CVE-2025-40090
📋 TL;DR
This CVE describes a deadlock vulnerability in the Linux kernel's ksmbd SMB server module. When clients attempt to open named pipes via RPC calls, the system can hang due to recursive locking, causing denial of service. This affects Linux systems with ksmbd enabled and SMB/CIFS file sharing active.
💻 Affected Systems
- Linux kernel with ksmbd module
📦 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 →⚠️ Risk & Real-World Impact
Worst Case
Complete denial of service for SMB/CIFS file sharing services, causing hung connections and potentially requiring system reboot to restore functionality.
Likely Case
SMB/CIFS clients experience connection hangs when attempting to open named pipes, disrupting file sharing operations.
If Mitigated
No impact if ksmbd module is not loaded or SMB/CIFS services are disabled.
🎯 Exploit Status
Exploitation requires SMB/CIFS access and ability to open named pipes. The vulnerability is triggered by normal client operations.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Fixed in kernel commits: 1891abe832cbf5a11039e088766131d0f1642d02, 3412fbd81b46b9cfae013817b61d4bbd27e09e36, 4602b8cee1481dbb896182e5cb1e8cf12910e9e7, 5493571f4351f74e11db9943e98a07c56467cf7e, 88f170814fea74911ceab798a43cbd7c5599bed4
Vendor Advisory: https://git.kernel.org/stable/c/1891abe832cbf5a11039e088766131d0f1642d02
Restart Required: Yes
Instructions:
1. Update Linux kernel to version containing fix commits. 2. Reboot system to load new kernel. 3. Verify ksmbd module loads correctly.
🔧 Temporary Workarounds
Disable ksmbd module
linuxUnload ksmbd kernel module to prevent exploitation
sudo rmmod ksmbd
echo 'blacklist ksmbd' | sudo tee /etc/modprobe.d/ksmbd-blacklist.conf
Disable SMB/CIFS services
linuxStop SMB/CIFS file sharing services
sudo systemctl stop smbd
sudo systemctl disable smbd
🧯 If You Can't Patch
- Disable ksmbd module if not required for operations
- Restrict SMB/CIFS access to trusted networks only using firewall rules
🔍 How to Verify
Check if Vulnerable:
Check if ksmbd module is loaded: lsmod | grep ksmbd. If loaded and kernel version is vulnerable range, system is vulnerable.Check Version:
uname -rVerify Fix Applied:
Check kernel version includes fix commits: uname -r. Verify ksmbd module loads without issues after patch.📡 Detection & Monitoring
Log Indicators:
- Kernel logs showing hung tasks related to ksmbd
- SMB/CIFS connection timeouts in application logs
Network Indicators:
- SMB/CIFS clients reporting connection hangs
- Increased TCP timeouts on SMB ports
SIEM Query:
source="kernel" AND "hung_task" AND "ksmbd" OR source="smbd" AND "timeout" AND "pipe"🔗 References
- https://git.kernel.org/stable/c/1891abe832cbf5a11039e088766131d0f1642d02
- https://git.kernel.org/stable/c/3412fbd81b46b9cfae013817b61d4bbd27e09e36
- https://git.kernel.org/stable/c/4602b8cee1481dbb896182e5cb1e8cf12910e9e7
- https://git.kernel.org/stable/c/5493571f4351f74e11db9943e98a07c56467cf7e
- https://git.kernel.org/stable/c/88f170814fea74911ceab798a43cbd7c5599bed4
