NAME¶

authprogs - SSH command authenticator

SYNOPSIS¶

authprogs --run [options]

authprogs --install_key [options]

authprogs --dump_config [options]

authprogs --help

DESCRIPTION¶

authprogs is an SSH command authenticator. It is invoked on an ssh server and decides if the command requested by the ssh client should be run or rejected based on logic in the authprogs configuration file.

Passwordless SSH using ssh identities or pubkeys can enable all sorts of wonderful automation, for example running unattended batch jobs, slurping down backups, or pushing out code. Unfortunately a key, once trusted, is allowed by default to run anything on that system, not just the small set of commands you actually need. If the key is compromised, you are at risk of a security breach. This could be catastrophic, for example if the access is to the root account.

Authprogs is run on the SSH server and compares the requested command against the authprogs configuration file/files. This enables authprogs to make intelligent decisions based on things such as the command itself, the SSH key that was used, the client IP, and such.

authprogs is enabled by using the command= option in the authorized_keys file.

KEY INSTALLATION¶

You can install your ssh identities/pubkeys manually, or allow authprogs to do the work for you.

MANUAL KEY INSTALLATION¶

You need to set up your ~/.ssh/authorized_keys file to force invocation of authprogs for the key or keys you wish to protect.

A line of an unrestricted authorized_key entry might look like this:

ssh-rsa AAAAxxxxx...xxxxx user@example.com

When setting up this key to use authprogs, you add a command= option to the very beginning of that line that points to the location where authprogs lives. For example if authprogs is in /usr/bin/authprogs, you would use this:

command="/usr/bin/authprogs --run" ssh-rsa AAAAxxxxx...xxxxx user@example.com

You must include --run to let authprogs know it is running in SSH command mode.

Authprogs has other command line options you may wish to include as well, for example

command="/usr/bin/authprogs --keyname=backups --run" ssh-rsa AAAA...xxxxx user@example.com

Lastly, if you wish, ssh offers a number of other helpful restrictions you may wish to include that are separate from authprogs. These can be appended right after (or before) the command="" section if you wish.

command="/usr/bin/authprogs --run",no-port-forwarding,no-pty ssh-rsa AAAA...xxxxx user@example.com

See the sshd(8) man page for more information about allowed authorized_keys configuration options.

AUTOMATED KEY INSTALLATION¶

Authprogs is capable of adding your key to your authorized_keys file (~/.ssh/authorized_keys by default) programmatically. It also disables ssh port forwarding by default for this key (a sensible default for most batch jobs.)

authprogs will refuse to install a key that is already present in the authorized_keys file.

For example the following

authprogs --install_key /path/to/backups_key.pub --keyname=backups

would cause the following line to be added to your ~/.ssh/authorized_keys file:

command="/usr/bin/authprogs --keyname backups --run",no-port-forwarding ssh-rsa AAAA...xxxxx user@example.com

RUN MODE OPTIONS¶

Authprogs can run in several modes, depending on which of these command line switches you provide.

--run

Act in run mode, as from an authorized_keys file.

--install_key filename

Install the key contained in the named file into your authorized_keys file.

--dump_config

Dump the configuration in a python-style view. Helpful only for debugging.

--silent

Do not inform the user if their command has been rejected. Default is to let them know it was rejected to prevent confusion.

--help

Show help information

OTHER OPTIONS¶

The following options may apply to multiple run modes, as appropriate.

--keyname key_name

This option ´names´ the key, for help in crafting your rules. Since an account may have multiple keys allowed, this helps us differentiate which one was used so we can make sensible choices.

In run mode, this specifies which name is used when matching in the configuration, e.g.

  command="/usr/bin/authprogs --keyname backups --run" ...

In key installation mode, this adds the --keyname option to the authorized_keys entry.

key_name may contain no whitespace.

--configfile

Specifies the authprogs configuration file to read. Defaults to ~/.ssh/authprogs.yaml.

