R-Services [512, 513, 514]

info

Basically it was before SSH, it was used to communicate between Unix devices, but the transmissions occured in an unencypted format

R-Command Suite Programs

The R-commands suite consists of the following programs:

• rcp (remote copy)

• rexec (remote execution)

• rlogin (remote login)

• rsh (remote shell)

• rstat

• ruptime

• rwho (remote who)

Command	Service Daemon	Port	Transport Protocol	Description
rcp	rshd	514	TCP	Copy a file or directory bidirectionally from the local system to the remote system (or vice versa) or from one remote system to another. It works like the cp command on Linux but provides no warning to the user for overwriting existing files on a system.
rsh	rshd	514	TCP	Opens a shell on a remote machine without a login procedure. Relies upon the trusted entries in the /etc/hosts.equiv and .rhosts files for validation.
rexec	rexecd	512	TCP	Enables a user to run shell commands on a remote machine. Requires authentication through the use of a username and password through an unencrypted network socket. Authentication is overridden by the trusted entries in the /etc/hosts.equiv and .rhosts files.
rlogin	rlogind	513	TCP	Enables a user to log in to a remote host over the network. It works similarly to telnet but can only connect to Unix-like hosts. Authentication is overridden by the trusted entries in the /etc/hosts.equiv and .rhosts files.

/etc/hosts.equiv

The /etc/hosts.equiv file contains a list of trusted hosts and is used to grant access to other systems on the network. When users on one of these hosts attempt to access the system, they are automatically granted access without further authentication.

Scanning for R-Services

sudo nmap -sV -p 512,513,514 10.0.17.

Access Control & Trusted Relationships

The primary concern for r-services, and one of the primary reasons SSH was introduced to replace it, is the inherent issues regarding access control for these protocols. R-services rely on trusted information sent from the remote client to the host machine they are attempting to authenticate to. By default, these services utilize Pluggable Authentication Modules (PAM) for user authentication onto a remote system; however, they also bypass this authentication through the use of the /etc/hosts.equiv and .rhosts files on the system. The hosts.equiv and .rhosts files contain a list of hosts (IPs or Hostnames) and users that are trusted by the local host when a connection attempt is made using r-commands. Entries in either file can appear like the following:

Note: The hosts.equiv file is recognized as the global configuration regarding all users on a system, whereas .rhosts provides a per-user configuration.

sample .rhosts

z3tssu@htb[/htb]$ cat .rhosts

htb-student     10.0.17.5
+               10.0.17.10
+               +

As we can see from this example, both files follow the specific syntax of <username> <ip address> or <username> <hostname> pairs. Additionally, the + modifier can be used within these files as a wildcard to specify anything. In this example, the + modifier allows any external user to access r-commands from the htb-student user account via the host with the IP address 10.0.17.10.

Misconfigurations in either of these files can allow an attacker to authenticate as another user without credentials, with the potential for gaining code execution. Now that we understand how we can potentially abuse misconfigurations in these files let's attempt to try logging into a target host using rlogin.

Login using R-login

z3tssu@htb[/htb]$ rlogin 10.0.17.2 -l htb-student

Last login: Fri Dec  2 16:11:21 from localhost

[htb-student@localhost ~]$

Listing Authenticated Users with Rwho

z3tssu@htb[/htb]$ rwho

root     web01:pts/0 Dec  2 21:34
htb-student     workstn01:tty1  Dec  2 19:57  2:25 

Listing Authentication Users with R-Users

rusers -al 10.0.17.5