CVE-2021-42377: Busybox Hush Applet RCE Vulnerability

CVE-2021-42377 Overview

CVE-2021-42377 is a critical attacker-controlled pointer free vulnerability in Busybox's hush applet that can lead to denial of service and potential code execution. The vulnerability occurs when the shell processes a crafted shell command containing the &&& string, which the shell mishandles. This memory corruption issue may be exploited for remote code execution under specific conditions where command input is filtered.

Busybox is often referred to as "The Swiss Army Knife of Embedded Linux" and is widely deployed in embedded systems, IoT devices, and containerized environments. The hush shell is a lightweight POSIX-compliant shell included in Busybox, making this vulnerability particularly concerning for environments where Busybox provides core system utilities.

Critical Impact

Attackers can achieve denial of service and potentially execute arbitrary code by sending crafted shell commands containing the &&& string to vulnerable Busybox hush shell instances.

Affected Products

• Busybox versions 1.33.0 and 1.33.1
• Fedora 33 and Fedora 34
• NetApp Cloud Backup
• NetApp HCI Management Node
• NetApp SolidFire
• NetApp H-Series Storage Systems (H300S, H500S, H700S, H300E, H500E, H700E, H410S) and associated firmware

Discovery Timeline

• November 15, 2021 - CVE-2021-42377 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-42377

Vulnerability Analysis

This vulnerability is classified under CWE-763 (Release of Invalid Pointer or Reference) and CWE-590 (Free of Memory not on the Heap). The root cause lies in how the hush shell parses and processes the &&& string within shell commands. When encountering this specific input pattern, the shell fails to properly validate memory pointers before freeing them, resulting in an attacker-controlled pointer free condition.

The attack can be initiated remotely over the network without requiring authentication or user interaction. If successfully exploited, an attacker could achieve complete compromise of confidentiality, integrity, and availability of the affected system. The vulnerability is particularly dangerous in embedded and IoT environments where Busybox serves as the primary shell interface.

Joint research by Claroty Team82 and JFrog identified this vulnerability as part of a larger disclosure involving 14 vulnerabilities in Busybox. For detailed technical analysis, refer to the Claroty Research Report and the JFrog Blog on Vulnerabilities.

Root Cause

The vulnerability stems from improper handling of the &&& string in the hush shell's command parser. When this specific character sequence is encountered, the shell incorrectly processes memory pointers, leading to a scenario where an attacker-controlled pointer is passed to the free() function. This type of memory corruption can have severe consequences:

1. Invalid Pointer Release: The shell attempts to free memory at an address controlled by the attacker rather than a legitimately allocated heap block
2. Heap Metadata Corruption: Freeing arbitrary memory locations can corrupt heap management structures
3. Code Execution Potential: Under specific heap layouts, attackers may leverage this primitive to achieve arbitrary code execution

Attack Vector

The attack vector involves sending crafted shell commands containing the &&& string to a vulnerable Busybox hush shell instance. Exploitation scenarios include:

• Direct Shell Access: If an attacker has any form of shell access (even restricted), they can trigger the vulnerability
• Remote Exploitation: In scenarios where shell commands are constructed from external input (such as CGI scripts, REST APIs, or device management interfaces), remote exploitation becomes possible
• Filtered Input Bypass: Even in environments with command filtering, the seemingly innocuous &&& string may pass through filters, enabling exploitation

The vulnerability manifests when the hush shell parser encounters the &&& character sequence and incorrectly handles the resulting parse tree, leading to memory being freed at an attacker-influenced address. For comprehensive technical details on the exploitation mechanism, consult the Claroty Research Report.

Detection Methods for CVE-2021-42377

Indicators of Compromise

• Shell commands or input containing the &&& character sequence in process logs or command history
• Unexpected crashes or segmentation faults in the hush shell process
• Memory corruption indicators in system logs associated with Busybox processes
• Anomalous process behavior following shell command execution on embedded devices

Detection Strategies

• Monitor for shell commands containing the &&& string pattern across all systems running Busybox hush shell
• Implement application-level logging to capture all shell command inputs on IoT and embedded devices
• Deploy endpoint detection rules that alert on Busybox process crashes or abnormal terminations
• Use file integrity monitoring to detect unauthorized modifications to Busybox binaries

Monitoring Recommendations

• Enable comprehensive logging for all shell activity on devices running Busybox
• Monitor network traffic for suspicious command sequences targeting embedded device management interfaces
• Implement real-time alerting for segmentation faults or crashes in Busybox-related processes
• Conduct regular vulnerability scans to identify unpatched Busybox installations across your infrastructure

How to Mitigate CVE-2021-42377

Immediate Actions Required

• Upgrade Busybox to version 1.34.0 or later, which contains the security fix for this vulnerability
• Audit all embedded systems, containers, and IoT devices for vulnerable Busybox versions
• Review and restrict access to shell interfaces on affected devices
• Implement input validation to filter or reject commands containing the &&& pattern as an interim measure

Patch Information

Users should upgrade to Busybox version 1.34.0 or later to address this vulnerability. Fedora users can obtain patched packages through the official Fedora package repositories as referenced in the Fedora Package Announcement. NetApp customers should consult the NetApp Security Advisory for specific guidance on affected products and available updates.

Workarounds

• If upgrading is not immediately possible, disable the hush shell applet and use an alternative shell if available
• Implement strict input validation on any interface that passes commands to the Busybox shell
• Restrict network access to devices running vulnerable Busybox versions using firewall rules
• Consider deploying Busybox configurations that exclude the hush applet entirely

# Check your Busybox version
busybox --help | head -1

# Verify if hush applet is enabled
busybox --list | grep hush

# Example: Restricting access to device management interface
iptables -A INPUT -p tcp --dport 80 -s trusted_network/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP