CVE-2021-42376: Busybox Hush Applet DoS Vulnerability

CVE-2021-42376 Overview

A NULL pointer dereference vulnerability exists in Busybox's hush applet that leads to denial of service when processing a crafted shell command. The vulnerability occurs due to missing validation after a \\x03 delimiter character is encountered during command parsing. While exploitability requires specific conditions involving filtered command input, successful exploitation can crash the affected application, causing service disruption.

Critical Impact

Attackers can cause denial of service by crashing the Busybox hush shell through specially crafted shell commands containing the \\x03 delimiter character, potentially disrupting embedded systems and IoT devices that rely on Busybox for core functionality.

Affected Products

• Busybox (all versions prior to patch)
• Fedora Project Fedora 33 and 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

• 2021-11-15 - CVE-2021-42376 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-42376

Vulnerability Analysis

This vulnerability is classified as CWE-476 (NULL Pointer Dereference), which occurs when an application dereferences a pointer that it expects to be valid but is actually NULL. In the context of Busybox's hush applet, the shell interpreter fails to properly validate input after encountering the \\x03 delimiter character during command parsing.

The hush shell is a lightweight POSIX-compliant shell implementation included in Busybox, commonly used in embedded Linux systems, IoT devices, and containerized environments where a minimal footprint is required. When the parser encounters the special \\x03 character (ASCII ETX - End of Text), it fails to check whether subsequent required data structures are properly initialized before attempting to access them.

The attack requires local access to execute shell commands, making remote exploitation impractical in most scenarios. However, in environments where user input is filtered and passed to shell commands, an attacker could potentially inject the crafted payload to trigger the crash.

Root Cause

The root cause is insufficient input validation in the hush applet's command parsing logic. When processing shell commands, the parser encounters the \\x03 delimiter character and proceeds to dereference a pointer without first verifying that the pointer references valid memory. This missing NULL check allows an attacker to trigger a segmentation fault by providing input that leaves the pointer in an uninitialized or NULL state.

Attack Vector

Exploitation requires local access to the system where Busybox is installed. The attacker must be able to execute or inject shell commands that will be processed by the hush applet.

The attack mechanism involves:

1. The attacker crafts a shell command containing the \\x03 delimiter character
2. The command is passed to the hush shell for processing
3. The parser encounters the \\x03 character and fails to properly initialize required data structures
4. The subsequent code attempts to dereference a NULL pointer
5. The shell process crashes, resulting in denial of service

While this vulnerability has a local attack vector, it is particularly concerning for embedded systems and IoT devices where Busybox is commonly deployed as the primary shell and system utility suite. A crash in these environments could disrupt critical device functionality or automated processes that rely on shell command execution.

Detection Methods for CVE-2021-42376

Indicators of Compromise

• Unexpected crashes or segmentation faults in Busybox hush shell processes
• Core dump files generated by hush applet crashes containing evidence of NULL pointer access
• Process termination signals (SIGSEGV) logged for Busybox-related processes
• Repeated shell process restarts in system logs

Detection Strategies

• Monitor system logs for segmentation fault events associated with Busybox or hush shell processes
• Implement application crash monitoring to detect abnormal termination patterns in shell processes
• Review input validation in applications that pass user-controlled data to shell commands
• Audit systems to identify Busybox installations and verify version information against known vulnerable versions

Monitoring Recommendations

• Configure logging to capture shell process crashes and associated command history when available
• Implement process monitoring on embedded systems and IoT devices running Busybox to detect service disruptions
• Set up alerts for repeated process restarts that may indicate exploitation attempts
• Monitor for unusual patterns of shell command execution failures

How to Mitigate CVE-2021-42376

Immediate Actions Required

• Inventory all systems running Busybox to identify potentially affected deployments
• Update Busybox to the latest patched version that addresses CVE-2021-42376
• For embedded systems with firmware-level Busybox installations, apply firmware updates from the respective vendor
• Review and restrict which users have local access to systems running vulnerable Busybox versions

Patch Information

Patches are available from the Busybox project and downstream distributions. Organizations should consult the following resources for patching guidance:

• Claroty Research Report - Original vulnerability research from Claroty Team82
• JFrog Blog Post - Detailed technical analysis and patch information
• NetApp Security Advisory - Guidance for NetApp product users
• Fedora Package Announcements - Updates for Fedora 33 and 34 users

Workarounds

• If patching is not immediately possible, consider using an alternative shell implementation (such as ash or sh) where available
• Implement input sanitization to filter or reject shell commands containing the \\x03 character before they reach the hush applet
• Restrict local access to systems running vulnerable Busybox versions to trusted users only
• For embedded devices, consider network segmentation to limit attacker access to vulnerable systems

# Check Busybox version to determine if vulnerable
busybox --help 2>&1 | head -1

# Verify which applets are enabled (check for hush)
busybox --list | grep hush

# If using package manager, check for available updates
# Fedora/RHEL:
dnf check-update busybox

# Debian/Ubuntu:
apt list --upgradable 2>/dev/null | grep busybox