CVE-2021-42379 in BusyBox
Summary
by MITRE
A use-after-free in Busybox's awk applet leads to denial of service and possibly code execution when processing a crafted awk pattern in the next_input_file function
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Analysis
by VulDB Data Team • 04/24/2025
The vulnerability identified as CVE-2021-42379 represents a critical use-after-free condition within Busybox's awk applet that can lead to both denial of service and potential code execution scenarios. This flaw exists in the next_input_file function where improper memory management allows attackers to manipulate the execution flow through carefully crafted awk patterns. The issue stems from the improper handling of memory allocation and deallocation sequences within the awk processing engine, creating opportunities for memory corruption that can be exploited by malicious actors. The vulnerability affects Busybox versions prior to 1.33.1 and impacts systems that rely on the awk applet for text processing tasks, making it particularly concerning for embedded systems and network appliances that commonly use Busybox implementations.
The technical root cause of this vulnerability lies in the improper memory management within the awk applet's next_input_file function where a pointer is freed but continues to be referenced in subsequent operations. This use-after-free condition occurs when the application processes a crafted awk pattern that triggers specific memory deallocation sequences followed by subsequent access to the freed memory region. According to CWE-416, this represents a classic use-after-free vulnerability where memory is accessed after it has been freed, potentially allowing attackers to manipulate program execution flow or cause system instability. The flaw demonstrates poor memory lifecycle management practices where the application fails to properly track and validate memory references throughout the processing pipeline, creating exploitable conditions that can be leveraged for privilege escalation or arbitrary code execution.
The operational impact of CVE-2021-42379 extends beyond simple denial of service scenarios to include potential remote code execution capabilities that can compromise system integrity. When exploited successfully, this vulnerability allows attackers to execute arbitrary code with the privileges of the user running the awk applet, which typically runs with elevated permissions in many embedded systems and network appliances. The vulnerability is particularly dangerous in environments where Busybox is used for critical system functions or where the awk applet is accessible to untrusted users. Systems running vulnerable versions of Busybox are at risk of being compromised through crafted input processing, potentially leading to complete system takeover or persistent backdoor installation. The impact is further amplified in IoT devices, routers, and network infrastructure where Busybox is commonly deployed as a lightweight system utility.
Mitigation strategies for CVE-2021-42379 focus primarily on updating to patched versions of Busybox where the memory management issues have been resolved through proper memory lifecycle handling and validation. Organizations should immediately upgrade to Busybox version 1.33.1 or later, which includes fixes for the use-after-free condition in the awk applet. Additionally, input validation should be implemented at the application level to prevent processing of untrusted awk patterns that could trigger the vulnerable code paths. System administrators should consider implementing network segmentation and access controls to limit exposure of systems running vulnerable Busybox versions. The remediation approach aligns with ATT&CK technique T1059.007 for command and script interpreters, where the vulnerability exploits interpreter flaws to achieve code execution. Security monitoring should include detection of unusual awk processing patterns and memory access anomalies that could indicate exploitation attempts. Regular vulnerability assessments and penetration testing should be conducted to identify and remediate similar memory management flaws across the organization's system infrastructure.