CVE-2026-89788
Use-after-free in Linux kernel ksmbd SMB server (CVE-2026-89788)
CVE-2026-89788 is a use-after-free vulnerability in the Linux kernel's ksmbd in-kernel SMB2/SMB3 server, in the smb2_tree_connect() handler. A newly created tree connection is published into the session's connection table with a single reference before it is fully initialized, so a concurrent session logoff can erase the entry and free the object while the connect handler is still using it; a remote client can trigger the race by sending a TREE_CONNECT request and a session LOGOFF close together. Successful triggering causes kernel memory corruption that can crash the server (denial of service) and may lead to arbitrary code execution with kernel privileges, consistent with the 9.8 CVSS score covering confidentiality, integrity and availability. Only Linux systems running the ksmbd module (CONFIG_SMB_SERVER) and exposing that service to untrusted clients are affected; Samba and Windows SMB servers are not. No public proof-of-concept, CISA KEV listing, or known in-the-wild exploitation is reported.
What to do: Apply a kernel update from your distribution containing the ksmbd tree-connection reference-count fix as soon as it is available; the source data does not name a fixed kernel version, so track your vendor's kernel security advisories for this CVE. Until patched, avoid using ksmbd (unload the ksmbd module or disable shares served by it) and restrict TCP port 445 to trusted networks with firewall rules if the server must remain in use.
| Linux kernel ksmbd in-kernel SMB server (fs/smb/server, CONFIG_SMB_SERVER) | unpatched kernels containing the ksmbd code prior to the fix commit; no specific affected/fixed version range is given in the source data |
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
In the Linux kernel, the following vulnerability has been resolved: ksmbd: fix tree connection use-after-free in smb2_tree_connect() ksmbd_tree_conn_connect() publishes a new tree connection in sess->tree_conns with a single reference and returns its pointer to smb2_tree_connect(). The handler continues to initialize the object and build the response after publication. A concurrent session logoff can erase the connection and drop that reference, freeing the object while the handler still uses it. BUG: KASAN: slab-use-after-free in smb2_tree_connect+0xe3d/0xf90 smb2_tree_connect (fs/smb/server/smb2pdu.c:2872) handle_ksmbd_work process_one_work worker_thread kthread After xa_store() succeeds, take a second reference before releasing tree_conns_lock. The original reference belongs to the xarray entry and the second belongs to the creating smb2_tree_connect() handler. Keep the references balanced in every path: - On normal exit or an error after publication, smb2_tree_connect() drops its creator reference. Error cleanup also calls ksmbd_tree_conn_disconnect(), which drops the xarray reference only if it removes the exact entry. - SMB2 TREE_DISCONNECT uses the same helper to remove the entry and drop its xarray reference. The request's existing lookup reference remains owned by the request and is released by the existing cleanup. - Session LOGOFF removes each entry and drops its xarray reference. If it wins the race, later cleanup sees that the entry is gone and does not drop that reference again. To enforce this ownership, claim the disconnected state and erase the exact entry atomically under tree_conns_lock. This guarantees one drop for the xarray reference and one drop by each in-flight user, regardless of which teardown path wins. If logoff removes the entry before initialization completes, fail the connect instead of marking the detached object TREE_CONNECTED.
- Vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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