CVE-2026-109411: 
Samba vulnerability analysis and mitigation

CVE-2026-109411 is an access control bypass vulnerability in Samba's experimental vfs_ceph_rgw module, specifically in its openat() function. An authenticated, low-privileged network attacker can exploit improper case-insensitive filename handling to read or overwrite files owned by other users on affected shares. The vulnerability was published on October 23, 2026, with a patch reference added on October 26, 2026. It carries a CVSS v3.1 base score of 8.1 (High) (Red Hat Advisory, Red Hat Bugzilla).

The root cause is classified as CWE-178 (Improper Handling of Case Sensitivity). The vfs_ceph_rgw module's openat() implementation ignores the case-corrected filename supplied by the Samba file server and instead re-opens the filename as originally sent by the client. When a client requests an existing file using mismatched character casing with a create disposition of FILE_OPEN_IF, the case-insensitive lookup fails inside the module, causing the server to treat the operation as a new file creation. This causes the server to check only the caller's permission to add a file to the parent directory — skipping the access check on the existing object — after which the open collides with and re-opens the existing file, granting unauthorized read or write access (Red Hat Bugzilla, Red Hat Advisory).

Successful exploitation allows an authenticated attacker to read sensitive files owned by other users or overwrite them despite restrictive file permissions, resulting in high confidentiality and high integrity impact. The vulnerability is scoped to shares configured with the experimental vfs_ceph_rgw module, limiting exposure to deployments using that specific module. There is no availability impact, but the ability to overwrite arbitrary files could facilitate privilege escalation or data destruction within the affected share (Red Hat Advisory, Red Hat Bugzilla).

As of the time of reporting, there is no public proof-of-concept exploit and no evidence of active in-the-wild exploitation (Red Hat Advisory). The CVE status remains "Reserved" and no CISA KEV catalog listing has been identified. Exploitation requires valid credentials (low-privileged authenticated user) and network access to a Samba server running the vfs_ceph_rgw module, which is described as experimental, limiting the overall attack surface. No threat actor attribution has been reported.

1. Reconnaissance: Identify Samba servers (e.g., via SMB port 445 scanning with Nmap or Shodan) that expose shares configured with the experimental vfs_ceph_rgw module.
2. Authentication: Authenticate to the target Samba share using any valid low-privileged account credentials.
3. Identify target file: Enumerate files on the share owned by other users that have restrictive permissions (e.g., files readable/writable only by their owner).
4. Craft mismatched-case request: Send an SMB open request for the target file using a different character casing than the file's actual name on disk (e.g., request Secret.txt when the file is stored as secret.txt), with a create disposition of FILE_OPEN_IF.
5. Trigger bypass: The vfs_ceph_rgw module's case-insensitive lookup fails, causing the server to treat the request as a new file creation and skip the access check on the existing file object.
6. Access or overwrite: The server opens the existing file object, granting the attacker read access to sensitive data or the ability to overwrite the file's contents (Red Hat Bugzilla).

• Network: SMB connections (port 445) from authenticated low-privileged users accessing shares configured with vfs_ceph_rgw; repeated file open requests with mismatched character casing for the same filename.
• Logs: Samba audit logs showing FILE_OPEN_IF create disposition requests where the requested filename casing differs from the on-disk filename; access log entries showing a user successfully opening files they do not own.
• File System: Unexpected modification timestamps on files owned by users other than the accessing account; file content changes on restricted files not initiated by the file owner.

The primary workaround is to disable or restrict use of the experimental vfs_ceph_rgw module on Samba shares until an official patch is released. Network-level access controls should be implemented to limit which clients can connect to affected Samba servers. Administrators should monitor Samba access logs for suspicious file access patterns indicative of exploitation attempts. A patch reference has been tracked in Red Hat Bugzilla (Bug 2552136), and users should monitor vendor channels for updated package releases (Red Hat Advisory, Red Hat Bugzilla).

Community coverage noted that Samba 4.25.1 addressed this CVE without an accompanying formal security advisory, which drew attention from Linux distribution communities (Warp2Search, LinuxCompatible). Slackware's changelog also reflected the update. No major vendor statements or notable researcher commentary beyond the Red Hat Bugzilla tracking entry have been identified at this time.