---
id: CVE-2026-64448
title: 'smb: client: restrict implied bcc[0] exemption to responses without data area'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  smb: client: restrict implied bcc[0] exemption to responses without data area

  smb2_check_message() has a long-standing quirk that accepts a response
  whose calculated l…
severity: high
cvss: 8.2
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 <
    8d0bbc78046d264bbf6a574ea6f9072258a43e35
  - >-
    Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 <
    b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0
  - >-
    Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 <
    31c6312608c60b72a1feb99a5afb680645a3e8a3
  - >-
    Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 <
    573e502d14714d2947e22e7eff40ec20a6a44a42
  - >-
    Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 <
    419ec1b604d7fb60c10aec2dc062371f9fcd4940
  - >-
    Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 <
    ceb875a375dedbf51c9425c1d13a2d7a8435c08c
  - >-
    Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 <
    6e9d10f62773b99bd927940fd9cbdfe7207e23ff
  - >-
    Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 <
    53b7c271f06be4dd5cfc8c6ef552a8355c891a7f
  - Linux 3.6
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-16T15:41:01.162608Z'
published: '2026-07-25'
updated: '2026-10-16'
sourceUpdated: '2026-10-16T15:42:37.026Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-64448'
references:
  - url: 'https://git.kernel.org/stable/c/8d0bbc78046d264bbf6a574ea6f9072258a43e35'
  - url: 'https://git.kernel.org/stable/c/b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0'
  - url: 'https://git.kernel.org/stable/c/31c6312608c60b72a1feb99a5afb680645a3e8a3'
  - url: 'https://git.kernel.org/stable/c/573e502d14714d2947e22e7eff40ec20a6a44a42'
  - url: 'https://git.kernel.org/stable/c/419ec1b604d7fb60c10aec2dc062371f9fcd4940'
  - url: 'https://git.kernel.org/stable/c/ceb875a375dedbf51c9425c1d13a2d7a8435c08c'
  - url: 'https://git.kernel.org/stable/c/6e9d10f62773b99bd927940fd9cbdfe7207e23ff'
  - url: 'https://git.kernel.org/stable/c/53b7c271f06be4dd5cfc8c6ef552a8355c891a7f'
tags:
  - cve.org
epss: 0.00463
epssPercentile: 0.38712
ingestedAt: '2026-10-16T17:24:46.821Z'
---

## Overview

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

smb: client: restrict implied bcc[0] exemption to responses without data area

smb2_check_message() has a long-standing quirk that accepts a response
whose calculated length is one byte larger than the bytes actually
received ("server can return one byte more due to implied bcc[0]").
This was introduced to accommodate servers that omit the trailing bcc[0]
overlap byte when no data area is present.

However, the exemption is applied unconditionally, regardless of whether
the command actually carries a data area (has_smb2_data_area[]).  When a
response with a data area is subject to the +1 exemption, the reported
data can extend one byte beyond the bytes actually received, yet
smb2_check_message() still accepts it.  The subsequent decoder then reads
past the end of the receive buffer.  This is reachable during NEGOTIATE
and SESSION_SETUP, before the session is established.

The resulting out-of-bounds reads are visible under KASAN when mounting
against a non-conforming server; both the SPNEGO/negTokenInit and the
NTLMSSP challenge decoders are affected:

