CVE-2021-42385: Busybox Awk Use-After-Free Vulnerability

CVE-2021-42385 Overview

A use-after-free vulnerability exists in BusyBox's awk applet that can lead to denial of service and potentially remote code execution when processing a specially crafted awk pattern in the evaluate function. BusyBox is a widely deployed software suite providing Unix utilities in a single executable, commonly found in embedded systems, IoT devices, and Linux-based appliances.

Critical Impact

This use-after-free vulnerability in BusyBox's awk implementation can be exploited by attackers with high privileges to achieve denial of service or potentially execute arbitrary code on affected systems, impacting embedded devices and Linux distributions that rely on BusyBox.

Affected Products

• BusyBox (all vulnerable versions)
• Fedora 33
• Fedora 34

Discovery Timeline

• November 15, 2021 - CVE-2021-42385 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2021-42385

Vulnerability Analysis

CVE-2021-42385 is a use-after-free (CWE-416) vulnerability located within the awk applet of BusyBox. The vulnerability manifests during the evaluation of crafted awk patterns within the evaluate function. When specific malformed input patterns are processed, the application references memory that has already been freed, leading to undefined behavior.

The vulnerability requires an attacker with high privileges but can be exploited over the network without user interaction. Successful exploitation could allow an attacker to cause a denial of service by crashing the BusyBox process or potentially achieve code execution by manipulating the freed memory contents before they are accessed.

This vulnerability was discovered as part of a broader security research effort that identified 14 vulnerabilities in BusyBox, as documented by Claroty Team82 Research and the JFrog Security Research Team.

Root Cause

The root cause of this vulnerability is improper memory management within the awk applet's evaluate function. When processing certain awk patterns, the code frees a memory object but continues to reference it in subsequent operations. This use-after-free condition arises from a failure to properly track memory lifecycle and nullify pointers after deallocation.

In use-after-free scenarios, the freed memory may be reallocated for other purposes, and when the dangling pointer is dereferenced, it can read or write corrupted data, leading to crashes or potentially allowing an attacker to control program execution flow.

Attack Vector

The attack vector for CVE-2021-42385 is network-based, though it requires high privileges to exploit. An attacker could craft a malicious awk pattern designed to trigger the use-after-free condition within the evaluate function. The attack scenario typically involves:

1. An attacker with elevated privileges on a system running BusyBox
2. Crafting a specially designed awk pattern that triggers the memory corruption
3. Sending the malicious input to a service or script that processes awk patterns using BusyBox
4. The evaluate function processes the malformed pattern, accessing freed memory
5. Resulting in denial of service or potential code execution depending on memory layout

The vulnerability mechanism involves memory object lifecycle mismanagement in BusyBox's awk implementation. When the evaluate function processes certain crafted patterns, it incorrectly frees memory while retaining references to that memory for subsequent use. For detailed technical analysis, refer to the Claroty Research publication.

Detection Methods for CVE-2021-42385

Indicators of Compromise

• Unexpected crashes or segmentation faults in BusyBox processes, particularly when executing awk commands
• Core dumps or error logs indicating memory access violations in the awk applet
• Anomalous awk pattern processing attempts from privileged accounts
• System instability in embedded devices or containers running BusyBox

Detection Strategies

• Monitor system logs for BusyBox crash events related to awk execution with patterns indicating memory corruption
• Implement file integrity monitoring to detect unauthorized modifications to BusyBox binaries
• Deploy runtime application self-protection (RASP) or memory safety tools on systems using BusyBox
• Use SentinelOne Singularity to detect anomalous process behavior and memory access patterns in BusyBox deployments

Monitoring Recommendations

• Enable detailed logging for awk command execution on critical systems running BusyBox
• Implement crash dump analysis to identify potential exploitation attempts
• Monitor network traffic for unusual patterns targeting services that utilize BusyBox awk functionality
• Establish baseline behavior for BusyBox processes and alert on deviations

How to Mitigate CVE-2021-42385

Immediate Actions Required

• Inventory all systems running BusyBox to determine exposure to CVE-2021-42385
• Prioritize patching embedded systems, IoT devices, and containers that include vulnerable BusyBox versions
• Apply vendor-provided patches from Fedora, Debian, and other distributions that package BusyBox
• Consider replacing BusyBox awk with alternative implementations in critical environments until patching is complete

Patch Information

Updates are available through multiple distribution channels. Fedora users should apply patches announced through the Fedora Package Announcement. Debian LTS users should refer to the Debian LTS Announcement. NetApp customers should review the NetApp Security Advisory NTAP-20211223-0002 for affected products and remediation guidance.

Workarounds

• Disable or remove the awk applet from BusyBox installations where it is not required
• Implement input validation for any awk patterns processed by BusyBox from external sources
• Use access controls to restrict which users can execute awk commands via BusyBox
• Consider compiling BusyBox with memory safety features or using hardened builds where available

# Check BusyBox version and installed applets
busybox --help | head -5

# List enabled applets to identify if awk is present
busybox --list | grep awk

# If awk is not required, consider building BusyBox without it
# In BusyBox configuration (make menuconfig):
# Editors -> awk -> disable