CVE-2026-64448

Status: UPDATED  |  Advisory ID: CVE-2026-64448

Key Details

Affected Products, Subsystems & Sectors

What to Know

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--- (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References