CVE-2025-39943

Q: What is the severity of CVE-2025-39943?

CVE-2025-39943 has a CVSS score of 7.1 (HIGH). A memory corruption vulnerability in the Linux kernel's ksmbd SMB server component allows attackers to trigger out-of-bounds memory access by sending specially crafted SMB Direct data transfer packets. This affects systems running vulnerable Linux kernel versions with ksmbd enabled. The vulnerability could lead to system crashes or potential code execution.

7.1 HIGH

Denial of Service

📋 TL;DR

A memory corruption vulnerability in the Linux kernel's ksmbd SMB server component allows attackers to trigger out-of-bounds memory access by sending specially crafted SMB Direct data transfer packets. This affects systems running vulnerable Linux kernel versions with ksmbd enabled. The vulnerability could lead to system crashes or potential code execution.

💻 Affected Systems

Products:

Linux kernel

Versions: Specific vulnerable kernel versions with ksmbd support (exact versions not specified in CVE, but patches available for stable branches)

Operating Systems: Linux distributions with ksmbd enabled

Default Config Vulnerable: ✅ No

Notes: Only vulnerable if ksmbd (kernel SMB server) is enabled and configured. Many distributions don't enable ksmbd 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...

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⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution leading to complete system compromise, data theft, or ransomware deployment.

🟠

Likely Case

Kernel panic causing system crashes and denial of service, potentially leading to data corruption.

🟢

If Mitigated

System remains stable with proper input validation preventing exploitation attempts.

🌐 Internet-Facing: MEDIUM - Requires SMB Direct protocol exposure and ksmbd service enabled, which is less common than standard SMB.

🏢 Internal Only: MEDIUM - Internal attackers could exploit if ksmbd is enabled and they have network access to the service.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires sending malformed SMB Direct packets to a vulnerable ksmbd instance. No authentication needed if service is exposed.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Patched in Linux kernel stable releases via commits referenced in CVE

Vendor Advisory: https://git.kernel.org/stable/c/5282491fc49d5614ac6ddcd012e5743eecb6a67c

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 ksmbd module loads correctly if needed.

🔧 Temporary Workarounds

Disable ksmbd module

linux

Prevent loading of vulnerable ksmbd kernel module

echo 'blacklist ksmbd' >> /etc/modprobe.d/blacklist-ksmbd.conf
rmmod ksmbd

Block SMB Direct ports

linux

Block SMB and SMB Direct traffic at network level

iptables -A INPUT -p tcp --dport 445 -j DROP
iptables -A INPUT -p tcp --dport 5445 -j DROP

🧯 If You Can't Patch

Disable ksmbd service if not required for business operations
Implement network segmentation to restrict access to ksmbd services

🔍 How to Verify

Check if Vulnerable:

Check if ksmbd module is loaded: lsmod | grep ksmbd. If loaded and kernel version is unpatched, system is vulnerable.

Check Version:

uname -r

Verify Fix Applied:

Check kernel version matches patched release from your distribution. Verify ksmbd module version if available.

📡 Detection & Monitoring

Log Indicators:

Kernel panic logs
ksmbd crash dumps in system logs
Unexpected SMB Direct connection attempts

Network Indicators:

Malformed SMB Direct packets on port 5445
Unusual SMB protocol anomalies

SIEM Query:

source="kernel" AND ("panic" OR "Oops" OR "ksmbd")

📊 Metadata

CVE ID: CVE-2025-39943

CVSS v3 Score: 7.1 (HIGH)

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

CWE: CWE-125

EPSS Score: 0.02% exploit probability (5.5th percentile)

Published: October 4, 2025

Last Updated: January 27, 2026

📋 TL;DR

💻 Affected Systems

📦 What is this software?

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

Linux Kernel by Linux

⚠️ Risk & Real-World Impact

Worst Case

Likely Case

If Mitigated

🎯 Exploit Status

🛠️ Fix & Mitigation

✅ Official Fix

Instructions:

🔧 Temporary Workarounds

Disable ksmbd module

Block SMB Direct ports

🧯 If You Can't Patch

🔍 How to Verify

Check if Vulnerable:

Check Version:

Verify Fix Applied:

📡 Detection & Monitoring

Log Indicators:

Network Indicators:

SIEM Query:

📊 Metadata

🔗 References

📤 Share & Export

🔗 Related Vulnerabilities