CVE-2025-21781 – A race condition vulnerability in the Linux ker... (How to Fix)

Q: What is the severity of CVE-2025-21781?

CVE-2025-21781 has a CVSS score of 5.5 (MEDIUM). A race condition vulnerability in the Linux kernel's batman-adv module causes kernel panic during network interface removal. This affects systems using batman-adv for mesh networking, potentially leading to system crashes and denial of service.

📋 TL;DR

A race condition vulnerability in the Linux kernel's batman-adv module causes kernel panic during network interface removal. This affects systems using batman-adv for mesh networking, potentially leading to system crashes and denial of service.

💻 Affected Systems

Products:

Linux kernel with batman-adv module

Versions: Linux kernel versions with vulnerable batman-adv implementation (specific versions not specified in CVE)

Operating Systems: Linux distributions using affected kernel versions

Default Config Vulnerable: ✅ No

Notes: Only vulnerable when batman-adv module is loaded and mesh networking is configured.

📦 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 →

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

System crash/kernel panic leading to complete denial of service and potential data loss or corruption.

🟠

Likely Case

System instability or crash during network interface changes, reboot operations, or mesh network reconfiguration.

🟢

If Mitigated

Minor service disruption during interface management operations.

🌐 Internet-Facing: LOW - Requires local access to trigger interface removal.

🏢 Internal Only: MEDIUM - Can be triggered by authorized users performing network administration or during system maintenance.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Requires ability to trigger network interface removal, typically requiring local access or administrative privileges.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Patched in kernel commits referenced in CVE (072b27873219, 167422a07096, 2c3fb7df4cc6, 522b1596ea19, 7eb5dd201695)

Vendor Advisory: https://git.kernel.org/stable/c/072b2787321903287a126c148e8db87dd7ef96fe

Restart Required: Yes

Instructions:

1. Update Linux kernel to version containing fixes. 2. Rebuild kernel if compiling from source. 3. Reboot system to load patched kernel.

🔧 Temporary Workarounds

Disable batman-adv module

linux

Unload batman-adv kernel module if not required for system functionality

sudo modprobe -r batman_adv

Blacklist batman-adv module

linux

Prevent batman-adv module from loading at boot

echo 'blacklist batman_adv' | sudo tee /etc/modprobe.d/blacklist-batman-adv.conf

🧯 If You Can't Patch

Avoid removing network interfaces while batman-adv is active
Schedule maintenance windows for interface changes and ensure proper backups

🔍 How to Verify

Check if Vulnerable:

Check if batman-adv module is loaded: lsmod | grep batman_adv

Check Version:

uname -r

Verify Fix Applied:

Check kernel version against patched versions in git commits, verify batman-adv module loads without errors

📡 Detection & Monitoring

Log Indicators:

Kernel panic messages in /var/log/kern.log or dmesg
batman-adv related crash reports
System crash during interface removal

Network Indicators:

Sudden loss of mesh network connectivity
Interface removal failures

SIEM Query:

kernel:panic OR kernel:Oops AND batman_adv

📊 Metadata

CVE ID: CVE-2025-21781

CVSS v3 Score: 5.5 (MEDIUM)

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

EPSS Score: 0.05% exploit probability (16.4th percentile)

Published: February 27, 2025

Last Updated: November 3, 2025

🔗 References

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