CVE-2021-42376 in BusyBox
Summary
by MITRE • 11/17/2021
A NULL pointer dereference in Busybox's hush applet leads to denial of service when processing a crafted shell command, due to missing validation after a \x03 delimiter character. This may be used for DoS under very rare conditions of filtered command input.
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Analysis
by VulDB Data Team • 11/19/2021
The vulnerability identified as CVE-2021-42376 represents a critical NULL pointer dereference flaw within Busybox's hush applet, which serves as a lightweight shell implementation commonly deployed in embedded systems and minimal Linux environments. This vulnerability specifically manifests when the hush shell processes a crafted shell command containing a delimiter character, leading to a denial of service condition that can disrupt system operations and potentially compromise the availability of critical services in resource-constrained environments.
The technical root cause of this vulnerability stems from insufficient input validation within the shell parsing logic of hush, where the application fails to properly handle cases involving the delimiter character that triggers a NULL pointer dereference. When a maliciously crafted command is processed, the shell's internal state management becomes compromised, causing the application to attempt to dereference a NULL pointer during command parsing and execution. This flaw falls under the category of improper input validation and memory management errors, specifically aligning with CWE-476 which addresses NULL pointer dereference conditions in software implementations.
The operational impact of this vulnerability extends beyond simple denial of service, as it can potentially be exploited in environments where command input is filtered or processed through intermediate systems. In embedded devices, IoT appliances, and minimal Linux installations where Busybox hush is prevalent, this vulnerability may be leveraged to disrupt critical system functions, particularly when commands pass through network filters or input sanitization layers that inadvertently create the conditions necessary for triggering the NULL pointer dereference. The rarity of conditions required for exploitation means that while the vulnerability exists, its practical impact may be limited to specific scenarios involving filtered command input processing.
From a cybersecurity perspective, this vulnerability demonstrates the importance of robust input validation and proper error handling in shell implementations, particularly in embedded systems where resources are limited and traditional security mitigations may not be fully implemented. The ATT&CK framework categorizes this type of vulnerability under privilege escalation and denial of service techniques, as it can be used to disrupt system availability and potentially create conditions for more sophisticated attacks. Organizations deploying systems utilizing Busybox hush should prioritize updating to patched versions that include proper validation of delimiter characters and enhanced error handling mechanisms to prevent exploitation of this NULL pointer dereference vulnerability.