CVE-2026-23220 Overview
CVE-2026-23220 is a Denial of Service vulnerability in the Linux kernel's ksmbd (kernel SMB server) component. The vulnerability occurs when a signed SMB2 request fails signature verification, causing an infinite loop due to improper handling of the next_smb2_rcv_hdr_off variable in error paths. This flaw can lead to high CPU usage and kernel log flooding, effectively causing a denial of service condition on affected systems.
Critical Impact
Attackers can trigger an infinite loop in the ksmbd module by sending malformed signed SMB2 requests, causing kernel log flooding with "bad smb2 signature" messages and sustained high CPU utilization.
Affected Products
- Linux kernel with ksmbd module enabled
- Systems using in-kernel SMB3 file server functionality
- Linux distributions with ksmbd support compiled into the kernel
Discovery Timeline
- 2026-02-18 - CVE-2026-23220 published to NVD
- 2026-02-19 - Last updated in NVD database
Technical Details for CVE-2026-23220
Vulnerability Analysis
This vulnerability represents a critical flaw in the error handling logic of the ksmbd module's SMB2 request processing chain. When the kernel SMB server receives a chained SMB2 request that requires signature verification, the check_sign_req() function validates the cryptographic signature. Upon signature verification failure, the set_smb2_rsp_status() function is invoked to return a STATUS_ACCESS_DENIED error to the client.
The root issue lies in how set_smb2_rsp_status() resets the work->next_smb2_rcv_hdr_off variable to zero as a side effect. This offset pointer is critical for tracking the position within chained SMB2 command sequences. When reset to zero, the kernel loses track of the current position in the command chain, causing is_chained_smb2_message() to incorrectly evaluate the same request header repeatedly.
If the original request's NextCommand field contains a non-zero value (indicating additional commands in the chain), the processing loop in __handle_ksmbd_work() never advances past the failed request. Instead, it continuously reprocesses the same malformed request, generating an endless stream of signature verification failures.
Root Cause
The vulnerability stems from improper state management during error handling in the SMB2 request processing pipeline. The set_smb2_rsp_status() function unconditionally resets next_smb2_rcv_hdr_off to zero without considering whether this invalidation would break chained message processing. The __process_request() function should have detected this error condition and returned SERVER_HANDLER_ABORT instead of SERVER_HANDLER_CONTINUE, which would properly terminate the processing loop.
Attack Vector
An attacker can exploit this vulnerability by sending specially crafted SMB2 requests to a system running the ksmbd module. The attack requires:
- Network access to the ksmbd service (typically TCP port 445)
- Crafting a chained SMB2 request with multiple commands where the NextCommand field is non-zero
- Intentionally providing an invalid or malformed signature for the request
When the signature verification fails, the kernel enters an infinite loop, consuming CPU resources and flooding the kernel log with error messages. This effectively creates a denial of service condition that persists until the affected process is manually terminated or the system is rebooted.
The vulnerability mechanism involves the following sequence in the ksmbd module's request processing:
- A signed chained SMB2 request arrives at __process_request()
- check_sign_req() validates the signature and returns an error for invalid signatures
- set_smb2_rsp_status() is called with STATUS_ACCESS_DENIED, resetting next_smb2_rcv_hdr_off to zero
- is_chained_smb2_message() checks if more commands exist in the chain
- Since NextCommand is non-zero and the offset was reset, the same header is evaluated repeatedly
- __handle_ksmbd_work() loops infinitely, reprocessing the failed request
For complete technical details, refer to the Linux kernel commit fixes.
Detection Methods for CVE-2026-23220
Indicators of Compromise
- Repeated "bad smb2 signature" messages in kernel logs (dmesg or /var/log/kern.log)
- Abnormally high CPU utilization by kernel threads associated with ksmbd
- ksmbd worker threads consuming 100% CPU on one or more cores
- System unresponsiveness or degraded performance on SMB file sharing services
Detection Strategies
- Monitor kernel logs for repeated signature verification failure messages using log aggregation tools
- Implement CPU usage alerting for kernel threads with names matching ksmbd patterns
- Deploy network intrusion detection rules to identify malformed SMB2 chained requests
- Use kernel tracing tools (ftrace, eBPF) to detect abnormal iteration counts in ksmbd processing functions
Monitoring Recommendations
- Configure syslog monitoring to alert on high-frequency "bad smb2 signature" messages
- Establish baseline CPU utilization metrics for ksmbd processes and alert on anomalies
- Monitor network traffic patterns for unusual SMB2 request volumes or malformed packets
- Implement kernel watchdog mechanisms to detect and recover from infinite loop conditions
How to Mitigate CVE-2026-23220
Immediate Actions Required
- Apply the kernel patches from the official Linux kernel git repository immediately
- If patching is not immediately possible, consider temporarily disabling the ksmbd module
- Implement network-level access controls to restrict SMB access to trusted networks only
- Monitor systems for signs of exploitation while awaiting patch deployment
Patch Information
The vulnerability has been resolved in the Linux kernel through multiple commits that change the return value from SERVER_HANDLER_CONTINUE to SERVER_HANDLER_ABORT when signature verification fails. This ensures the processing loop terminates immediately rather than attempting to continue from an invalidated offset.
Official patches are available from the following kernel commits:
Workarounds
- Disable the ksmbd kernel module if not required: modprobe -r ksmbd
- Use user-space Samba instead of ksmbd for SMB file sharing services
- Implement firewall rules to restrict SMB (port 445) access to trusted IP ranges
- Consider network segmentation to isolate systems running ksmbd from untrusted networks
# Disable ksmbd module temporarily
sudo modprobe -r ksmbd
# Prevent ksmbd from loading on boot
echo "blacklist ksmbd" | sudo tee /etc/modprobe.d/blacklist-ksmbd.conf
# Restrict SMB access via iptables (allow only trusted network)
sudo iptables -A INPUT -p tcp --dport 445 -s 192.168.1.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 445 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