In key installation mode, this adds the --configfile option to the authorized_keys entry.

--configdir

Specifies the authprogs configuration, in which multiple configuration files can be found. Defaults to ~/.ssh/authprogs.d if present.

Files in the configuration directory are read as rules in filename order. See CONFIGURATION for more info.

LIMITATIONS¶

Commands are executed via fork/exec, and are not processed through the shell. This means you cannot have multiple commands separated by semicolons, pipelines, redirections, backticks, shell builtins, wildcards, variables, etc.

Also, you cannot have spaces in any arguments your command runs. This is because the SSH server takes the command that was specified by the client and squashes it into the SSH_ORIGINAL_COMMAND variable. By doing this it makes it impossible for us to know what spaces in SSH_ORIGINAL_COMMAND were between arguments and which were part of arguments.

Here are some commands that would not work through authprogs:

○

ssh host "rm /tmp/foo; touch /tmp/success"

○

ssh host "rm /tmp/*.html"

○

ssh host "cut -d: -f 1 /etc/passwd > /tmp/users"

○

ssh host "touch ´/tmp/file with spaces´"

○

ssh host "for file in /tmp/*.html; do w3m -dump $file > $file.txt; done"

You can work around these limitations by writing a shell script that does what you need and calling that from authprogs, rather than attempting to run complicated command lines via ssh directly.

CONFIGURATION FILES¶

authprogs rules are maintained in one or more configuration files in YAML format.

The rules allow you to decide whether the client´s command should be run based on criteria such as the command itself, the client IP address, and ssh key in use.

Rules can be read from a single file (~/.ssh/authprogs.yaml by default) or by putting files in a configuration directory (~/.ssh/authprogs.d). The configuration directory method is most useful when you want to be able to easily add or remove rules without manually editing a single configuration file, such as when installing rules via your configuration tool of choice.

All the authprogs configuration files are concatenated together into one large yaml document which is then processed. The files are concatenated in the following order:

○

~/.ssh/authprogs.yaml, if present

○

files in ~/.ssh/authprogs.d/ directory, in asciibetical order

Dotfiles contained in a configuration directory are ignored. The configuration directory is not recursed; only those files directly contained are processed.

Each rule in the configuration file/files is tested in order and once a match is found, processing stops and the command is run.

Rules are made of rule selection options (e.g. client IP address) and subrules (e.g. a list of allowed commands). All pieces must match for the command to be run.

The general format of a rule is as follows:

# First rule
-


  # Selection options


  #


  # All must match or we stop processing this rule.


  selection_option_1: value


  selection_option_2: value


  # The allow block, aka subrules


  #


  # This lets us group a bunch of possible commands


  # into one rule. Otherwise we´d need a bunch of


  # rules where you repeat selection options.


  allow:


    -


      rule_type: value


      rule_param_1: value


      rule_param_2: value


    -


      rule_type: value2


      rule_param_1: value


      rule_param_2: value
# Next rule
-


  selection_option_3: value
...

Some of the keys take single arguments, while others may take lists. See the definition of each to understand the values it accepts.

RULE SELECTION OPTIONS¶

These configuration options apply to the entire rule, and help you limit under what conditions the rule matches.

○

from: This is a single value or list of values that define what SSH client IP addresses are allowed to match this rule. The client IP address is gleaned by environment variables set by the SSH server. Any from value may be an IP address or a CIDR network.

Examples:

-


  from: 192.168.1.5


  ...
-


  from: [192.168.0.1, 10.0.0.3]


  ...
-


  from:


    - 192.168.0.0/24


    - 10.10.0.3


  ...

○

keynames: This is a single value or list of values that define which SSH pubkeys are allowed to match this rule. The keyname is specified by the --keyname foo parameter in the authprogs command line in the entry in authorized_keys.

Examples:

-


  keynames: backups


  ...
-


  keynames: [repo_push, repo_pull]


  ...
-


  keynames:


    - repo_push


    - repo_pull


  ...

