Kuba/ssh/services disabled by default hardening guide update/otp 19969/otp 20078/otp 20079#10970
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IngelaAndin
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Mikaka27
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Apr 10, 2026
Mikaka27
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Apr 10, 2026
Change the shell and exec daemon options to default to disabled
instead of enabling the Erlang shell and Erlang term evaluation
respectively. This implements the "secure by default" principle,
preventing authenticated users from executing arbitrary Erlang code
unless explicitly configured.
The new erlang_eval exec option enables Erlang term evaluation via
exec requests, replacing the previous implicit behavior.
Applications requiring these services must now explicitly enable them:
%% Enable Erlang shell
ssh:daemon(Port, [{shell, {shell, start, []}} | Options])
%% Enable Erlang term evaluation via exec
ssh:daemon(Port, [{exec, erlang_eval} | Options])
Also fix a client-side crash in ssh:shell/1 where the missing exec
key in options caused channel_cb_init_args/1 to append undefined to
the callback init args. Both erlang_eval and undefined are now
filtered out, as neither is a callback init argument.
The subsystems daemon option now defaults to an empty list instead
of enabling the SFTP subsystem via ssh_sftpd:subsystem_spec([]).
This extends the "secure by default" principle to also cover SFTP,
complementing the shell and exec default changes.
Previously, all authenticated SSH users had access to the entire
file system visible to the Erlang VM process through the default
SFTP subsystem, with no path restrictions.
Changes:
- Change subsystems default from [ssh_sftpd:subsystem_spec([])] to []
- Update subsystem_spec doc string in ssh.hrl
- Remove dead code in ssh_connection:check_subsystem/2
- Update test suites to explicitly enable SFTP where needed
- Update documentation (hardening.md, introduction.md, using_ssh.md,
ssh_app.md, terminology.md)
Applications requiring SFTP must now explicitly enable it:
ssh:daemon(Port, [{subsystems, [ssh_sftpd:subsystem_spec([])]}
| Options])
Add sections on SFTP enablement, network-level security and advanced authentication: - SFTP subsystem enablement with root option and rights model reference - Non-root VM recommendation in introduction - IP binding restrictions (daemon/3 vs daemon/2 default behavior) - MFA limitations and VPN alternative recommendation - Public key validation via key_cb with pk_check_user - Account lockout using pwdfun with cross-connection ETS tracking
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