CVE-2021-46936 – This is a use-after-free vulnerability in the L... (How to Fix)

Q: What is the severity of CVE-2021-46936?

CVE-2021-46936 has a CVSS score of 7.8 (HIGH). This is a use-after-free vulnerability in the Linux kernel's networking subsystem that can cause kernel panic and system crashes. It affects Linux systems running vulnerable kernel versions when network namespaces are destroyed. The vulnerability allows attackers with local access to crash the system, potentially leading to denial of service.

📋 TL;DR

This is a use-after-free vulnerability in the Linux kernel's networking subsystem that can cause kernel panic and system crashes. It affects Linux systems running vulnerable kernel versions when network namespaces are destroyed. The vulnerability allows attackers with local access to crash the system, potentially leading to denial of service.

💻 Affected Systems

Products:

Linux kernel

Versions: Linux kernel versions from 5.4 onward where commit 61a7e26028b9 was introduced, up to versions containing the fix

Operating Systems: All Linux distributions using affected kernel versions

Default Config Vulnerable: ⚠️ Yes

Notes: Requires network namespace functionality to be triggered. Systems using containers, virtualization, or network namespaces are particularly vulnerable.

📦 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...

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 complete system crash and denial of service, potentially requiring physical reboot of affected systems.

🟠

Likely Case

System crash or instability when network namespaces are destroyed, causing service disruption on affected Linux systems.

🟢

If Mitigated

Limited impact if systems are patched or don't use network namespace features extensively.

🌐 Internet-Facing: LOW - This is a local vulnerability requiring access to the system, not directly exploitable over the network.

🏢 Internal Only: HIGH - Local attackers or malicious users with system access can trigger the vulnerability to crash the system.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ✅ No

Complexity: LOW

The vulnerability has been publicly discussed since 2017 and a reliable trigger exists. Exploitation requires local access to trigger the use-after-free condition.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Fixed in stable kernel releases via commits referenced in CVE details

Vendor Advisory: https://git.kernel.org/stable/c/08eacbd141e2495d2fcdde84358a06c4f95cbb13

Restart Required: Yes

Instructions:

1. Update Linux kernel to patched version from your distribution vendor. 2. Reboot system to load new kernel. 3. Verify kernel version after reboot.

🔧 Temporary Workarounds

Disable network namespaces

linux

Prevent creation of new network namespaces to avoid triggering the vulnerability

echo 0 > /proc/sys/user/max_user_namespaces

Restrict user namespace creation

linux

Limit ability to create user namespaces which can create network namespaces

sysctl -w user.max_user_namespaces=0

🧯 If You Can't Patch

Restrict local user access to systems, implement strict privilege separation
Monitor for system crashes and implement high availability/failover for critical systems

🔍 How to Verify

Check if Vulnerable:

Check kernel version and compare with affected versions. Vulnerable if running Linux kernel 5.4+ without patches.

Check Version:

uname -r

Verify Fix Applied:

Check kernel version after update matches patched versions from distribution vendor.

📡 Detection & Monitoring

Log Indicators:

Kernel panic messages in /var/log/messages or dmesg
System crash/reboot events
Network namespace destruction events

Network Indicators:

Sudden loss of network connectivity on affected systems

SIEM Query:

source="kernel" AND ("panic" OR "BUG" OR "use-after-free" OR "tw_timer_handler")

📊 Metadata

CVE ID: CVE-2021-46936

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-416

Published: February 27, 2024

Last Updated: November 21, 2024

🔗 References

📤 Share & Export

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🔗 Related Vulnerabilities

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