ALLOW SUBRULE SECTION¶

The allow section of a rule is a single subrule or list of subrules.

Subrules can be simple, for example the explicit command match, or be more program-aware such as scp support. You specify which kind of subrule you want with the rule_type option:

-


  allow:


    -


      rule_type: command


      command: /bin/touch /tmp/timestamp


    -


      command: /bin/rm /tmp/bar


    -


      rule_type: scp


      allow_upload: true
...

See the separate subrules sections below for how to craft each type.

COMMAND SUBRULES¶

This section applies if rule_type is set to command or is not present at all.

The command requested by the client is compared to the command listed in the rule. (Spaces are squashed together.) If it matches, then the command is run.

Note that the command must be exactly the same; authprogs is not aware of arguments supported by a command, so it cannot realise that "ls -la" and "ls -a -l" and "ls -al" and "ls -l -a" are all the same. You can list multiple commands to allow you to accept variants of a command if necessary.

The simplest configuration looks like this:

-


  allow:


    command: /bin/true

Or you can provide a list of commands:

-


  allow:


    - command: /bin/true


    - command: /bin/false

A number of optional settings can tweak how command matching is performed.

○

allow_trailing_args: true: This setting allows you to specify a partial command that will match as long as the command requested by the client is the same or longer. This allows you to avoid listing every variant of a command that the client may wish to run.

Examples:

-


  allow:


    -


      command: /bin/echo


      allow_trailing_args: true


    -


      command: /bin/ls


      allow_trailing_args: true


    -


      command: /bin/rm -i


      allow_trailing_args: true

○

pcre_match: true: Compare the command using pcre regular expressions, rather than doing an explicit match character by character. The regex is not anchored at the beginning nor end of the string, so if you wish to anchor it is your responsibility to do so.

Caution: never underestimate the sneakiness of an adversary who may find a way to match your regex and still do something nasty.

Examples:

-


  allow:


    -


      # Touch the foo file, allowing any


      # optional command line params


      # before the filename


      command: ^touch\\s+(-\\S+\\s+)*foo$


      pcre_match: true


    -


      # attempt to allow rm of files in /var/tmp


      # but actually would fail to catch malicious


      # commands e.g. /var/tmp/../../etc/passwd


      #


      # As I said, be careful with pcre matching!!!


      command: ^/bin/rm\\s+(-\\S+\\s+)*/var/tmp/\\S*$


      pcre_match: true

RSYNC SUBRULES¶

authprogs has special support for rsync file transfer. You are not required to use this - you could use a simple command subrules to match explicit rsync commands - but using an rsync-specific subrule offers you greater flexibility.

Rsync support is in beta, so please raise any bugs found. Supporting the full set of rsync command line options is a moving target.

To specify rsync mode, use rule_type: rsync.

The rsync options are as follows.

○

rule_type: rsync: This indicates that this is an rsync subrule.

○

allow_upload: false|true: Allow files to be uploaded to the ssh server. Defaults to false.

○

allow_download: false|true: Allow files to be downloaded from the ssh server. Defaults to false.

○

allow_archive: false|true: Allow file archive, i.e. the options that are set when using -a or --archive. This is used to simplify authprogs configuration files. Specifying this and negating one of he associated options (e.g. allow_recursive: false) is considered an error. Defaults to false.

○

paths: a list of explicit files/directories that are allowed to match. Files specified by the client will be resolved via realpath to avoid any symlink trickery, so members of paths must be the real paths.

WARNING: specifying a directory in paths would allow rsync to act on any files therein at potentially infinite depth, e.g. when allow_recursive is set, or the client uses --files-from. If you want to restrict to specific files you must name them explicitly.

See RSYNC SYMLINK SUPPORT for potential limitations to paths.

○

path_startswith: a list of pathname prefixes that are allowed to match. Files specified by the client will be resolved via realpath and if they start with the name provided then they will be allowed.

