CVE-2026-31707
Published: 01 May 2026
Summary
CVE-2026-31707 is a high-severity Out-of-bounds Write (CWE-787) vulnerability in Linux Linux Kernel. Its CVSS base score is 7.1 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 2.4th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).
Threat & Defense at a Glance
Threat & Defense Details
Mitigating Controls (NIST 800-53 r5)AI
Mandates validating attacker-controlled inputs like payload_sz and ngroups to detect integer overflows in message size calculations within ipc_validate_msg(), preventing bypass of size checks.
Implements memory protections such as stack canaries and address space randomization to mitigate corruption from memcpy() and kmemdup() using unverified lengths.
Requires timely flaw remediation via kernel patches like check_add_overflow() and ngroups bounds checks to directly address the integer overflow vulnerability.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Integer overflow bypasses kernel size validation in IPC path, directly enabling memcpy/kmemdup memory corruption that can be abused for local DoS via kernel crash (T1499.004 Application or System Exploitation) or to disclose kernel memory contents (T1005 Data from Local System).
NVD Description
In the Linux kernel, the following vulnerability has been resolved: ksmbd: validate response sizes in ipc_validate_msg() ipc_validate_msg() computes the expected message size for each response type by adding (or multiplying) attacker-controlled fields from the daemon response to a fixed struct…
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size in unsigned int arithmetic. Three cases can overflow: KSMBD_EVENT_RPC_REQUEST: msg_sz = sizeof(struct ksmbd_rpc_command) + resp->payload_sz; KSMBD_EVENT_SHARE_CONFIG_REQUEST: msg_sz = sizeof(struct ksmbd_share_config_response) + resp->payload_sz; KSMBD_EVENT_LOGIN_REQUEST_EXT: msg_sz = sizeof(struct ksmbd_login_response_ext) + resp->ngroups * sizeof(gid_t); resp->payload_sz is __u32 and resp->ngroups is __s32. Each addition can wrap in unsigned int; the multiplication by sizeof(gid_t) mixes signed and size_t, so a negative ngroups is converted to SIZE_MAX before the multiply. A wrapped value of msg_sz that happens to equal entry->msg_sz bypasses the size check on the next line, and downstream consumers (smb2pdu.c:6742 memcpy using rpc_resp->payload_sz, kmemdup in ksmbd_alloc_user using resp_ext->ngroups) then trust the unverified length. Use check_add_overflow() on the RPC_REQUEST and SHARE_CONFIG_REQUEST paths to detect integer overflow without constraining functional payload size; userspace ksmbd-tools grows NDR responses in 4096-byte chunks for calls like NetShareEnumAll, so a hard transport cap is unworkable on the response side. For LOGIN_REQUEST_EXT, reject resp->ngroups outside the signed [0, NGROUPS_MAX] range up front and report the error from ipc_validate_msg() so it fires at the IPC boundary; with that bound the subsequent multiplication and addition stay well below UINT_MAX. The now-redundant ngroups check and pr_err in ksmbd_alloc_user() are removed. This is the response-side analogue of aab98e2dbd64 ("ksmbd: fix integer overflows on 32 bit systems"), which hardened the request side.
Deeper analysisAI
CVE-2026-31707 is an integer overflow vulnerability in the Linux kernel's ksmbd module, specifically in the ipc_validate_msg() function used for validating response sizes from the userspace ksmbd daemon. The issue arises during unsigned int arithmetic when computing expected message sizes for certain response types—KSMBD_EVENT_RPC_REQUEST, KSMBD_EVENT_SHARE_CONFIG_REQUEST, and KSMBD_EVENT_LOGIN_REQUEST_EXT—by adding or multiplying attacker-controlled fields like resp->payload_sz (__u32) or resp->ngroups (__s32) to fixed struct sizes. These operations can wrap around, producing a msg_sz that matches entry->msg_sz and bypasses the size check, causing downstream code to trust unverified lengths in operations such as memcpy() in smb2pdu.c and kmemdup() in ksmbd_alloc_user().
A local attacker with low privileges (AV:L/AC:L/PR:L) can exploit this vulnerability by controlling fields in daemon responses sent over the IPC channel to the kernel. Successful exploitation bypasses validation, enabling memory corruption through memcpy() with an unverified payload_sz or kmemdup() with manipulated ngroups, potentially leading to high confidentiality impact (e.g., information disclosure) and high availability impact (e.g., denial of service via kernel crash), as reflected in the CVSS v3.1 base score of 7.1 (C:H/I:N/A:H/S:U).
Kernel patches address the issue via commits such as 299db777ea0c, 7dd0c858e190, 99c631d0366c, and d6a6aa81eac2. For RPC_REQUEST and SHARE_CONFIG_REQUEST paths, check_add_overflow() detects overflows without limiting functional payload sizes, accommodating userspace growth in 4096-byte chunks. For LOGIN_REQUEST_EXT, ngroups values outside the signed [0, NGROUPS_MAX] range are rejected early at the IPC boundary, ensuring multiplication and addition remain below UINT_MAX; redundant checks in ksmbd_alloc_user() were removed. This fix mirrors prior hardening of request-side overflows (commit aab98e2dbd64).
Details
- CWE(s)