Kernel-level ksmbd flaw enables unauthorized file overwrites on Linux systems; impacts data integrity across deployments and requires org-wide patching.
CVE-2026-64399
Linux ksmbd flaw permits unauthorized file overwrite via FSCTL_DUPLICATE_EXTENTS_TO_FILE
Is CVE-2026-64399 being exploited?
Not confirmed. CVE-2026-64399 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.49% probability of exploitation in the next 30 days.
How severe is CVE-2026-64399?
CVE-2026-64399 is rated Critical with a CVSS score of 9.8. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-64399?
Yes. A fix has been recorded for CVE-2026-64399. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-64399 affect?
CVE-2026-64399 affects Linux kernel (ksmbd), Linux distributions using in-kernel SMB server, Systems exposing SMB shares via ksmbd. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-64399?
Install kernel ksmbd security updates or disable ksmbd until patched.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote SMB client can overwrite destination file data on read-only shares or via handles lacking FILE_WRITE_DATA, causing data corruption and integrity loss.
Immediate action required
- affected>= eb817368f50c1cbe1bd07044124aad7db6330e3a and < bf460ad5958d506492de4524a656439da3f99c51
- affected>= eb817368f50c1cbe1bd07044124aad7db6330e3a and < 620d133d469295ee7c017ca6aafac335f65c4a5a
- affected>= eb817368f50c1cbe1bd07044124aad7db6330e3a and < 9b9cf7e65cbeaae1b6636144bacee611cdd7a5d6
- affected>= eb817368f50c1cbe1bd07044124aad7db6330e3a and < baae7b39673ec21073a25e3d14f8feaada01d5df
- affected>= eb817368f50c1cbe1bd07044124aad7db6330e3a and < c917e4522d251071dde9871b9142d8ea1186ebfe
- affected>= eb817368f50c1cbe1bd07044124aad7db6330e3a and < 388e4139db27a9e3612c9d356b826f5b1ff6a9e3
- affected5.15
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.