CVE-2026-109411 is an access control bypass vulnerability in Samba's experimental vfs_ceph_rgw module affecting its openat() function. Authenticated users can bypass access controls to read or overwrite restricted files on shares configured with this module due to improper case-insensitive filename handling. This causes the server to treat existing files as new, skipping permission checks. The vulnerability is limited to Samba deployments using this experimental S3-to-SMB gateway feature introduced in Samba 4.25. Red Hat's Samba packages do not include this module, so they are not affected. Mitigation involves disabling the vfs_ceph_rgw module or enabling case-sensitive file handling on affected shares.