CVE-2021-42375: Busybox ash Applet DoS Vulnerability

CVE-2021-42375 Overview

CVE-2021-42375 is a denial of service vulnerability in BusyBox's ash applet caused by incorrect handling of special elements when processing shell commands. The vulnerability occurs when the shell mistakenly interprets specific characters as reserved characters, leading to improper command processing. This flaw can be exploited to cause a denial of service condition, though exploitation requires specific conditions where command input is filtered.

Critical Impact

Attackers with local access can craft malicious shell commands that cause the ash applet to crash, resulting in denial of service on embedded systems and IoT devices running BusyBox.

Affected Products

• BusyBox version 1.33.1
• Fedora Project Fedora 33 and 34
• NetApp Cloud Backup
• NetApp HCI Management Node
• NetApp SolidFire
• NetApp H300S/H500S/H700S Series (firmware and hardware)
• NetApp H300E/H500E/H700E Series (firmware and hardware)
• NetApp H410S (firmware and hardware)

Discovery Timeline

• November 15, 2021 - CVE-2021-42375 published to NVD
• April 23, 2025 - Last updated in NVD database

Technical Details for CVE-2021-42375

Vulnerability Analysis

This vulnerability stems from improper input validation in the BusyBox ash shell applet, classified under CWE-159 (Improper Handling of Invalid Use of Special Elements). The ash shell is a lightweight POSIX-compliant shell commonly used in embedded Linux systems and IoT devices due to BusyBox's minimal footprint.

When processing shell commands, the ash applet incorrectly interprets certain characters as reserved shell metacharacters. This misinterpretation leads to improper parsing of command input, which under specific conditions can cause the shell to enter an invalid state, resulting in a denial of service.

The vulnerability requires local access to exploit, meaning an attacker must have the ability to execute shell commands on the target system. While the impact is limited to availability (no confidentiality or integrity breach), the widespread deployment of BusyBox across embedded systems, network appliances, and IoT devices makes this vulnerability particularly relevant for critical infrastructure.

Root Cause

The root cause lies in the ash applet's command parsing logic, which fails to properly distinguish between user-supplied characters and shell-reserved metacharacters. When specially crafted input containing specific character sequences is processed, the parser misidentifies these characters as having special meaning, leading to malformed command interpretation.

This input validation flaw is exacerbated in scenarios where command input undergoes filtering before reaching the shell, as the filter-escape sequence combinations can create unexpected parsing states.

Attack Vector

The attack requires local access to the vulnerable system with the ability to execute shell commands through the ash applet. An attacker must craft a malicious shell command containing specific character patterns that trigger the incorrect special element handling.

The attack scenario involves:

1. Attacker gains local access to a system running BusyBox with the ash applet
2. Attacker crafts a shell command with specific character sequences designed to confuse the parser
3. When the ash shell processes this command, it misinterprets characters as reserved elements
4. The mishandling causes the shell to crash or become unresponsive, resulting in denial of service

The vulnerability is most impactful in environments where filtered command input is processed by the ash shell, as the filtering can inadvertently create the conditions needed for exploitation.

Detection Methods for CVE-2021-42375

Indicators of Compromise

• Unexpected crashes or restarts of the BusyBox ash shell process
• Abnormal shell command patterns in system logs containing unusual character sequences
• Service interruptions on embedded systems running BusyBox
• Repeated shell initialization failures without clear cause

Detection Strategies

• Monitor for unusual patterns in shell command execution logs that may indicate crafted input
• Implement process monitoring to detect unexpected termination of ash shell processes
• Deploy endpoint detection and response (EDR) solutions to identify abnormal shell behavior
• Audit systems to identify BusyBox version 1.33.1 or earlier installations across your infrastructure

Monitoring Recommendations

• Enable comprehensive logging for shell activity on embedded systems running BusyBox
• Configure alerts for repeated shell crashes or abnormal restart patterns
• Monitor network appliances and IoT devices for unexpected service interruptions
• Review system logs for patterns of malformed command processing

How to Mitigate CVE-2021-42375

Immediate Actions Required

• Identify all systems running vulnerable BusyBox versions, particularly version 1.33.1
• Update BusyBox to the latest patched version available from your distribution or vendor
• Review NetApp security advisories if using affected NetApp products
• Restrict local access to embedded systems and IoT devices to authorized personnel only

Patch Information

Organizations should consult the following resources for patch information:

• Claroty Research Report for detailed vulnerability analysis
• JFrog Blog Post on Vulnerabilities for technical details and remediation guidance
• NetApp Security Advisory for NetApp product-specific patches
• Fedora users should apply updates from Fedora Package Announcements

Workarounds

• Restrict shell access to trusted users only on systems where patching is not immediately possible
• Implement strict input validation and sanitization before commands reach the ash shell
• Consider using an alternative shell implementation where BusyBox cannot be updated
• Segment affected embedded systems and IoT devices from critical network resources
• Monitor shell activity closely on unpatched systems until remediation is complete

# Configuration example - Identify BusyBox version on affected systems
busybox --help | head -n 1

# Check if ash applet is enabled
busybox --list | grep ash

# Restrict shell access (adjust permissions as needed)
chmod 750 /bin/busybox