CVE-2026-64394: ksmbd: add a WRITE_DAC/WRITE_OWNER check to SMB2 SET_INFO SECURITY

Severity: high (CVSS 8.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-25 · updated 2026-09-04

Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: add a WRITE_DAC/WRITE_OWNER check to SMB2 SET_INFO SECURITY commit cc57232cae23 ("ksmbd: fix FSCTL permission bypass by adding a permission check for FSCTL_SET_SPARSE") added a fp->daccess gate to fsctl_set_sparse and noted that "similar handle-level checks exist in other functions but are missing here." The SMB2 SET_INFO SECURITY arm is one of the missing ones, and the most security-relevant: smb2_set_info_sec() calls set_info_sec() with no per-handle access check. set_info_sec() (fs/smb/server/smbacl.c) re-permissions the file: it rewrites owner/group/mode via notify_change(), rewrites the POSIX ACL via set_posix_acl(), and on KSMBD_SHARE_FLAG_ACL_XATTR shares removes and rewrites the Windows security descriptor via ksmbd_vfs_set_sd_xattr(). Every other persistent-mutation arm of the sibling handler smb2_set_info_file() checks fp->daccess first (FILE_WRITE_DATA / FILE_DELETE / FILE_WRITE_EA / FILE_WRITE_ATTRIBUTES); the SECURITY arm — which mutates the access control itself — is the only one with no gate. A client can therefore open a handle with FILE_WRITE_ATTRIBUTES only (no FILE_WRITE_DAC / FILE_WRITE_OWNER) and use SMB2_SET_INFO with InfoType SMB2_O_INFO_SECURITY to rewrite the file's DACL and owner, granting itself access the handle's daccess never carried. Unlike the FSCTL data arms this is a metadata/xattr operation, so there is no FMODE_WRITE VFS backstop — the missing fp->daccess check is the entire gate. Setting a security descriptor is the WRITE_DAC / WRITE_OWNER operation, so require at least one of those on the handle before re-permissioning the file. -EACCES is mapped to STATUS_ACCESS_DENIED by smb2_set_info().

Affected products

Linux kernel: from 5.15 before 6.1.178; from 6.2 before 6.6.145; from 6.7 before 6.12.96; from 6.13 before 6.18.39; from 6.19 before 7.1.4 — fixed in 6.1.178, 6.12.96, 6.18.39, 6.6.145, 7.1.4

How to fix

Upgrade to a fixed version: Linux kernel 6.1.178, 6.12.96, 6.18.39, 6.6.145. Check the vendor advisory for mitigations.

References

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