CVE-2026-31707
Analyzed Analyzed - Analysis Complete
Integer Overflow in Linux Kernel ksmbd

Publication date: 2026-05-01

Last updated on: 2026-05-23

Assigner: kernel.org

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 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.
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Meta Information
Published
2026-05-01
Last Modified
2026-05-23
Generated
2026-06-16
AI Q&A
2026-05-01
EPSS Evaluated
2026-06-15
NVD
CVE-2026-31707
EUVD
EUVD-2026-26516
Affected Vendors & Products
Showing 3 associated CPEs
Vendor Product Version / Range
linux linux_kernel From 6.13 (inc) to 6.18.25 (exc)
linux linux_kernel From 6.19 (inc) to 7.0.2 (exc)
linux linux_kernel From 5.15 (inc) to 6.12.84 (exc)
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Exploitability
CWE
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KEV
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CWE ID Description
CWE-787 The product writes data past the end, or before the beginning, of the intended buffer.
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Executive Summary

This vulnerability exists in the Linux kernel's ksmbd component, specifically in the ipc_validate_msg() function that validates response sizes. The function calculates expected message sizes by adding or multiplying attacker-controlled fields from daemon responses to fixed structure sizes using unsigned integer arithmetic. Three cases can cause integer overflow: KSMBD_EVENT_RPC_REQUEST, KSMBD_EVENT_SHARE_CONFIG_REQUEST, and KSMBD_EVENT_LOGIN_REQUEST_EXT.

The overflow occurs because resp->payload_sz and resp->ngroups fields, which are attacker-controlled, can wrap around the unsigned integer limit or convert negative values to very large unsigned values. This causes the computed message size (msg_sz) to wrap and potentially match an expected size, bypassing size checks. As a result, downstream code trusts unverified lengths, leading to potential memory corruption or other issues.

The fix involves using overflow checks (check_add_overflow()) on the first two cases and rejecting out-of-range values for ngroups in the third case to prevent overflow and ensure message sizes are properly validated.

Impact Analysis

This vulnerability can lead to integer overflow in message size validation, allowing an attacker to bypass size checks and cause the system to trust incorrect message lengths. This can result in memory corruption, such as buffer overflows or improper memory allocations, potentially leading to denial of service, information disclosure, or arbitrary code execution within the Linux kernel's ksmbd service.

Mitigation Strategies

The vulnerability is addressed by validating response sizes in the ipc_validate_msg() function within the ksmbd component of the Linux kernel.

Immediate mitigation involves ensuring that the kernel version in use includes the fix that applies check_add_overflow() on the RPC_REQUEST and SHARE_CONFIG_REQUEST paths to detect integer overflow without limiting functional payload size.

Additionally, for LOGIN_REQUEST_EXT responses, the fix rejects resp->ngroups values outside the signed [0, NGROUPS_MAX] range early, preventing overflow.

Therefore, the recommended immediate step is to update the Linux kernel to a version that contains this fix.

Detection Guidance

The vulnerability involves integer overflow in the ksmbd daemon's response size validation in the Linux kernel. Detection would require monitoring or analyzing ksmbd responses for abnormal or suspicious message sizes that could indicate an overflow.

Since the vulnerability is in kernel code related to ksmbd's IPC message validation, direct detection on the network or system using simple commands is not straightforward from the provided information.

No specific detection commands or tools are provided in the available context or resources.

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