CVE-2026-31707: 
Chainguard vulnerability analysis and mitigation

CVE-2026-31707 is an integer overflow vulnerability in the Linux kernel's ksmbd (kernel SMB daemon) IPC message validation function ipc_validate_msg(). Attacker-controlled fields from daemon responses — specifically payload_sz (__u32) and ngroups (__s32) — are used in unsigned integer arithmetic that can wrap around, bypassing size checks and causing downstream kernel code to trust unverified payload lengths. Affected Linux kernel versions span 5.15 through 6.12 (fixed in 6.12.84), 6.13 through 6.18 (fixed in 6.18.25), and 6.19 and later (fixed in 7.0.2). It was published on May 1, 2026, and carries a CVSS v3.1 base score of 7.1 (High) (Red Hat Advisory, Red Hat Bugzilla).

The root cause is an incorrect calculation of buffer size (CWE-131) leading to out-of-bounds write (CWE-787) in ipc_validate_msg(). Three event types are vulnerable: KSMBD_EVENT_RPC_REQUEST and KSMBD_EVENT_SHARE_CONFIG_REQUEST compute msg_sz by adding a __u32 payload_sz to a fixed struct size in unsigned int arithmetic, which can wrap; KSMBD_EVENT_LOGIN_REQUEST_EXT multiplies a __s32 ngroups by sizeof(gid_t), where a negative ngroups is implicitly converted to SIZE_MAX before the multiply. A wrapped msg_sz that coincidentally equals entry->msg_sz bypasses the size check, and downstream consumers — a memcpy in smb2pdu.c:6742 using rpc_resp->payload_sz and a kmemdup in ksmbd_alloc_user using resp_ext->ngroups — then operate on the unverified lengths. The fix applies check_add_overflow() for the RPC and share config paths, and enforces a signed [0, NGROUPS_MAX] range check for ngroups before any arithmetic (Red Hat Bugzilla, Kernel Patch).

A local user with low privileges who can interact with the ksmbd daemon can craft malformed IPC response messages that bypass size validation, triggering out-of-bounds kernel memory writes via memcpy and kmemdup. This can result in high confidentiality impact (kernel memory disclosure) and high availability impact (kernel crash or instability), with no integrity impact per the CVSS assessment. Successful exploitation could escalate to kernel code execution, enabling full system compromise (Red Hat Advisory, Red Hat Bugzilla).

As of the time of publication, there is no known public proof-of-concept exploit and no evidence of in-the-wild exploitation. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score is approximately 0.018% (0.000180), indicating a very low probability of exploitation in the near term. Exploitation requires local access with low privileges, limiting the attack surface compared to remotely exploitable vulnerabilities (Red Hat Advisory).

Update to a patched Linux kernel version: 6.12.84 or later for the 5.15–6.12 branch, 6.18.25 or later for the 6.13–6.18 branch, or 7.0.2 or later for the 6.19+ branch. Patches are available via the upstream kernel stable tree. If immediate upgrade is not possible, consider disabling the ksmbd module (modprobe -r ksmbd) on systems where SMB kernel serving is not required. Monitor kernel changelogs for the specific commits implementing check_add_overflow() validation and ngroups range restrictions (Red Hat Advisory, Kernel Patch).

Red Hat has tracked this vulnerability via Bugzilla (Bug 2464366) and assigned it a medium severity rating. The upstream fix was referenced in the linux-cve-announce mailing list and the Yocto Project security mailing list has issued multiple advisories covering this CVE. No notable independent researcher commentary or significant social media discussion has been identified beyond standard CVE tracking (Red Hat Bugzilla).