NRPE SECURITY README

TCP WRAPPER SUPPORT

NRPE 2.x includes native support for TCP wrappers. Once you compile NRPE you can check to see if it has wrapper support built in by running the daemon from the command line without any arguments like this:

./nrpe --help

COMMAND ARGUMENTS

NRPE 2.0 includes the ability for clients to supply arguments to commands which should be run. Please note that this feature should be considered a security risk, and you should only use it if you know what you're doing!

BASH COMMAND SUBSTITUTION

Even with the metacharacter restrictions below, if command arguments are enabled, it is still possible to send bash command substitutions in the form $(...) as an argument. This is explicitly disabled by default, but can be enabled by a configure-time option and a configuration file option. Enabling this option is VERY RISKY and its use is HIGHLY DISCOURAGED.

ENABLING ARGUMENTS

To enable support for command argument in the daemon, you must do two things:

Run the configure script with the --enable-command-args option
Set the dont_blame_nrpe directive in the NRPE config file to 1.

ENABLING BASH COMMAND SUBSTITUTION

To enable support for arguments containing bash command substitions, you must do two things:

Enable arguments as described above
Include the --enable-bash-command-substitution configure option when running the configure script
Set the allow_bash_command_substitutions directive in the NRPE config file to 1.

ILLEGAL METACHARS

To help prevent some nasty things from being done by evil clients, the following metacharacters are not allowed in client command arguments:

| ` & > < ' " \ [ ] { } ; !

Any client request which contains the above mentioned metachars is discarded.

USER/GROUP RESTRICTIONS

The NRPE daemon cannot be run with (effective) root user/group privileges. You must run the daemon with an account that does not have superuser rights. Use the nrpe_user and nrpe_group directives in the config file to specify which user/group the daemon should run as.

ENCRYPTION

If you do enable support for command arguments in the NRPE daemon, make sure that you encrypt communications either by using:

Stunnel (see http://www.stunnel.org for more info)
Native SSL support (See the README.SSL.md file for more info)

Do NOT assume that just because the daemon is behind a firewall that you are safe! Always encrypt NRPE traffic!

USING ARGUMENTS

How do you use command arguments? Well, lets say you define a command in the NRPE config file that looks like this:

command[check_users]=/usr/local/nagios/libexec/check_users -w $ARG1$ -c $ARG2$

You could then call the check_nrpe plugin like this:

./check_nrpe -H <host> -c check_users -a 5 10

The arguments '5' and '10' get substituted into the appropriate ARGx macros in the command (ARG1 and ARG2, respectively). The command that would be executed by the NRPE daemon would look like this:

/usr/local/nagios/libexec/check_users -w 5 -c 10

You can supply up to 16 arguments to be passed to the command for substitution in ARG macros (ARG1 - ARG16).

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