CVE-2026-53383 (GCVE-0-2026-53383)
Vulnerability from cvelistv5
Published
2026-07-19 11:59
Modified
2026-08-17 04:50
Severity ?
7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
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.
References
Impacted products
Vendor Product Version
Linux Linux Version: eb947403518ea3d93f6d89264bb1f5416bb0c7d0
Version: 854156d12caa9d36de1cf5f084591c7686cc8a9d
Version: 5005bcb4219156f1bf7587b185080ec1da08518e
Version: 5005bcb4219156f1bf7587b185080ec1da08518e
Version: 5005bcb4219156f1bf7587b185080ec1da08518e
Version: 5005bcb4219156f1bf7587b185080ec1da08518e
Version: 5005bcb4219156f1bf7587b185080ec1da08518e
Version: 5005bcb4219156f1bf7587b185080ec1da08518e
Version: d1066c1b3663401cd23c0d6e60cdae750ce00c0f
Version: 5.15.121   ≤
Version: 6.1.36   ≤
Version: 6.3.10   ≤
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{
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nksmbd: reject non-VALID session in compound request branch\n\nsmb2_check_user_session() takes a shortcut for any operation that is not\nthe first in a COMPOUND request: it reuses work-\u003esess (the session bound by\nthe first operation) and validates only the SessionId, then returns\n\"valid\". It never re-checks work-\u003esess-\u003estate == SMB2_SESSION_VALID, and a\nSessionId of 0xFFFFFFFFFFFFFFFF (ULLONG_MAX, the MS-SMB2 related-operation\nvalue) skips even the id comparison. The standalone path\n(ksmbd_session_lookup_all() plus the SESSION_SETUP state machine) does\nenforce the VALID state; the compound branch bypasses all of it.\n\nA SESSION_SETUP carrying only an NTLM Type-1 (NtLmNegotiate) blob publishes\na fresh SMB2_SESSION_IN_PROGRESS session whose sess-\u003euser is still NULL\n(-\u003euser is assigned later, by ntlm_authenticate()). Used as operation 1 of\na COMPOUND with operation 2 = TREE_CONNECT (related, SessionId=ULLONG_MAX,\n\\\\host\\IPC$), the tree-connect then runs on that IN_PROGRESS session and\nreaches ksmbd_ipc_tree_connect_request(), which dereferences\nuser_name(sess-\u003euser) with sess-\u003euser == NULL (transport_ipc.c:687/701/704)\n-\u003e remote NULL-pointer dereference and a kernel Oops that wedges the ksmbd\nworker for all clients.\n\nReject any non-first compound operation that lands on a session which is\nnot SMB2_SESSION_VALID, mirroring the validity the standalone lookup path\nenforces. SESSION_SETUP itself legitimately runs on an IN_PROGRESS session,\nbut it is never carried as a non-first compound operation, so multi-leg\nauthentication is unaffected by this check."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.5,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - The bug is in ksmbd\u0027s SMB2 compound-request handler, reachable by any remote attacker who can send crafted SMB2 packets to TCP port 445 on a host running the in-kernel SMB server.\nAC:L - An attacker can reliably trigger the bug by sending a compound SMB2 request with SESSION_SETUP (NTLM Type-1 only) as the first operation and TREE_CONNECT (SessionId=0xFFFFFFFFFFFFFFFF, \\\\host\\IPC$) as the second; no special timing, memory layout, or victim state is required.\nPR:N - Exploitation requires only network access to ksmbd during the SMB2 authentication handshake, before sess-\u003euser is assigned; no Linux local account, capability, or prior authenticated OS session is needed.\nUI:N - No victim user action (opening files, mounting shares, clicking links) is required; the attacker triggers the crash solely by sending malicious SMB2 protocol traffic.\nS:U - The impact is a kernel oops and ksmbd worker disruption within the kernel/SMB server security authority; it does not cross a VM, container, or IOMMU boundary.\nC:N - The failure is a NULL pointer dereference on sess-\u003euser in ksmbd_ipc_tree_connect_request() when user_name(sess-\u003euser) is called with sess-\u003euser == NULL; this is a pure crash with no information disclosure.\nI:N - The bug causes a NULL pointer dereference only; there is no memory corruption, arbitrary write, or code-execution primitive\u2014only a kernel fault.\nA:H - The NULL dereference causes a kernel oops that wedges the shared ksmbd-io workqueue worker, denying SMB service to all clients connected through that worker and potentially panicking the host if panic_on_oops is enabled."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-17T04:50:33.785Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/25ff12b82a376ff5c4583102a63d2456a6b9ebb9"
        },
        {
          "url": "https://git.kernel.org/stable/c/fc578523a72cb8b329d32070b95898e81613cc3f"
        },
        {
          "url": "https://git.kernel.org/stable/c/d2bbbb6c55812220fee5d801c275cc267ea3cbeb"
        },
        {
          "url": "https://git.kernel.org/stable/c/8f0302fb691537d33ec8f668565257ea9d340ffe"
        },
        {
          "url": "https://git.kernel.org/stable/c/7cad3ceaf679c55bc9946685dacafce78ce6b51a"
        },
        {
          "url": "https://git.kernel.org/stable/c/06e1f05a1dbe8bbd054c0927b17fc0a61cc8bef7"
        },
        {
          "url": "https://git.kernel.org/stable/c/5f983b864d3d473ac533b2f4f44a1bbe5dcbccf4"
        },
        {
          "url": "https://git.kernel.org/stable/c/609ca17d869d04ba249e32cdcbf13c0b1c66f43c"
        }
      ],
      "title": "ksmbd: reject non-VALID session in compound request branch",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-53383",
    "datePublished": "2026-07-19T11:59:31.616Z",
    "dateReserved": "2026-06-09T07:44:35.401Z",
    "dateUpdated": "2026-08-17T04:50:33.785Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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