Is it possible to grant users sftp access without shell access? If yes, how is it implemented?

I like the following setup for managing SSH access, which I use at work to manage a group of users on small fleet of servers. Security and ease of management is high on the list of my priorities.

Its key features are easily managing SSH rights through Unix group membership, having tightly defined permissions, and being secure by default.

Setting up

Install software (optional but useful):

yum install members   # or apt install members

Add groups:

addgroup --system allowssh
addgroup --system sftponly

In /etc/ssh/sshd_config, ensure that the following to settings are No:

PermitRootLogin no
PubkeyAuthentication no
PasswordAuthentication no

And at the end of /etc/ssh/sshd_config, add these two stanzas:

Match Group allowssh
    PubkeyAuthentication yes

Match Group sftponly
    ChrootDirectory %h
    X11Forwarding no
    AllowTcpForwarding no
    ForceCommand internal-sftp

(don't forget to restart SSH after editing the file)

Explanation

So, what does all this do?

  • It always disables root logins, as an extra security measure.
  • It always disables password-based logins (weak passwords are a big risk for servers running sshd).
  • It only allows (pubkey) login for users in the allowssh group.
  • Users in the sftponly group cannot get a shell over SSH, only SFTP.

Managing who has access is then simply done by managing group membership (these changes take effect immediately, no SSH restart required). members allowssh will show all users that are allowed to log in over SSH, and members sftponly will show all users that are limited to SFTP.

# adduser marcelm allowssh
# members allowssh
marcelm
# deluser marcelm allowssh
# members allowssh
#

Note that your sftp users need to be members of both sftponly (to ensure they won't get a shell), and of allowssh (to allow login in the first place).

Further information

  1. Please note that this configuration does not allow password logins; all accounts need to use public key authentication. This is probably the single biggest security win you can get with SSH, so I argue it's worth the effort even if you have to start now.

    If you really don't want this, then also add PasswordAuthentication yes to the Match Group allowssh stanza. This will allow both pubkey and password auth for allowssh users. Alternatively, you can add another group (and Match Group stanza) to selectively grant users password-based logins.

  2. This configuration limits any sftponly user to their home directory. If you do not want that, remove the ChrootDirectory %h directive.

    If you do want the chrooting to work, it's important that the user's home directory (and any directory above it) is owned by root:root and not writable by group/other. It's OK for subdirectories of the home directory to be user-owned and/or writable.

    Yes, the user's home directory must be root-owned and unwritable to the user. Sadly, there are good reasons for this limitation. Depending on your situation, ChrootDirectory /home might be a good alternative.

  3. Setting the shell of the sftponly users to /sbin/nologin is neither necessary nor harmful for this solution, because SSH's ForceCommand internal-sftp overrides the user's shell.

    Using /sbin/nologin may be helpful to stop them logging in via other ways (physical console, samba, etc) though.

  4. This setup does not allow direct root logins over SSH; this forms an extra layer of security. If you really do need direct root logins, change the PermitRootLogin directive. Consider setting it to forced-commands-only, prohibit-password, and (as a last resort) yes.

  5. For bonus points, have a look at restricting who can su to root; add a system group called wheel, and add/enable auth required pam_wheel.so in /etc/pam.d/su.


Edit your /etc/ssh/sshd_config to contain:

Match User [SFTP user]
ForceCommand internal-sftp

Restart sshd. If you have multiple users put them all on the match user line separated by commas like so:

Match User User1,User2,User3

The key to configuring sftp to not allow shell access is to limit users via the ForceCommand option.

Tags:

Sftp

Nologin

Recent Posts