AUTHPROGS(1)

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 identies 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 AAAAB3NzaC1yc2E.....OgQ7Pm1X8= 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 AAAAB3NzaC1yc2E.....OgQ7Pm1X8= user@example.com

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

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

    command="/usr/bin/authprogs --keyname=backups --run" ssh-rsa AAAA...Pm1X8= 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..Pm1X8= 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) programatically. It also disableds 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..Pm1X8= 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.

--help
    Show help information

OTHER OPTIONS
The folowing 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_COMAND 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 commandlines via ssh directly.

CONFIGURATION FILES
authprogs configuration files are written as a series of yaml documents. Each yaml document represents a single authprogs rule. These 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.

Rules are read and processed in the following order:

•   rules in the configuration file, from top to bottom.

•   rules in the configuration directory, processed in asciibetical order

Dotfiles contained in a configuration directory are not processed. 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:

    --
    # 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 begins here
    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.

Examples:

    --
    from: 192.168.1.5
    --
    from: [192.168.0.1, 10.0.0.3]
    --
    from:
        - 192.168.0.1
        - 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 comamnd, so it cannot realize 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

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

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

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

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

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

    Examples:

      --
      allow:
          - rule_type: scp
            allow_download: true
            files:
                - /etc/group
                - /etc/passwd
          - rule_type: scp
            allow_upload: true
            files: [/tmp/file1, /tmp/file2]

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

    --
    # 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
    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
          files:
              - /srv/backups/host1.tgz
              - /srv/backups/host2.tgz
              - /srv/backups/host3.tgz

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.

EXIT STATUS
authprogs returns 0 on success, non-zero on errors. In run mode it exits with the exit code of the command that was requested, or 126 on unexpected errors.

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 http://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 bri@ifokr.org