CVE-2025-21683 – A memory leak vulnerability in the Linux kernel... (How to Fix)

Q: What is the severity of CVE-2025-21683?

CVE-2025-21683 has a CVSS score of 5.5 (MEDIUM). A memory leak vulnerability in the Linux kernel's BPF subsystem allows unprivileged local users to cause denial of service by exhausting kernel memory. The issue occurs when bpf_sk_select_reuseport() fails to properly release socket references in error paths, leading to kernel memory exhaustion over time. This affects Linux systems with BPF socket reuseport functionality enabled.

📋 TL;DR

A memory leak vulnerability in the Linux kernel's BPF subsystem allows unprivileged local users to cause denial of service by exhausting kernel memory. The issue occurs when bpf_sk_select_reuseport() fails to properly release socket references in error paths, leading to kernel memory exhaustion over time. This affects Linux systems with BPF socket reuseport functionality enabled.

💻 Affected Systems

Products:

Linux kernel

Versions: Specific affected versions not specified in CVE, but fix commits indicate recent kernel versions with BPF socket reuseport support

Operating Systems: Linux distributions using vulnerable kernel versions

Default Config Vulnerable: ✅ No

Notes: Requires BPF socket reuseport functionality to be used. Not all systems enable this 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...

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

Local unprivileged attacker causes kernel memory exhaustion leading to system instability, crashes, or denial of service affecting all users and services.

🟠

Likely Case

Local user or misconfigured application gradually consumes kernel memory, eventually causing performance degradation or system instability.

🟢

If Mitigated

With proper memory limits and monitoring, impact is limited to performance degradation of affected processes.

🌐 Internet-Facing: LOW - Requires local access to exploit, not directly exploitable over network.

🏢 Internal Only: MEDIUM - Local users or compromised containers can exploit this to cause denial of service.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Requires local access and ability to create BPF programs with socket reuseport functionality. Exploitation requires specific conditions to trigger the error paths.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Kernel versions containing fixes from git commits: 0ab52a8ca6e156a64c51b5e7456cac9a0ebfd9bf, b02e70be498b138e9c21701c2f33f4018ca7cd5e, b3af60928ab9129befa65e6df0310d27300942bf, bb36838dac7bb334a3f3d7eb29875593ec9473fc, cccd51dd22574216e64e5d205489e634f86999f3

Vendor Advisory: https://git.kernel.org/stable/c/0ab52a8ca6e156a64c51b5e7456cac9a0ebfd9bf

Restart Required: Yes

Instructions:

1. Update Linux kernel to version containing the fix commits. 2. Check with your distribution vendor for specific patched kernel versions. 3. Reboot system after kernel update.

🔧 Temporary Workarounds

Disable BPF socket reuseport

linux

Prevent use of the vulnerable functionality by disabling BPF socket reuseport support

sysctl -w net.core.bpf_jit_enable=0
echo 0 > /proc/sys/net/core/bpf_jit_enable

Restrict BPF capabilities

linux

Use Linux capabilities or SELinux/AppArmor to restrict unprivileged BPF usage

setcap cap_bpf=ep /path/to/application
Or configure SELinux/AppArmor policies to restrict bpf() syscall

🧯 If You Can't Patch

Implement strict memory limits using cgroups to contain memory exhaustion
Monitor kernel memory usage and restart affected services if abnormal consumption detected

🔍 How to Verify

Check if Vulnerable:

Check kernel version and if BPF socket reuseport is in use. Vulnerable if using affected kernel version with BPF socket reuseport functionality.

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version includes the fix commits or check with 'uname -r' and compare against patched versions from your distribution.

📡 Detection & Monitoring

Log Indicators:

Kernel OOM (Out of Memory) messages in dmesg
Abnormal memory consumption by kernel processes
System instability or crashes

Network Indicators:

None - this is a local memory leak vulnerability

SIEM Query:

Search for: 'Out of memory: Kill process' OR 'kernel: Memory cgroup out of memory' in system logs

📊 Metadata

CVE ID: CVE-2025-21683

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-401

Published: January 31, 2025

Last Updated: November 3, 2025

🔗 References

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🔗 Related Vulnerabilities

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