanning procedures. The vulnerability's presence in the parsing layer of the engine means that even seemingly benign input files can become attack vectors when they contain crafted equality expressions. Remediation requires updating to a patched version of Cesanta MJS that implements proper bounds checking and input validation mechanisms. The mitigation strategy should also include input sanitization at multiple layers, network segmentation to limit exposure, and monitoring for unusual parsing patterns. Organizations should also consider implementing intrusion detection systems that can identify potential exploitation attempts through abnormal memory access patterns or stack manipulation behaviors. The vulnerability highlights the importance of robust input validation and memory safety practices in embedded scripting engines, particularly those used in resource-constrained environments where traditional security mitigations may not be fully applicable. Security teams should prioritize patch management for this vulnerability as it represents a significant risk to system availability and potentially system integrity. The ATT&CK framework categorizes this type of vulnerability under defensive evasion techniques where attackers leverage memory corruption to bypass security controls and establish persistent access to affected systems.