Hi, I have researched the bug 12997(CVE-2017-12150) and found there also is a code path which ignores smb signing requirement and falls back to anonymous. SMBC_server_internal uses cli_session_setup_anon when login fails. It sees NO_AUTO_ANONYMOUS_LOGON option flag but ignores for example, client signing = mandatory in smb.conf. Is this by design? Don't we need to see smbXcli_conn_signing_mandatory also? Sample session on Fedora 25 follows. $ smbd --version Version 4.5.14 # smbclient does not fall back to anonymous due to fix on do_connect. $ smbclient -U kanda //kandaw2016/temp Enter kanda's password: session setup failed: NT_STATUS_LOGON_FAILURE # testread uses SMBC_server via smbc_open. $ pwd /home/kanda/rpmbuild/BUILD/samba-4.5.14/bin/default/examples/libsmbclient $ ./testread Path: smb://kandaw2016/temp/hello Workgroup: [KANDA] Username: [kanda] Password: smbc_open: File exists Path: ^C $ Packet monitor shows that after ntlm session setup error, libsmbclient attempts anonymous login and succeeds. It then tries tree connect to "temp" and gets STATUS_ACCESS_DENIED from Windows server 2016. It has restrictnullsessaccess=1 and maybe that's the cause of the error. The point is, libsmbclient sometimes falls back to anonymous session and messages go unsigned on wire, unless the caller sets NO_AUTO_ANONYMOUS_LOGON option.