This is a simple prefix match. For example if you had

  path_startswith: [ /tmp ]

then it would match all of the following

  /tmp


  /tmp/


  /tmpfiles      # may not be what you meant!


  /tmp/foo.txt


  /tmp/dir1/dir2/bar.txt

If you want it to match only a directory (and any infinite subdirectories) be sure to include a trailing slash, e.g. /tmp/

See RSYNC SYMLINK SUPPORT for potential limitations to paths.

○

allow_acls: false|true: Allow syncing of file ACLs. (--acls). Defaults to false.

○

allow_checksum: true|false: Allow checksum method for identifying files that need syncing. (-c / --checksum) Defaults to true.

○

allow_debug: true|false: Allow fine-grained debug verbosity. (--debug FLAGS). No support for sanity checking the debug flags that are specified. Defaults to true.

○

allow_delete: false|true: Allow any of the delete options. (--del --delete --delete-after --delete-before --delete-delay --delete-during --delete-excluded --delete-missing-args). Defaults to false.

○

allow_devices: false|true: Allow syncing of device files. (--devices). Defaults to false.

○

allow_group: false|true: Allow group change. (-g --group). Defaults to false.

○

allow_info: true|false: Allow fine-grained info verbosity. (-info FLAGS). No support for sanity checking the info flags that are specified. Defaults to true.

○

allow_links: false|true: Allow copying symlinks as symlinks. (-l --links). Defaults to false.

○

allow_group: false|true: Allow ownership change. (-o --owner). Defaults to false.

○

allow_perms: false|true: Allow perms change. (-p --perms). Defaults to false.

○

allow_recursive: false|true: Allow recursive sync. (-r --recursive). Defaults to false.

○

allow_specials: false|true: Allow syncing of special files, e.g. fifos. (--specials). Defaults to false.

○

allow_times: true|false: Allow setting synced file times. (-t --times). Defaults to true.

○

allow_verbose: true|false|#: Allow verbose output. (-v --verbose). Rsync allows multiple -v options, so this option accepts true (allow any verbosity), false (deny any verbosity), or a number which indicates the maximum number of -v option that are allowed, e.g. 2 would allow -v or -vv but not -vvv. Defaults to true.

SCP SUBRULES¶

authprogs has special support for scp file transfer. You are not required to use this - you could use a simple command subrules to match explicit scp commands - but using an scp-specific subrule offers you greater flexibility.

To specify scp mode, use rule_type: scp.

The scp options are as follows.

○

rule_type: scp: This indicates that this is an scp subrule.

○

allow_upload: false|true: Allow files to be uploaded to the ssh server. Defaults to false.

○

allow_download: false|true: Allow files to be downloaded from the ssh server. Defaults to false.

○

allow_recursive: false|true: Allow recursive (-r) file up/download. Defaults to false.

○

allow_recursion: false|true: Deprecated version of allow_recursive. will be removed in 1.0 release.

○

allow_permissions: true|false: Allow scp to get/set the permissions of the file/files being transferred. Defaults to false.

○

paths: The paths option allows you to specify which file or files are allowed to be transferred. If this is not specified then transfers are not restricted based on filename.

Examples:

-


  allow:


    - rule_type: scp


      allow_download: true


      paths:


        - /etc/group


        - /etc/passwd


    - rule_type: scp


      allow_upload: true


      paths: [/tmp/file1, /tmp/file2]

EXAMPLES¶

Here is a sample configuration file with multiple rules, going from simple to more complex.

Note that this config can be spread around between the ~/.ssh/authprogs.yaml and ~/.ssh/authprogs.d directory.

