CVE-2021-4439 – This CVE-2021-4439 is an array index out-of-bou... (How to Fix)

Q: What is the severity of CVE-2021-4439?

CVE-2021-4439 has a CVSS score of 7.8 (HIGH). This CVE-2021-4439 is an array index out-of-bounds vulnerability in the Linux kernel's ISDN CAPI subsystem. It allows local attackers to trigger a kernel panic or potentially execute arbitrary code by exploiting improper bounds checking in the cmtp_session thread. Systems running affected Linux kernel versions with ISDN/CAPI functionality are vulnerable.

📋 TL;DR

This CVE-2021-4439 is an array index out-of-bounds vulnerability in the Linux kernel's ISDN CAPI subsystem. It allows local attackers to trigger a kernel panic or potentially execute arbitrary code by exploiting improper bounds checking in the cmtp_session thread. Systems running affected Linux kernel versions with ISDN/CAPI functionality are vulnerable.

💻 Affected Systems

Products:

Linux kernel

Versions: Versions before the fix commits (specific versions vary by distribution, but generally Linux kernel versions before the fixes in 5.15-rc2 and stable branches)

Operating Systems: Linux distributions with affected kernel versions

Default Config Vulnerable: ✅ No

Notes: Only vulnerable if ISDN/CAPI subsystem is enabled and used. Many modern systems don't use ISDN hardware by default.

📦 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 →

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 denial of service, or potential privilege escalation to kernel-level code execution.

🟠

Likely Case

Kernel panic causing system crash and denial of service.

🟢

If Mitigated

No impact if ISDN/CAPI functionality is disabled or system is patched.

🌐 Internet-Facing: LOW - Requires local access to trigger the vulnerability.

🏢 Internal Only: MEDIUM - Local users or processes could crash the kernel, affecting system availability.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Requires local access and ability to trigger ISDN/CAPI operations. No known public exploits as of analysis.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Fixed in Linux kernel commits: 1f3e2e97c003f80c4b087092b225c8787ff91e4d, 24219a977bfe3d658687e45615c70998acdbac5a, 285e9210b1fab96a11c0be3ed5cea9dd48b6ac54, 7d91adc0ccb060ce564103315189466eb822cc6a, 7f221ccbee4ec662e2292d490a43ce6c314c4594

Vendor Advisory: https://git.kernel.org/stable/c/1f3e2e97c003f80c4b087092b225c8787ff91e4d

Restart Required: Yes

Instructions:

1. Update Linux kernel to version containing the fix commits. 2. For distributions: Use package manager to update kernel package. 3. Reboot system to load new kernel.

🔧 Temporary Workarounds

Disable ISDN/CAPI module

linux

Prevent loading of vulnerable ISDN/CAPI kernel modules

echo 'blacklist capi' >> /etc/modprobe.d/blacklist.conf
echo 'blacklist isdn' >> /etc/modprobe.d/blacklist.conf

Remove ISDN hardware

linux

Physically remove or disable ISDN hardware if present

🧯 If You Can't Patch

Disable ISDN/CAPI functionality in kernel configuration
Restrict local user access to systems with ISDN hardware

🔍 How to Verify

Check if Vulnerable:

Check kernel version and if ISDN/CAPI modules are loaded: lsmod | grep -E '^(capi|isdn)'

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version includes fix commits and ISDN/CAPI modules are not loaded or patched

📡 Detection & Monitoring

Log Indicators:

Kernel panic messages
UBSAN array-index-out-of-bounds errors in kernel logs
Process crashes related to kcmtpd_ctr threads

Network Indicators:

Unusual ISDN/CAPI network traffic if hardware present

SIEM Query:

source="kernel" AND "array-index-out-of-bounds" AND "capi"

📊 Metadata

CVE ID: CVE-2021-4439

CVSS v3 Score: 7.8 (HIGH)

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

CWE: CWE-129

Published: June 20, 2024

Last Updated: November 21, 2024

🔗 References

📤 Share & Export

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🔗 Related Vulnerabilities

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