CVE-2026-53383

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: reject non-VALID session in compound request branch

smb2_check_user_session() takes a shortcut for any operation that is not

the first in a COMPOUND request: it reuses work->sess (the session bound by

the first operation) and validates only the SessionId, then returns

"valid". It never re-checks work->sess->state == SMB2_SESSION_VALID, and a

SessionId of 0xFFFFFFFFFFFFFFFF (ULLONG_MAX, the MS-SMB2 related-operation

value) skips even the id comparison. The standalone path

(ksmbd_session_lookup_all() plus the SESSION_SETUP state machine) does

enforce the VALID state; the compound branch bypasses all of it.

A SESSION_SETUP carrying only an NTLM Type-1 (NtLmNegotiate) blob publishes

a fresh SMB2_SESSION_IN_PROGRESS session whose sess->user is still NULL

(->user is assigned later, by ntlm_authenticate()). Used as operation 1 of

a COMPOUND with operation 2 = TREE_CONNECT (related, SessionId=ULLONG_MAX,

\\host\IPC$), the tree-connect then runs on that IN_PROGRESS session and

reaches ksmbd_ipc_tree_connect_request(), which dereferences

user_name(sess->user) with sess->user == NULL (transport_ipc.c:687/701/704)

-> remote NULL-pointer dereference and a kernel Oops that wedges the ksmbd

worker for all clients.

Reject any non-first compound operation that lands on a session which is

not SMB2_SESSION_VALID, mirroring the validity the standalone lookup path

enforces. SESSION_SETUP itself legitimately runs on an IN_PROGRESS session,

but it is never carried as a non-first compound operation, so multi-leg

authentication is unaffected by this check.