  BUG: KASAN: slab-out-of-bounds in asn1_ber_decoder+0x16a7/0x1b00
  Read of size 1 at addr ffff8880084d67c0 by task mount.cifs/81
  CPU: 1 UID: 0 PID: 81 Comm: mount.cifs Not tainted 7.1.0-rc6 #1
  Call Trace:
   <TASK>
   dump_stack_lvl+0x4e/0x70
   print_report+0x157/0x4c9
   kasan_report+0xce/0x100
   asn1_ber_decoder+0x16a7/0x1b00
   decode_negTokenInit+0x19/0x30
   SMB2_negotiate+0x31d9/0x4c90
   cifs_negotiate_protocol+0x1f2/0x3f0
   cifs_get_smb_ses+0x93f/0x17e0
   cifs_mount_get_session+0x7f/0x3a0
   cifs_mount+0xb4/0xcf0
   cifs_smb3_do_mount+0x23a/0x1500
   smb3_get_tree+0x3b0/0x630
   vfs_get_tree+0x82/0x2d0
   fc_mount+0x10/0x1b0
   path_mount+0x50d/0x1de0
   __x64_sys_mount+0x20b/0x270
   do_syscall_64+0xee/0x590
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  Allocated by task 85:
   kmem_cache_alloc_noprof+0x106/0x380
   mempool_alloc_noprof+0x116/0x1e0
   cifs_small_buf_get+0x31/0x80
   allocate_buffers+0x10d/0x2b0
   cifs_demultiplex_thread+0x1d5/0x1d50
   kthread+0x2c6/0x390
   ret_from_fork+0x36e/0x5a0
   ret_from_fork_asm+0x1a/0x30
  The buggy address is located 0 bytes to the right of
   allocated 448-byte region [ffff8880084d6600, ffff8880084d67c0)
   which belongs to the cache cifs_small_rq of size 448

  BUG: KASAN: slab-out-of-bounds in kmemdup_noprof+0x36/0x50
  Read of size 329 at addr ffff88800726c678 by task mount.cifs/89
  CPU: 0 UID: 0 PID: 89 Comm: mount.cifs Tainted: G    B      7.1.0-rc6 #1
  Call Trace:
   <TASK>
   dump_stack_lvl+0x4e/0x70
   print_report+0x157/0x4c9
   kasan_report+0xce/0x100
   kasan_check_range+0x10f/0x1e0
   __asan_memcpy+0x23/0x60
   kmemdup_noprof+0x36/0x50
   decode_ntlmssp_challenge+0x457/0x680
   SMB2_sess_auth_rawntlmssp_negotiate+0x6f0/0xcb0
   SMB2_sess_setup+0x219/0x4f0
   cifs_setup_session+0x248/0xaf0
   cifs_get_smb_ses+0xf79/0x17e0
   cifs_mount_get_session+0x7f/0x3a0
   cifs_mount+0xb4/0xcf0
   cifs_smb3_do_mount+0x23a/0x1500
   smb3_get_tree+0x3b0/0x630
   vfs_get_tree+0x82/0x2d0
   fc_mount+0x10/0x1b0
   path_mount+0x50d/0x1de0
   __x64_sys_mount+0x20b/0x270
   do_syscall_64+0xee/0x590
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  Allocated by task 93:
   kmem_cache_alloc_noprof+0x106/0x380
   mempool_alloc_noprof+0x116/0x1e0
   cifs_small_buf_get+0x31/0x80
   allocate_buffers+0x10d/0x2b0
   cifs_demultiplex_thread+0x1d5/0x1d50
   kthread+0x2c6/0x390
   ret_from_fork+0x36e/0x5a0
   ret_from_fork_asm+0x1a/0x30
  The buggy address is located 120 bytes inside of
   allocated 448-byte region [ffff88800726c600, ffff88800726c7c0)
   which belongs to the cache cifs_small_rq of size 448

Restrict the +1 exemption to responses that have no data area, so that
it still covers the bcc[0] omission it was meant for.  When a data area
is present, the +1 discrepancy instead means the reported data length
overruns the
---truncated---

## Affected

- `Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 < 8d0bbc78046d264bbf6a574ea6f9072258a43e35`
- `Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 < b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0`
- `Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 < 31c6312608c60b72a1feb99a5afb680645a3e8a3`
- `Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 < 573e502d14714d2947e22e7eff40ec20a6a44a42`
- `Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 < 419ec1b604d7fb60c10aec2dc062371f9fcd4940`
- `Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 < ceb875a375dedbf51c9425c1d13a2d7a8435c08c`
- `Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 < 6e9d10f62773b99bd927940fd9cbdfe7207e23ff`
- `Linux >= 093b2bdad3221e3fae3c26d89387e7297a157664 < 53b7c271f06be4dd5cfc8c6ef552a8355c891a7f`
- `Linux 3.6`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.