CVE-2026-64448· NVD / CVE Program· CNA Linux

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

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---

HighCVSS 8.2 · v3.1—No exploit Fix available

Published: Jul 25, 2026
Updated: Sep 3, 2026
EPSS: 0.6% · 49th percentile
CWE: CWE-125

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Low priority for now.

CVSS: 32 / 40 · 8.2 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.6%

CNA vs NVD score

NVD: —
CNA · Linux: 8.2; NVD has not scored this yet; the score shown is the CNA’s.

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Affected systems

Vendor	Product	CPE
linux	linux kernel	cpe:2.3:o:linux:linux_kernel

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

linux linux kernel3.6 and later · before 5.10.261
linux linux kernel5.11 and later · before 5.15.212
linux linux kernel5.16 and later · before 6.1.178
linux linux kernel6.2 and later · before 6.6.145
linux linux kernel6.7 and later · before 6.12.96
linux linux kernel6.13 and later · before 6.18.39
linux linux kernel6.19 and later · before 7.1.4
linux linux kernel7.2

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Linux). Independent of NVD's CPE analysis and usually ahead of it.

Linux Linux
- 3.6affected
- 093b2bdad3221e3fae3c26d89387e7297a157664 and later · before 8d0bbc78046d264bbf6a574ea6f9072258a43e35affected · git
- 093b2bdad3221e3fae3c26d89387e7297a157664 and later · before b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0affected · git
- 093b2bdad3221e3fae3c26d89387e7297a157664 and later · before 31c6312608c60b72a1feb99a5afb680645a3e8a3affected · git
- 093b2bdad3221e3fae3c26d89387e7297a157664 and later · before 573e502d14714d2947e22e7eff40ec20a6a44a42affected · git
- 093b2bdad3221e3fae3c26d89387e7297a157664 and later · before 419ec1b604d7fb60c10aec2dc062371f9fcd4940affected · git
- 093b2bdad3221e3fae3c26d89387e7297a157664 and later · before ceb875a375dedbf51c9425c1d13a2d7a8435c08caffected · git
- 093b2bdad3221e3fae3c26d89387e7297a157664 and later · before 6e9d10f62773b99bd927940fd9cbdfe7207e23ffaffected · git
- 093b2bdad3221e3fae3c26d89387e7297a157664 and later · before 53b7c271f06be4dd5cfc8c6ef552a8355c891a7faffected · git
- before 3.6not affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
Debian:12	linux	before 6.1.180-1	6.1.180-1
Debian:12	linux-6.12	before 6.12.100-1~deb12u1	6.12.100-1~deb12u1
Debian:13	linux	before 6.12.96-1	6.12.96-1
Debian:14	linux	before 7.1.4-1	7.1.4-1

Same product

linux: all records

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Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
Linux Linux	31c6312608c60b72a1feb99a5afb680645a3e8a3	Vendor (CNA)
Linux Linux	419ec1b604d7fb60c10aec2dc062371f9fcd4940	Vendor (CNA)
Linux Linux	53b7c271f06be4dd5cfc8c6ef552a8355c891a7f	Vendor (CNA)
Linux Linux	573e502d14714d2947e22e7eff40ec20a6a44a42	Vendor (CNA)
Linux Linux	6e9d10f62773b99bd927940fd9cbdfe7207e23ff	Vendor (CNA)
Linux Linux	8d0bbc78046d264bbf6a574ea6f9072258a43e35	Vendor (CNA)
Linux Linux	b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0	Vendor (CNA)
Linux Linux	ceb875a375dedbf51c9425c1d13a2d7a8435c08c	Vendor (CNA)
azl3 kernel 6.6.144.1-1 on Azure Linux 3.0	6.6.145.2-1 · CBL-Mariner Releases	Microsoft (MSRC)
debian:linux	6.1.180-1 · Debian:12	Package registry (OSV)
debian:linux-6.12	6.12.100-1~deb12u1 · Debian:12	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

Anyone who can reach it over the internet can trigger it.
No account or password is required.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: low · data can be read
Integrity: none
Availability: high · the service can be disrupted

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality impact: Low
Integrity impact: None
Availability impact: High

Weakness class (CWE)

CWE-125 · Out-of-bounds Read

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Attack patterns (CAPEC)

Overread BuffersCAPEC-540likelihood: Low · High

ATT&CK techniques

—

Change log

Fix✗ → ✓Sep 28, 2026, 10:11 PM UTC

For records you follow, these changes also arrive as notifications. →

References

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records

Noroxi analysis

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Data sources and verification

CNA: Linux

CVE.org (CNA)Aug 17, 2026, 4:55 AM UTC
NVDSep 3, 2026, 11:34 AM UTC
FIRST EPSSOct 4, 2026, 12:00 AM UTC
OSV / GHSAOct 5, 2026, 3:22 AM UTC
MSRCOct 1, 2026, 3:24 AM UTC

Each line is when that source was last processed for this record.

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