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CVE-2026-53383 is a denial-of-service vulnerability in the Linux kernel's ksmbd in-kernel SMB server caused by improper session-state validation in SMB2 compound request processing. In the non-first operation path of smb2_check_user_session(), ksmbd reuses the session associated with the first compound operation and treats it as valid after checking only the SessionId. This shortcut fails to re-validate that the session is in SMB2_SESSION_VALID state. In addition, use of the SMB2 related-operation SessionId value can bypass even the SessionId comparison in this branch. An attacker can first create a session that remains in SMB2_SESSION_IN_PROGRESS by sending a SESSION_SETUP request containing only an NTLM negotiate message, leaving the session's user object unset. If that request is used as the first operation in a compound request and followed by a related TREE_CONNECT operation to IPC$, ksmbd can process the tree connect against the still-in-progress session. The IPC tree-connect path then dereferences the session user pointer while it is still NULL, resulting in a kernel NULL-pointer dereference. The flaw stems from inconsistent enforcement of session validity between the standalone session lookup path and the compound-request branch.
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