CVE-2026-23220: 
Linux Kernel vulnerability analysis and mitigation

CVE-2026-23220 is an infinite loop vulnerability in the Linux kernel's ksmbd (kernel SMB server) subsystem that allows a local low-privileged attacker to cause a denial of service via CPU exhaustion and kernel log flooding. The flaw was published on February 18, 2026, and affects multiple Linux kernel versions: 5.15.145 up to (not including) 5.16; 6.1.71 through 6.1.163; 6.6-rc6 and later; 6.6.1 through 6.6.124; 6.7 through 6.12.71; 6.13 through 6.18.10; and 6.19 through 6.19.0. Microsoft Azure Linux (azl3_kernel_6.6.121.1-1) is also affected. It carries a CVSS v3.1 base score of 5.5 (Medium) (Red Hat, Microsoft MSRC).

The root cause is classified as CWE-835 (Loop with Unreachable Exit Condition / Infinite Loop). In the ksmbd subsystem's __process_request() function, when check_sign_req() fails SMB2 signature verification, set_smb2_rsp_status(work, STATUS_ACCESS_DENIED) is called, which inadvertently resets work->next_smb2_rcv_hdr_off to zero. This causes is_chained_smb2_message() to lose track of the next command in the SMB2 compound request chain and instead continue pointing to the same (failed) request header. If the header's NextCommand field is non-zero, is_chained_smb2_message() returns true, causing __handle_ksmbd_work() to repeatedly process the same failed request indefinitely. The fix changes the return value from SERVER_HANDLER_CONTINUE to SERVER_HANDLER_ABORT to terminate processing immediately upon error (Red Hat, Kernel Patch).

Successful exploitation results in a denial-of-service condition affecting availability only — there is no confidentiality or integrity impact. An attacker can cause the kernel to enter an infinite loop, flooding kernel logs with "bad smb2 signature" messages and driving CPU usage to sustained high levels, rendering SMB/CIFS file sharing services (ksmbd) degraded or completely unresponsive. Systems providing network file shares to other hosts may experience cascading availability impacts for dependent clients (Red Hat).

1. Identify a target: Locate a Linux system running a vulnerable kernel version (e.g., 6.6.1–6.6.124, 6.7–6.12.71, 6.13–6.18.10) with the ksmbd module loaded and active, providing SMB2 file sharing services.
2. Obtain local low-privileged access: Authenticate to the target system as any local user (no root or special privileges required).
3. Craft a malicious SMB2 compound request: Construct an SMB2 compound (chained) request packet where the NextCommand field in the SMB2 header is set to a non-zero value, and the request is signed with an invalid/incorrect signature that will fail check_sign_req().
4. Send the crafted request to ksmbd: Transmit the malformed SMB2 request to the ksmbd listener (typically TCP port 445) on the local or accessible network interface.
5. Trigger the infinite loop: The failed signature verification causes next_smb2_rcv_hdr_off to reset to zero; with NextCommand non-zero, is_chained_smb2_message() returns true repeatedly, locking the kernel worker into an infinite processing loop.
6. Observe denial of service: Monitor for kernel log flooding with "bad smb2 signature" messages and sustained high CPU usage on the target, confirming successful exploitation and DoS condition (Red Hat).

• Logs: Kernel logs (/var/log/kern.log, dmesg) flooded with repeated "bad smb2 signature" messages originating from the ksmbd subsystem.
• System Performance: Sustained abnormally high CPU usage on one or more kernel worker threads associated with ksmbd, observable via top, htop, or ps aux.
• Network: Repeated SMB2 compound requests (TCP port 445) from the same source with non-zero NextCommand fields and invalid signatures, detectable via packet capture (e.g., Wireshark/tcpdump with SMB2 dissection).
• Process: ksmbd worker threads consuming near 100% CPU without corresponding legitimate SMB workload, identifiable via perf top or kernel thread monitoring.

Apply the patched kernel versions released via the stable kernel tree: 6.1.164, 6.6.125, 6.12.72, 6.18.11, and 6.19.1 (commits: 010eb01ce23b, 5accdc5b7f28, 71b5e7c52831, 9135e791ec27, f7b1c2f5642b, fb3b66bd72de). Debian has released DSA-6163-1 and DLA-4499-1 addressing this issue. For systems that cannot be patched immediately: disable the ksmbd module if SMB file sharing is not required (modprobe -r ksmbd), restrict local user access to minimize the attack surface, and monitor for sustained high CPU usage and repeated "bad smb2 signature" kernel log messages as indicators of exploitation attempts (Red Hat, Microsoft MSRC, Kernel Patch).

Red Hat and Microsoft both published advisories for this vulnerability shortly after disclosure. Debian issued security advisories (DSA-6163-1 for stable and DLA-4499-1 for oldstable/LTS) and accepted linux 6.1.164-1 into its security repositories. The openSUSE project noted the fix in its Tumbleweed March 2026 monthly update. The Yocto Project security mailing list also flagged the issue for embedded Linux maintainers. Overall community reaction has been measured, consistent with a medium-severity local DoS in a kernel subsystem (Red Hat, Microsoft MSRC).