CVE-2021-47284 – This vulnerability is a use-after-free bug in t... (How to Fix)

Q: What is the severity of CVE-2021-47284?

CVE-2021-47284 has a CVSS score of 4.7 (MEDIUM). This vulnerability is a use-after-free bug in the Linux kernel's ISDN mISDN netjet driver that can cause a kernel panic during device probe failure. It affects systems using the netjet driver for ISDN hardware. The bug occurs when nj_setup fails, leaving an invalid IRQ value that causes a crash when nj_release attempts to free it.

📋 TL;DR

This vulnerability is a use-after-free bug in the Linux kernel's ISDN mISDN netjet driver that can cause a kernel panic during device probe failure. It affects systems using the netjet driver for ISDN hardware. The bug occurs when nj_setup fails, leaving an invalid IRQ value that causes a crash when nj_release attempts to free it.

💻 Affected Systems

Products:

Linux kernel

Versions: Kernel versions before the fix commits (specific versions vary by distribution)

Operating Systems: Linux distributions using vulnerable kernel versions

Default Config Vulnerable: ✅ No

Notes: Only vulnerable if the mISDN netjet driver is loaded (typically requires specific ISDN hardware or manual loading).

📦 What is this software?

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

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Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Kernel panic leading to system crash and denial of service, requiring physical or remote console access to reboot.

🟠

Likely Case

System crash during device initialization, causing boot failure or service disruption on affected hardware.

🟢

If Mitigated

No impact if system doesn't use the vulnerable driver or has been patched.

🌐 Internet-Facing: LOW - Requires local access or specific hardware configuration to trigger.

🏢 Internal Only: MEDIUM - Could affect servers or workstations with ISDN hardware, but requires specific driver usage.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: NO

Unauthenticated Exploit: ✅ No

Complexity: HIGH

Exploitation requires triggering the specific error condition during device probe, which typically requires physical access or specific hardware configuration.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Fixed in kernel commits: 143fc7220961220eecc04669e5909af8847bf8c8 and related backports

Vendor Advisory: https://git.kernel.org/stable/c/143fc7220961220eecc04669e5909af8847bf8c8

Restart Required: Yes

Instructions:

1. Update Linux kernel to version containing the fix. 2. For distributions: Use package manager (apt/yum/dnf) to update kernel. 3. Reboot system to load new kernel.

🔧 Temporary Workarounds

Blacklist netjet driver

linux

Prevent loading of vulnerable driver module

echo 'blacklist netjet' >> /etc/modprobe.d/blacklist.conf
update-initramfs -u

Disable ISDN hardware

all

Remove or disable ISDN hardware that triggers the driver

🧯 If You Can't Patch

Ensure ISDN hardware is not present or not being used
Monitor system logs for kernel panic events related to netjet driver

🔍 How to Verify

Check if Vulnerable:

Check if netjet module is loaded: lsmod | grep netjet. If loaded, check kernel version against patched versions.

Check Version:

uname -r

Verify Fix Applied:

After update, verify kernel version includes fix commit or is newer than vulnerable versions. Check dmesg for successful driver operation.

📡 Detection & Monitoring

Log Indicators:

Kernel panic messages mentioning netjet, nj_probe, or free_irq
WARNING: CPU: 0 PID: 1 at kernel/irq/manage.c:1826 free_irq

SIEM Query:

event_source="kernel" AND (message CONTAINS "netjet" OR message CONTAINS "nj_probe" OR message CONTAINS "free_irq" AND message CONTAINS "WARNING")

📊 Metadata

CVE ID: CVE-2021-47284

CVSS v3 Score: 4.7 (MEDIUM)

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

CWE: CWE-400

Published: May 21, 2024

Last Updated: April 2, 2025

🔗 References

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