CVE-2021-46526 in MJS
Summary
by MITRE • 01/28/2022
Cesanta MJS v2.20.0 was discovered to contain a global buffer overflow via snquote at src/mjs_json.c.
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Analysis
by VulDB Data Team • 01/30/2022
The vulnerability identified as CVE-2021-46526 affects Cesanta MJS version 2.20.0, a lightweight JavaScript engine designed for embedded systems and IoT devices. This global buffer overflow represents a critical security flaw that could potentially allow attackers to execute arbitrary code or cause system instability. The vulnerability specifically manifests within the snquote function located in the src/mjs_json.c source file, which is responsible for handling JSON string quoting operations. The flaw occurs when the JavaScript engine processes malformed JSON input containing specially crafted string literals that exceed the allocated buffer boundaries during the quoting process.
The technical implementation of this buffer overflow stems from inadequate input validation and boundary checking within the JSON parsing routines. When the snquote function encounters certain string patterns, it fails to properly verify that the destination buffer can accommodate the expanded string representation during the quoting operation. This deficiency allows attackers to supply input data that overflows the predetermined buffer limits, potentially corrupting adjacent memory regions. The vulnerability is classified as a global buffer overflow because it affects memory locations beyond the intended scope of the function, making it particularly dangerous in embedded environments where memory corruption can lead to complete system compromise. The flaw aligns with CWE-121, which describes stack-based buffer overflow conditions, and represents a classic example of insufficient boundary checking in memory management operations.
The operational impact of CVE-2021-46526 extends beyond simple denial of service scenarios, as it creates potential attack vectors for remote code execution in systems utilizing the affected MJS engine. Given that Cesanta MJS is commonly deployed in IoT devices, network appliances, and embedded systems, exploitation of this vulnerability could enable attackers to gain unauthorized access to critical infrastructure. The buffer overflow could be leveraged to overwrite function pointers, return addresses, or other critical program data structures, potentially allowing for privilege escalation or complete system takeover. This vulnerability is particularly concerning in environments where the JavaScript engine processes untrusted input from network sources, as it could be exploited through web interfaces, API endpoints, or configuration files that utilize JSON data formats. The ATT&CK framework categorizes this vulnerability under T1059.007 for scripting languages and T1203 for exploitation for privilege escalation, highlighting its potential for both initial access and lateral movement within compromised systems.
Mitigation strategies for CVE-2021-46526 should prioritize immediate patching of affected systems to version 2.21.0 or later, which contains the necessary fixes for the buffer overflow vulnerability. Organizations should implement input validation measures at multiple layers, including JSON parser level checks and application-level sanitization of all incoming data. Network segmentation and access controls can help limit the potential impact of exploitation attempts, while monitoring systems should be configured to detect anomalous JSON processing patterns that might indicate exploitation attempts. Security teams should also conduct comprehensive vulnerability assessments of all systems utilizing Cesanta MJS, particularly those handling external input, and implement runtime protections such as address space layout randomization and stack canaries to mitigate exploitation success rates. Regular security updates and patch management processes should be established to prevent similar vulnerabilities from emerging in other components of the embedded system ecosystem.