<!--
%CopyrightBegin%

SPDX-License-Identifier: Apache-2.0

Copyright Ericsson AB 2023-2026. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

%CopyrightEnd%
-->
# Hardening

## Introduction

The Erlang/OTP SSH application is intended to be used in other applications as a
library.

Different applications using this library may have very different requirements.
One application could be running on a high performance server, while another is
running on a small device with very limited cpu capacity. For example, the first
one may accept many users simultaneously logged in, while the second one wants
to limit them to only one.

That simple example shows that it is impossible to deliver the SSH application
with default values on hardening options as well on other options that suites
every need.

The purpose of this guide is to discuss the different hardening options
available, as a guide to the reader. Configuration in general is described in
the [Configuration in SSH](configurations.md) chapter.

## Resilience to DoS attacks

The following applies to daemons (servers).

DoS (Denial of Service) attacks are hard to fight at the node level. Here are
firewalls and other means needed, but that is out of scope for this guide.
However, some measures could be taken in the configuration of the SSH server to
increase the resilence. The options to use are:

### Counters and parallelism

- **[max_sessions](`m:ssh#hardening_daemon_options-max_sessions`)** - The
  maximum number of simultaneous sessions that are accepted at any time for this
  daemon. This includes sessions that are being authorized. The default is that
  an unlimited number of simultaneous sessions are allowed. It is a good
  candidate to set if the capacity of the server is low or a capacity margin is
  needed.

- **[max_channels](`m:ssh#hardening_daemon_options-max_channels`)** - The
  maximum number of channels that are accepted for each connection. The default
  is unlimited.

- **[parallel_login](`m:ssh#hardening_daemon_options-parallel_login`)** - If set
  to false (the default value), only one login is handled at a time. If set to
  true, the number of simultaneous login attempts are limited by the value of
  the [max_sessions](`m:ssh#hardening_daemon_options-max_sessions`) option.

### Timeouts

- **[hello_timeout](`t:ssh:hello_timeout_daemon_option/0`)** - If the client
  fails to send the first ssh message after a tcp connection setup within this
  time (in milliseconds), the connection is closed. The default value is 30
  seconds. This is actually a generous time, so it can lowered to make the
  daemon less prone to DoS attacks.

- **[negotiation_timeout](`t:ssh:negotiation_timeout_daemon_option/0`)** -
  Maximum time in milliseconds for the authentication negotiation counted from
  the TCP connection establishment. If the client fails to log in within this
  time the connection is closed. The default value is 2 minutes. It is quite a
  long time, but can lowered if the client is supposed to be fast like if it is
  a program logging in.

- **[idle_time](`t:ssh:max_idle_time_common_option/0`)** - Sets a time-out on a
  connection when no channels are left after closing the final one. It defaults
  to infinity.

- **[max_initial_idle_time](`t:ssh:max_initial_idle_time_daemon_option/0`)** -
  Sets a time-out on a connection that will expire if no channel is opened on
  the connection. The timeout is started when the authentication phase is
  completed. It defaults to infinity.

A figure clarifies when a timeout is started and when it triggers:

![SSH server timeouts](assets/ssh_timeouts.jpg "SSH server timeouts")

### Resilience to compression-based attacks

SSH supports compression of the data stream.

Reasonable finite [max_sessions](`m:ssh#hardening_daemon_options-max_sessions`)
option is highly recommended if compression is used to prevent excessive resource
usage by the compression library.
See [Counters and parallelism](#counters-and-parallelism).

The `'zlib@openssh.com'` algorithm is recommended because it only activates
after successful authentication.

The `'zlib'` algorithm is not recommended because it activates before
authentication completes, allowing unauthenticated clients to expose potential
vulnerabilities in compression libraries, and increases attack surface of
compression-based side-channel and traffic-analysis attacks.

In both algorithms decompression is protected by a size limit that prevents
excessive memory consumption.

## Verifying the remote daemon (server) in an SSH client

Every SSH server presents a public key - the _host key_ \- to the client while
keeping the corresponding private key in relatively safe privacy.

The client checks that the host that presented the public key also possesses the
private key of the key-pair. That check is part of the SSH protocol.

But how can the client know that the host _really_ is the one that it tried to
connect to and not an evil one impersonating the expected one using its own
valid key-pair? There are two alternatives available with the default key
handling plugin `m:ssh_file`. The alternatives are:

- **Pre-store the host key** - \* For the default handler ssh_file, store the
  valid host keys in the file [`known_hosts`](`m:ssh_file#FILE-known_hosts`) and
  set the option
  [silently_accept_hosts](`m:ssh#hardening_client_options-silently_accept_hosts`)
  to `false`.

  - or, write a specialized key handler using the
    [SSH client key API](`m:ssh_client_key_api`) that accesses the pre-shared
    key in some other way.

