CVE-2026-31707: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31707 Overview

CVE-2026-31707 is an integer overflow vulnerability in the Linux kernel's ksmbd SMB server subsystem. The flaw resides in ipc_validate_msg(), which computes expected response message sizes using attacker-influenced fields from the userspace ksmbd daemon. Three code paths perform unsigned arithmetic that can wrap, allowing a crafted response to bypass the size validation check. Downstream consumers then trust the unverified length, leading to out-of-bounds memory operations [CWE-787].

Critical Impact

A wrapped msg_sz value matching entry->msg_sz enables out-of-bounds writes via memcpy in smb2pdu.c and kmemdup in ksmbd_alloc_user(), threatening kernel memory integrity and availability.

Affected Products

• Linux kernel ksmbd subsystem (multiple stable branches)
• Systems running the in-kernel SMB3 server with ksmbd-tools userspace daemon
• Distributions shipping vulnerable kernels prior to the four upstream stable backports

Discovery Timeline

• 2026-05-01 - CVE-2026-31707 published to NVD
• 2026-05-06 - Last updated in NVD database

Technical Details for CVE-2026-31707

Vulnerability Analysis

The ksmbd kernel module communicates with a userspace daemon over a Netlink-based IPC channel. The function ipc_validate_msg() calculates the expected response size for each event type before forwarding the buffer to consumers. Three response types perform arithmetic on attacker-controlled fields without overflow checks.

For KSMBD_EVENT_RPC_REQUEST and KSMBD_EVENT_SHARE_CONFIG_REQUEST, the code adds resp->payload_sz (a __u32) to a fixed struct size in unsigned int arithmetic. A sufficiently large payload_sz causes the sum to wrap below UINT_MAX. For KSMBD_EVENT_LOGIN_REQUEST_EXT, the code multiplies resp->ngroups (a __s32) by sizeof(gid_t). A negative ngroups is converted to SIZE_MAX before the multiplication, producing an attacker-controlled wrapped value.

Root Cause

The root cause is unchecked integer arithmetic mixing signed, unsigned, and size_t types when computing buffer length expectations. When the wrapped msg_sz happens to equal entry->msg_sz, the size check at the next line passes. Consumers in smb2pdu.c:6742 then call memcpy using the original rpc_resp->payload_sz, and ksmbd_alloc_user() calls kmemdup using resp_ext->ngroups, both reading or writing past the allocated buffer.

Attack Vector

Exploitation requires local access with privileges sufficient to influence the ksmbd userspace daemon's response messages, or compromise of the daemon process itself. The attacker crafts an IPC response whose payload_sz or ngroups field triggers integer wraparound such that the computed msg_sz matches the transport-layer length. The kernel then trusts the unverified field, producing out-of-bounds memory access in kernel context.

The upstream fix introduces check_add_overflow() on the RPC and SHARE_CONFIG paths and rejects ngroups values outside [0, NGROUPS_MAX] at the IPC boundary for the LOGIN_REQUEST_EXT path. This mirrors commit aab98e2dbd64, which previously hardened the request side against the same class of overflow. Refer to the upstream patch series for the complete diff.

Detection Methods for CVE-2026-31707

Indicators of Compromise

• Unexpected crashes, panics, or KASAN reports referencing ipc_validate_msg, ksmbd_alloc_user, or smb2pdu.c in kernel logs.
• Anomalous ksmbd-tools daemon behavior, including unexpected restarts or memory growth in NDR response paths.
• SMB session activity correlated with kernel oops events on hosts running ksmbd.

Detection Strategies

• Audit kernel versions across the fleet and flag hosts running ksmbd builds prior to the upstream fixes.
• Enable KASAN and kernel panic-on-oops in test environments to surface overflow-driven memory corruption during validation.
• Monitor dmesg and journald for ksmbd: warnings, particularly entries originating from ipc_validate_msg() or login/RPC handling paths.

Monitoring Recommendations

• Forward kernel logs to a centralized log platform and alert on ksmbd errors and unexpected SMB service restarts.
• Track integrity of the ksmbd-tools userspace binary using file integrity monitoring to identify tampering that could weaponize this flaw.
• Baseline SMB traffic patterns and alert on anomalous session counts or authentication storms preceding kernel instability.

How to Mitigate CVE-2026-31707

Immediate Actions Required

• Apply the stable kernel updates referenced in the vendor advisories as soon as distribution packages are available.
• Disable the ksmbd kernel module on hosts that do not require an in-kernel SMB server, falling back to Samba if file sharing is needed.
• Restrict local access and tighten privilege boundaries on systems that must continue running ksmbd until patched.

Patch Information

The Linux kernel maintainers released fixes across multiple stable branches. The patches add check_add_overflow() on the KSMBD_EVENT_RPC_REQUEST and KSMBD_EVENT_SHARE_CONFIG_REQUEST paths and validate ngroups against NGROUPS_MAX for KSMBD_EVENT_LOGIN_REQUEST_EXT. See the stable commit 299db777, commit 7dd0c858, commit 99c631d0, and commit d6a6aa81.

Workarounds

• Unload the ksmbd module with modprobe -r ksmbd and blacklist it where SMB serving is not required.
• Block inbound SMB traffic at host and network firewalls to reduce the attack surface against vulnerable kernels.
• Limit which local accounts can interact with the ksmbd IPC interface and audit the ksmbd-tools daemon configuration for least privilege.

# Configuration example
# Unload and blacklist the ksmbd module pending patch deployment
sudo modprobe -r ksmbd
echo 'blacklist ksmbd' | sudo tee /etc/modprobe.d/blacklist-ksmbd.conf
sudo update-initramfs -u

# Verify the running kernel no longer exposes the module
lsmod | grep ksmbd