CVE-2025-39920 – A NULL pointer dereference vulnerability in the... (How to Fix)

Q: What is the severity of CVE-2025-39920?

CVE-2025-39920 has a CVSS score of 5.5 (MEDIUM). A NULL pointer dereference vulnerability in the Linux kernel's PCMCIA subsystem could allow local attackers to cause a kernel panic or potentially execute arbitrary code. This affects systems with PCMCIA/CardBus hardware support enabled in the kernel. Only local users with access to PCMCIA operations are affected.

📋 TL;DR

A NULL pointer dereference vulnerability in the Linux kernel's PCMCIA subsystem could allow local attackers to cause a kernel panic or potentially execute arbitrary code. This affects systems with PCMCIA/CardBus hardware support enabled in the kernel. Only local users with access to PCMCIA operations are affected.

💻 Affected Systems

Products:

Linux kernel

Versions: Specific affected versions not specified in CVE, but patches exist in stable kernel trees

Operating Systems: Linux distributions with vulnerable kernel versions

Default Config Vulnerable: ⚠️ Yes

Notes: Only vulnerable if PCMCIA/CardBus support is compiled into the kernel (CONFIG_PCMCIA=y) and hardware is present or emulated.

📦 What is this software?

Debian Linux by Debian

View all CVEs affecting Debian Linux →

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 →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Local privilege escalation to kernel-level code execution, leading to complete system compromise.

🟠

Likely Case

Kernel panic causing denial of service (system crash) when triggered by a local user.

🟢

If Mitigated

Minimal impact if proper access controls prevent local users from accessing PCMCIA operations.

🌐 Internet-Facing: LOW - Requires local access to exploit, not remotely exploitable.

🏢 Internal Only: MEDIUM - Local users could crash systems or potentially escalate privileges.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Requires local access and ability to trigger PCMCIA memory validation operations. No public exploit code known.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Patched in stable kernel versions via commits referenced in CVE

Vendor Advisory: https://git.kernel.org/stable/c/06b26e3099207c94b3d1be8565aedc6edc4f0a60

Restart Required: Yes

Instructions:

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

🔧 Temporary Workarounds

Disable PCMCIA support

linux

Remove PCMCIA kernel module if not needed

sudo modprobe -r pcmcia_core
echo 'blacklist pcmcia_core' | sudo tee /etc/modprobe.d/blacklist-pcmcia.conf

🧯 If You Can't Patch

Restrict local user access to systems with PCMCIA hardware
Implement strict access controls to prevent unauthorized local users from accessing PCMCIA operations

🔍 How to Verify

Check if Vulnerable:

Check if PCMCIA module is loaded: lsmod | grep pcmcia. Check kernel version against patched releases.

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version after update matches patched version. Check that PCMCIA operations work without crashes.

📡 Detection & Monitoring

Log Indicators:

Kernel panic messages in /var/log/kern.log or dmesg
NULL pointer dereference errors in kernel logs

Network Indicators:

None - local exploit only

SIEM Query:

source="kernel" AND ("NULL pointer dereference" OR "kernel panic" OR "Oops") AND process="pcmcia"

📊 Metadata

CVE ID: CVE-2025-39920

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-476

EPSS Score: 0.02% exploit probability (5.1th percentile)

Published: October 1, 2025

Last Updated: January 16, 2026

🔗 References

https://git.kernel.org/stable/c/06b26e3099207c94b3d1be8565aedc6edc4f0a60 Patch
https://git.kernel.org/stable/c/289b58f8ff3198d091074a751d6b8f6827726f3e Patch
https://git.kernel.org/stable/c/369bf6e241506583f4ee7593c53b92e5a9f271b4 Patch
https://git.kernel.org/stable/c/4a81f78caa53e0633cf311ca1526377d9bff7479 Patch
https://git.kernel.org/stable/c/5b60ed401b47897352c520bc724c85aa908dedcc Patch
https://git.kernel.org/stable/c/85be7ef8c8e792a414940a38d94565dd48d2f236 Patch
https://git.kernel.org/stable/c/8699358b6ac99b8ccc97ed9e6e3669ef8958ef7b Patch
https://git.kernel.org/stable/c/ae184024ef31423e5beb44cf4f52999bbcf2fe5b Patch
https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html Third Party Advisory,Mailing List
https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html Third Party Advisory,Mailing List

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