- **Pre-store the "fingerprint" (checksum) of the host key** - \*
  [silently_accept_hosts](`m:ssh#hardening_client_options-silently_accept_hosts`)
  - [`accept_callback()`](`t:ssh:accept_callback/0`)
  - [`{HashAlgoSpec, accept_callback()}`](`t:ssh:accept_hosts/0`)

## Verifying the remote client in a daemon (server)

- **Password checking** - The default password checking is with the list in the
  [user_passwords](`m:ssh#option-user_passwords`) option in the SSH daemon. It
  could be replaced with a [pwdfun](`m:ssh#option-pwdfun`) plugin. The arity
  four variant ([`pwdfun_4()`](`t:ssh:pwdfun_4/0`)) can also be used for
  introducing delays after failed password checking attempts. Here is a simple
  example of such a pwdfun:

  ```erlang
  fun(User, Password, _PeerAddress, State) ->
          case lists:member({User,Password}, my_user_pwds()) of
              true ->
                  {true, undefined}; % Reset delay time
              false when State == undefined ->
                  timer:sleep(1000),
                  {false, 2000}; % Next delay is 2000 ms
              false when is_integer(State) ->
                  timer:sleep(State),
                  {false, 2*State} % Double the delay for each failure
          end
  end.
  ```

  If a public key is used for logging in, there is normally no checking of the
  user name. It could be enabled by setting the option
  [`pk_check_user`](`m:ssh#option-pk_check_user`) to `true`. In that case the
  pwdfun will get the atom `pubkey` in the password argument.

## Hardening in the cryptographic area

### Algorithm selection

One of the cornerstones of security in SSH is cryptography. The development in
crypto analysis is fast, and yesterday's secure algorithms are unsafe today.
Therefore some algorithms are no longer enabled by default and that group grows
with time. See the
[SSH (App)](ssh_app.md#supported-specifications-and-standards) for a list of
supported and of disabled algorithms. In the User's Guide the chapter
[Configuring algorithms in SSH](configure_algos.md) describes the options for
enabling or disabling algorithms -
[preferred_algorithms](`t:ssh:preferred_algorithms_common_option/0`) and
[modify_algorithms](`t:ssh:modify_algorithms_common_option/0`).

### Re-keying

In the setup of the SSH connection a secret cipher key is generated by
co-operation of the client and the server. Keeping this key secret is crucial
for keeping the communication secret. As time passes and encrypted messages are
exchanged, the probability that a listener could guess that key increases.

The SSH protocol therefore has a special operation defined - _key
re-negotiation_ or _re-keying_. Any side (client or server) could initiate the
re-keying and the result is a new cipher key. The result is that the
eves-dropper has to restart its evil and dirty craftmanship.

See the option [rekey_limit](`t:ssh:rekey_limit_common_option/0`) for a
description.

## Hardening of the SSH protocol - both daemons and clients

### Disabling shell and exec in a daemon

A daemon has two services for evaluating tasks on behalf of a remote client. The
_exec_ server-side service takes a string provided by the client, evaluates it
and returns the result. The _shell_ function enables the client to open a shell
in the shell host.

Those service could - and should - be disabled when they are not needed. The
options [exec](`t:ssh:exec_daemon_option/0`) and
[shell](`t:ssh:shell_daemon_option/0`) are enabled per default but could be set
to `disabled` if not needed. The same options could also install handlers for
the string(s) passed from the client to the server.

### The id string

One way to reduce the risk of intrusion is to not convey which software and
which version the intruder is connected to. This limits the risk of an intruder
exploiting known faults or peculiarities learned by reading the public code.

Each SSH client or daemon presents themselves to each other with brand and
version. This may look like

```text
SSH-2.0-Erlang/4.10
```

or

```text
SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.3
```

This brand and version may be changed with the option
[id_string](`t:ssh:id_string_common_option/0`). We start a daemon with that
option:

```erlang
	ssh:daemon(1234, [{id_string,"hi there"}, ... ]).
```

and the daemon will present itself as:

```text
SSH-2.0-hi there
```

It is possible to replace the string with one randomly generated for each
connection attempt. See the reference manual for
[id_string](`t:ssh:id_string_common_option/0`).

## SFTP symlink risks

Symbolic links inside the SFTP root that point outside it can be followed by
file operations such as `open`, `read`, and `stat`.  The `root` option
confines path resolution done by the daemon itself, but it does not prevent
the operating system from following symlinks when the daemon opens the
resulting path.

Symlinks created by SFTP clients are not exploitable this way: the daemon
converts the target to an absolute path and clamps it to the configured root
before writing the link.  However, symlinks that already exist inside the root
directory, or are created through other access channels (shell access, another
service, etc.), can point outside the root and will be followed.

Mitigations:

- **Filesystem permissions** - Run the SSH daemon as a dedicated user with
  minimal privileges.  Ensure that user cannot read or write sensitive files
  outside the root.

- **Control the root contents** - Ensure no pre-existing symlinks in the root
  point outside it, and that no other process can create such links.

- **OS-level protections** - On Linux, mount the root directory with the
  `nosymfollow` mount option, or use filesystem namespaces to prevent
  symlink traversal outside the root.

## Client connection options

A client could limit the time for the initial tcp connection establishment with
the option [connect_timeout](`t:ssh:connect_timeout_client_option/0`). The time
is in milliseconds, and the initial value is infinity.

The negotiation (session setup time) time can be limited with the _parameter_
`NegotiationTimeout` in a call establishing an ssh session, for example
`ssh:connect/3`.

## SFTP Security

### Root Directory Isolation

The `root` option (see `m:ssh_sftpd`) restricts SFTP users to a
specific directory tree, preventing access to files outside that directory.

**Example:**

```erlang
ssh:daemon(Port, [
    {subsystems, [ssh_sftpd:subsystem_spec([{root, "/home/sftpuser"}])]},
    ...
]).
```

**Important:** The `root` option is configured per daemon, not per user. All
users connecting to the same daemon share the same root directory. For per-user
isolation, consider running separate daemon instances on different ports or
using OS-level mechanisms (PAM chroot, containers, file permissions).

**Defense-in-depth:** For high-security deployments, combine the `root` option
with OS-level isolation mechanisms such as chroot jails, containers, or
mandatory access control (SELinux, AppArmor).
