CVE-2021-42377 in BusyBox
Summary
by MITRE • 11/17/2021
An attacker-controlled pointer free in Busybox's hush applet leads to denial of service and possible code execution when processing a crafted shell command, due to the shell mishandling the &&& string. This may be used for remote code execution under rare conditions of filtered command input.
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Analysis
by VulDB Data Team • 11/19/2021
The vulnerability identified as CVE-2021-42377 represents a critical memory corruption flaw within Busybox's hush shell applet that can lead to both denial of service and potential remote code execution scenarios. This vulnerability specifically manifests when the shell processes crafted input containing the &&& string pattern, creating a condition where an attacker-controlled pointer is improperly freed during command parsing operations. The issue stems from inadequate input validation and memory management within the shell's parsing logic, particularly affecting how it handles sequential logical operators in command chains.
The technical exploitation of this vulnerability leverages a heap-based memory corruption condition that occurs during shell command interpretation. When processing commands containing the &&& pattern, the hush shell fails to properly validate or sanitize the input before attempting memory operations on what it believes to be valid command structure elements. This improper handling results in a use-after-free condition where memory that has already been released is accessed, potentially allowing an attacker to manipulate the freed memory pointer to execute arbitrary code or cause system instability. The vulnerability is categorized under CWE-415 as an improper free condition, which directly relates to the insecure handling of memory resources in the shell's command processing pipeline.
The operational impact of this vulnerability extends beyond simple denial of service to encompass potential remote code execution capabilities under specific conditions. While the vulnerability requires the attacker to control command input that gets processed by the hush shell, the possibility of remote exploitation exists when the target system processes untrusted input from network sources or when input filtering mechanisms can be bypassed. The rare conditions mentioned in the vulnerability description suggest that successful exploitation may require specific environmental factors or input filtering scenarios that allow the malicious command pattern to reach the vulnerable shell processing logic. This vulnerability particularly affects embedded systems and IoT devices that rely on Busybox for shell functionality, as these systems often have limited security controls and may process untrusted input from network services.
Mitigation strategies for CVE-2021-42377 should focus on immediate patching of affected Busybox versions, with particular attention to implementing proper input validation and sanitization mechanisms before shell command processing. System administrators should consider implementing strict input filtering and command validation at network boundaries to prevent malicious commands from reaching vulnerable shell processing components. The remediation approach aligns with ATT&CK technique T1059.004 for defensive measures against shell injection attacks, emphasizing the importance of proper command parsing and input validation. Organizations should also implement monitoring for unusual command patterns and consider deploying intrusion detection systems that can identify potential exploitation attempts involving the &&& string pattern. Additionally, system hardening measures including disabling unnecessary shell functionality and implementing least privilege principles for shell access can significantly reduce the attack surface and potential impact of this vulnerability.