CVE-2021-42381 – A use-after-free vulnerability in BusyBox's awk... (How to Fix)

Q: What is the severity of CVE-2021-42381?

CVE-2021-42381 has a CVSS score of 7.2 (HIGH). A use-after-free vulnerability in BusyBox's awk applet allows attackers to cause denial of service or potentially execute arbitrary code by providing a specially crafted awk pattern. This affects systems using BusyBox with the awk applet enabled, particularly embedded devices, routers, and Linux distributions that include BusyBox.

📋 TL;DR

A use-after-free vulnerability in BusyBox's awk applet allows attackers to cause denial of service or potentially execute arbitrary code by providing a specially crafted awk pattern. This affects systems using BusyBox with the awk applet enabled, particularly embedded devices, routers, and Linux distributions that include BusyBox.

💻 Affected Systems

Products:

BusyBox

Versions: BusyBox versions prior to 1.34.0

Operating Systems: Linux distributions with BusyBox, Embedded systems, Network devices, IoT devices

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects systems where BusyBox awk applet is compiled and available. Many embedded devices use BusyBox as their core utilities.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution leading to complete system compromise, privilege escalation, and persistent access to affected devices.

🟠

Likely Case

Denial of service causing BusyBox awk to crash, potentially disrupting system functionality in embedded environments.

🟢

If Mitigated

Limited impact with proper input validation and sandboxing, potentially only causing application crashes.

🌐 Internet-Facing: MEDIUM - Exploitation requires awk processing of attacker-controlled input, which may occur through web interfaces, APIs, or file uploads.

🏢 Internal Only: MEDIUM - Internal users or compromised systems could exploit this via local command execution or automated scripts.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires crafting specific awk patterns to trigger the use-after-free. Public research and advisories provide technical details.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: BusyBox 1.34.0 and later

Vendor Advisory: https://git.busybox.net/busybox/commit/?id=bd4c42a69c5c

Restart Required: No

Instructions:

1. Update BusyBox to version 1.34.0 or later. 2. For embedded systems: Rebuild firmware with patched BusyBox. 3. For Linux distributions: Use package manager to update busybox package.

🔧 Temporary Workarounds

Disable awk applet

linux

Remove or disable the awk applet from BusyBox build configuration

# Reconfigure BusyBox without awk support
# During build: make menuconfig, deselect awk under Coreutils

Input validation

all

Validate and sanitize awk pattern inputs before processing

🧯 If You Can't Patch

Restrict access to systems using BusyBox awk, especially from untrusted networks
Implement strict input validation for any data that gets processed by awk

🔍 How to Verify

Check if Vulnerable:

Check BusyBox version: busybox --help | head -1. If version is earlier than 1.34.0, system may be vulnerable.

Check Version:

busybox --help | head -1

Verify Fix Applied:

Verify BusyBox version is 1.34.0 or later: busybox --help | head -1

📡 Detection & Monitoring

Log Indicators:

BusyBox awk process crashes
Segmentation faults in awk processes
Unexpected termination of scripts using awk

Network Indicators:

Unusual network traffic to/from devices running BusyBox
Attempts to send crafted patterns to awk-processing services

SIEM Query:

process_name:"busybox" AND (event_type:"crash" OR event_type:"segfault") AND process_args:"awk"

📊 Metadata

CVE ID: CVE-2021-42381

CVSS v3 Score: 7.2 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H

CWE: CWE-416

Published: November 15, 2021

Last Updated: November 3, 2025

🔗 References

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