CVE-2025-21681 – A denial-of-service vulnerability in the Linux ... (How to Fix)

Q: What is the severity of CVE-2025-21681?

CVE-2025-21681 has a CVSS score of 5.5 (MEDIUM). A denial-of-service vulnerability in the Linux kernel's Open vSwitch (OVS) module causes system lockups when transmitting packets to network devices that are unregistering. This affects Linux systems using OVS, particularly those with dummy network interfaces used for debugging. The infinite loop in skb_tx_hash() can only be resolved by rebooting the affected system.

📋 TL;DR

A denial-of-service vulnerability in the Linux kernel's Open vSwitch (OVS) module causes system lockups when transmitting packets to network devices that are unregistering. This affects Linux systems using OVS, particularly those with dummy network interfaces used for debugging. The infinite loop in skb_tx_hash() can only be resolved by rebooting the affected system.

💻 Affected Systems

Products:

Linux kernel with Open vSwitch (OVS) module

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

Operating Systems: Linux distributions with OVS support

Default Config Vulnerable: ✅ No

Notes: Requires OVS to be configured and actively transmitting packets to network devices (particularly dummy interfaces) during unregistration.

📦 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

Complete system lockup requiring hard reboot, causing extended service downtime and potential data loss.

🟠

Likely Case

System becomes unresponsive when OVS attempts to send packets to unregistering dummy interfaces during debugging or maintenance.

🟢

If Mitigated

Minimal impact with proper monitoring and avoidance of packet transmission to unregistering interfaces.

🌐 Internet-Facing: LOW - Requires local system access and specific OVS configuration with dummy interfaces.

🏢 Internal Only: MEDIUM - Can affect internal systems using OVS for network virtualization, particularly in development/testing environments.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: NO

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Exploitation requires local access and ability to trigger packet transmission to unregistering network devices. Not easily weaponized for remote attacks.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Linux kernel with commits 47e55e4b410f7d552e43011baa5be1aab4093990, 82f433e8dd0629e16681edf6039d094b5518d8ed, 87fcf0d137c770e6040ebfdb0abd8e7dd481b504, 930268823f6bccb697aa5d2047aeffd4a497308c, or b5c73fc92f8d15c16e5dc87b5c17d2abf1e6d092

Vendor Advisory: https://git.kernel.org/stable/c/47e55e4b410f7d552e43011baa5be1aab4093990

Restart Required: Yes

Instructions:

1. Update Linux kernel to version containing the fix commits. 2. Check distribution-specific security advisories. 3. Reboot system after kernel update. 4. Verify OVS module loads correctly.

🔧 Temporary Workarounds

Avoid dummy interface usage

linux

Prevent using dummy network interfaces with OVS during debugging operations

Monitor network device state

linux

Implement monitoring to detect when network devices are unregistering and prevent OVS from transmitting to them

🧯 If You Can't Patch

Avoid using dummy network interfaces with OVS for packet capture/debugging
Implement strict change control to prevent network device unregistration while OVS is actively transmitting

🔍 How to Verify

Check if Vulnerable:

Check kernel version and verify if OVS module is loaded: 'lsmod | grep openvswitch' and 'uname -r'

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version includes fix commits: 'git log --oneline | grep -E "(47e55e4b|82f433e8|87fcf0d1|93026882|b5c73fc9)"' or check distribution patch notes

📡 Detection & Monitoring

Log Indicators:

System becoming unresponsive
Kernel panic messages
High CPU usage in kernel processes
OVS process hanging

Network Indicators:

Sudden loss of network connectivity through OVS
Packet transmission failures

SIEM Query:

source="kernel" AND ("panic" OR "lockup" OR "hung") AND ("ovs" OR "openvswitch")

📊 Metadata

CVE ID: CVE-2025-21681

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

CWE: CWE-835

Published: January 31, 2025

Last Updated: November 3, 2025

🔗 References

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🔗 Related Vulnerabilities

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