CVE-2026-53383

Summary

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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/25ff12b82a376ff5c4583102a63d2456a6b9ebb9
• https://git.kernel.org/stable/c/fc578523a72cb8b329d32070b95898e81613cc3f
• https://git.kernel.org/stable/c/d2bbbb6c55812220fee5d801c275cc267ea3cbeb
• https://git.kernel.org/stable/c/8f0302fb691537d33ec8f668565257ea9d340ffe
• https://git.kernel.org/stable/c/7cad3ceaf679c55bc9946685dacafce78ce6b51a
• https://git.kernel.org/stable/c/06e1f05a1dbe8bbd054c0927b17fc0a61cc8bef7
• https://git.kernel.org/stable/c/5f983b864d3d473ac533b2f4f44a1bbe5dcbccf4
• https://git.kernel.org/stable/c/609ca17d869d04ba249e32cdcbf13c0b1c66f43c

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