# CVE-2026-64448: smb: client: restrict implied bcc[0] exemption to responses without data area

> A malicious SMB server can trigger out-of-bounds reads in the Linux kernel SMB client during negotiation, potentially causing crashes or leaking memory.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Jul 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-93782, BELL-CVE-2026-64448, DEBIAN-CVE-2026-64448, GHSA-36gp-77hh-hxwm, ROOT-OS-DEBIAN-12-CVE-2026-64448, ROOT-OS-UBUNTU-2204-CVE-2026-64448, UBUNTU-CVE-2026-64448

## What is CVE-2026-64448?

CVE-2026-64448 is a high-severity memory corruption vulnerability in Linux kernel, published on 25 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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---

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-64448?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64448?

Apply the vendor's patch.

## Is CVE-2026-64448 being exploited?

There is no known exploitation of CVE-2026-64448 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64448 and fix it if it is affected.

What it is: CVE-2026-64448 is a high-severity memory corruption vulnerability in Linux kernel, published on 25 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-64448 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-64448/ (Markdown: https://insights.sbomify.com/CVE-2026-64448/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64448
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64448
- OSV: https://osv.dev/vulnerability/CVE-2026-64448

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64448/
Summaries are generated and may contain mistakes; the official description above is the record.