# All files should start with an initial solo dash -
# remember, we´re being concatenated with all other
# files!
# Simple commands, no IP restrictions.
-


  allow:


    - command: /bin/tar czvf /backups/www.tgz /var/www/


    - command: /usr/bin/touch /var/www/.backups.complete
# Similar, but with IP restrictions
-


  from: [192.168.0.10, 192.168.0.15, 172.16.3.3]


  allow:


    - command: git --git-dir=/var/repos/foo/.git pull


    - command: sudo /etc/init.d/apache2 restart
# Some more complicated subrules
-


  # All of these ´allows´ have the same ´from´ restrictions


  from:


    - 10.1.1.20


    - 10.1.1.21


    - 10.1.1.22


    - 10.1.1.23


  allow:


    # Allow unrestricted ls


    - command: /bin/ls


      allow_trailing_args: true


    # Allow any ´service apache2 (start|stop)´ commands via sudo


    - command: sudo service apache2


      allow_trailing_args:true


    # How about a regex? Allow wget of any https url, outputting


    #  to /tmp/latest


    - command: ^/usr/bin/wget\\s+https://\\S+\\s+-O\\s+/tmp/latest$


      pcre_match: true


    # Allow some specific file uploads


    - rule_type: scp


      allow_upload: true


      paths:


        - /srv/backups/host1.tgz


        - /srv/backups/host2.tgz


        - /srv/backups/host3.tgz


    # Allow rsync to upload everything, deny any download


    - rule_type: rsync


      allow_upload: true


    # Allow rsync to recursively sync /tmp/foo/ to the server


    # in archive mode (-a, or any subset of -logptrD)


    # but do not allow download


    - rule_type: rsync


      allow_upload: true


      allow_recursive: true


      allow_archive: true


      paths:


        - /tmp/foo


    # Allow rsync to write some specific files and any individual


    #   files under /data/lhc/ directory, such as /data/lhc/foo


    #   or /data/lhc/subdir/foo.


    #


    # Disallow download (explicitly listed) or recursive


    #    upload (default false).


    - rule_type: rsync


      allow_upload: true


      allow_download: false


      paths:


        - /srv/htdocs/index.html


        - /srv/htdocs/status.html


      path_startswith:


        - /data/lhc/

TROUBLESHOOTING¶

--dump_config is your friend. If your yaml config isn´t parsing, consider --dump_config --logfile=/dev/tty for more debug output to find the error.

FILES¶

○

~/.ssh/authorized_keys: The default place your key should be installed and configured to call authprogs. The actual location can differ if your administrator has changed it.

○

~/.ssh/authprogs.yaml: Default authprogs configuration file. Override with --configfile.

○

~/.ssh/authprogs.d: Default authprogs configuration directory. Override with --configdir.

ENVIRONMENT¶

authprogs uses the following environment variables that are set by the sshd(8) binary:

○

SSH_CONNECTION: This is used to determine the client IP address.

○

SSH_CLIENT: This is used to determine the client IP address if SSH_CONNECTION was not present.

○

SSH_ORIGINAL_COMMAND: The (squashed) original SSH command that was issued by the client.

authprogs sets the following environment variables for use by the authenticated process

○

AUTHPROGS_KEYNAME: the value of the --keyname command line. Will be set to an empty string if no --keyname was set.

EXIT STATUS¶

On unexpected error or rejecting the command authprogs will exit 126.

If the command was accepted then it returns the exit code of the command that was run.

Note that if you´re invoking ssh via another tool that program may provide a different exit status and provide a misleading error message when authprogs returns a failure, For example rsync will exit 12 and assume a "protocol problem" rather than a rejection on the server side.

LOGGING AND DEBUGGING¶

If a --logfile is specified then it will be opened in append mode and a line about each command that is attempted to be run will be written to it. The line itself is in the form of a python dictionary.

If authprogs is run with --debug, then this logfile will get increased debugging information, including the configuration, rule matching status as they are checked, etc.

HISTORY¶

A perl version of authprogs was originally published at https://www.hackinglinuxexposed.com/articles/20030115.html in 2003. This is a complete rewrite in python, with a more extensible configuration, and avoiding some of the limitations of the former.

SEE ALSO¶

ssh(1), sshd(8), scp(1).

AUTHOR¶

Bri Hatch mailto:bri@ifokr.org